UNIVERSITY OF WATERLOO

Faculty of Engineering

Nanotechnology Engineering

The Effects of Water Concentration on Solder Voiding in Lead-Free Solder

Prepared by:

Graeme Williams

2A Nanotechnology

UNIVERSITY OF WATERLOO

Faculty of Engineering

Nanotechnology Engineering

The Effects of Water Concentration on Solder Voiding in Lead-Free Solder

Research In Motion – Materials Interconnect Lab

451 Phillip St., Waterloo, ON

Prepared by:

Graeme Williams

USER ID g3willia

2A Nanotechnology

April 24, 2007

354 Tallwood Dr.

Orillia, Ontario

L3V 2G9

April 24, 2007

Dr. Marios Ioannidis, director

Nanotechnology Engineering

University of Waterloo

Waterloo, Ontario

N2L 3B9

Dear Sir,

This report, entitled, “The Effects of Water Concentration on Solder Voiding in Lead-Free Solder”,

was prepared in fulfillment of the course WKRPT 200 and the work report requirements of my 2A

co-op term. This is my second work term report. The purpose of this report is to investigate methods

to alter solder voiding in the solder joints on a printed circuit pack. This project was completed at the

Materials Interconnect Lab at Research In Motion. The report is a confidential-1 report.

Research In Motion is recognized worldwide as a leader in wireless technology and innovation. RIM

is also renowned for its successful corporate personal digital assistant, the BlackBerry. Recently,

RIM has delved into the consumer market with its release of the Blackberry Pearl, a smartphone

developed for the average cell phone user. Research In Motion has continued to be successful as a

result of its dedication to quality and innovation.

As a member of the Materials Interconnect Lab, I was responsible for the conduction of quality

assurance tasks for the RIM manufacturing plant based in Waterloo. In this position, I received test

requests from personnel in various areas in the plant. These test requests required me to analyze

many aspects of the RIM products, including functionality, reliability and fulfillment of industry

regulations. Throughout the term, I also worked on a larger voiding project, directly related to this

report.

I would like to thank Bev Christian and David Connell for their assistance on this project. David‟s

provision of crucial materials needed to conduct this experiment was especially helpful. I hereby

confirm that I have received no further help other than what is mentioned above in writing this report.

I also confirm this report has not been previously submitted for academic credit at this or any other

academic institution.

Sincerely,

Graeme M. Williams

iii

Contributions

The team that I worked for at Research In Motion is relatively small. The entire Materials

Interconnect Lab consists of fourteen people. I was the sole person working directly on the voiding

project; however, I received periodic assistance from various people within the manufacturing plant

and the lab.

The Materials Interconnect Lab is a critical aspect of the Manufacturing Quality Assurance group at

Research In Motion. The primary responsibility of the lab is to provide accurate and reliable results

regarding any test requests from within the manufacturing plant. These test requests can range from

tests of the solderability of a single component to the x-ray analysis of a fully assembled device. The

test requests are completed with the aid of a number of technical pieces of equipment within the lab.

Completing these test requests provided me with a great understanding of the electronics industry. It

also gave me invaluable experience with many pieces important, industry-standard equipment.

The primary tasks that were my responsibility included: solderability tests, x-ray analysis, x-ray

fluorescence analysis, and tests involving component cross-sectioning and further visual inspection.

In addition to these basic tasks, my role at RIM also included lab maintenance. In particular, I was

responsible for conducting lab chemical inventories and updating chemical data sheets.

Solderability tests were conducted with the MUST System II Solderability Tester. This machine

allows one to use either a solder globule or a solder bath to determine the ability of an individual

component to solder to a board. In this test, the component is slowly lowered into the solder globule

or bath, and a balance within the cantilever on the machine measures the forces experienced by the

component. With the aid of this test, it is possible to determine if the wetting force of the component

is sufficient, such that the component will remain on the circuit board through and after the reflow

process. This test is especially useful for older components, where a thick oxide layer formed on the

edge of the component may prevent it from wetting to the board. Consequently, the test was used

extensively when the manufacturing warehouse was being cleaned and older reels of components

were found.

X-Ray analysis was completed with the Fein Focus X-Ray System. Using the x-ray machine, it was

my duty to examine populated circuit boards and fully assembled devices for defects. These defects

iv

usually manifested in the form of solder-related issues, such as shorts and irregularly-shaped solder

balls. The x-ray machine was especially useful because it is a non-destructive test, allowing one to

keep the device intact while gathering crucial information. X-Ray analysis was performed as a

preliminary task for many test requests brought into the Materials Interconnect Lab, including all test

requests involving the cross-sectioning of components on a populated circuit board.

X-Ray Fluorescence Analysis was completed using the Fischer XRF. This test is useful to determine

the compositions of materials. It is also effective to find metal-plating thicknesses. In my role in the

lab, I spent a large amount of time using the XRF in order to detect industry-recognized, potentially-

harmful elements, such as lead, mercury, cadmium and chromium (VI). These elements are

potentially toxic and should not be present in significant concentrations in any electronic devices that

are required to meet Chinese and European Union regulations. In my role, I would often test single

components to be used on circuit boards. Occasionally, I would have to disassemble entire devices in

order to test every aspect of the devices for unwanted levels of these metals. I also used XRF as a

method to determine metal coating thicknesses on various plated contacts, such as those used for

batteries and headset plugs.

Cross-sectioning is one of the key tasks performed in the Materials Interconnect Lab. In this test, a

component from a populated circuit board is cut and mounted in epoxy. The epoxy sample is then

ground and polished in order to look at the component‟s solder joints. This task is especially useful

for less-accessible components, such as ball-grid arrays. In my role, it was necessary to use a

metallurgical microscope to examine the solder joint integrity of many components. More

specifically, the solder joints were examined for possible micro-cracks, adequate wetting and a

passable intermetallic layer. This task was often performed after specific devices had undergone

rigorous testing, such as drop-testing or thermal cycling.

An overall lab liquid inventory was completed in the beginning of this term. Following the inventory,

the Material Safety Data Sheet binder was updated to account for all chemicals present in the lab.

These actions were necessary in order to ensure proper safety and control measures could be made for

each chemical. Additionally, a complete lab inventory was necessary as one of the aspects required

for RIM to achieve ISO 14001 certification – an important recognition of Research In Motion‟s

environmental awareness.

v

While conducting these basic tasks, I also worked on a solder voiding project using a batch reflow

oven and the previously mentioned equipment. Although a great amount of effort has been dedicated

to understanding the chemistry and metallurgy of the electronics industry, solder voiding has

remained a source of ambiguity. Large amounts of solder voiding can lead to cracks and problems

within the solder joints of board components. The ultimate goal of this project was to determine if it

would be possible to control the degree of voiding within solder joints. This was believed to be

possible by changing the temperature reflow profile for the circuit board, and altering the water

concentration in the solder paste.

This report is a direct result of the aforementioned solder voiding project. This project was a benefit

to my development as an engineer, as it provided me with both practical and academic experience and

knowledge. As a Nanotechnology Engineering student, it is necessary to specialize in my final two

years of education. Since I intend to specialize in nano-electronics and nano-materials, this task

provided insight into the problems that I may encounter in my future career. Nanotechnology may

provide a key role in controlling solder voiding to an even greater extent in the future. Furthermore,

this experiment gave me critical experience with many industry-standard pieces of equipment.

Although Research In Motion has been successful in manufacturing products for a commercial

market, background research truly drives these products. The solder voiding project is an example of

RIM‟s dedication to a further understanding of the electronics industry. This project does not provide

a direct impact on RIM‟s product line; however, the knowledge gained from the experiments may

prove to be useful for developing future solder pastes and reflow profiles. Alterations to the current

solder pastes and reflow profiles could potentially create more reliable end products.

vi

Summary

The main purpose of this report is to investigate methods to augment and control voiding in solder

joints through adjustments to the moisture within solder paste. In order to obtain data for this report,

a solder voiding experiment was completed. The details and results of this experiment are presented

and analyzed throughout this report. The voiding percentage, number of voids, position of voids and

the percent of large voids were examined in various test samples to provide a thorough analysis of

solder voiding.

The intended audience of this report should have a strong grasp of solder and flux chemistry. The

individual reading this report should also have basic knowledge of the surface mount technology

assembly process and reflow profiling.

The key points addressed in this document include an overview of surface mount technology, an

explanation of the presence of solder voiding in surface mount technology, a description of the

experiment design, details on the development of the experiment reflow profile, and an analysis of the

experiment data. Recommendations for further research in solder voiding are also made throughout

various portions of the body of the report.

It is concluded that, for water concentrations equal to or less than 0.10%, the solder paste water

concentration does not significantly affect the number of voids and the percentage of voiding within

the solder. For water concentrations greater than 0.50%, the solder paste is unstable and experiences

major defects, resulting in a large degree of voiding. It was also found that the position of the voids

in all samples for this experiment was random. Furthermore, as the water concentration of the solder

paste increased, the presence of small voids within the solder increased. It can also be concluded that

for the purpose of this experiment, the ramp-to-spike profile was the optimum reflow profile.

The major recommendation of this report is to continue research on various aspects of solder voiding.

In particular, it is recommended that this experiment is duplicated using high boiling point solvents

instead of water, modified test coupons, and various different devices, such as ball grid arrays.

Further testing on solder voiding with the use of different reflow profiles would also be of interest.

An additional recommendation in this report is to examine the differences between solder reflowed in

an assembly-line reflow oven and a batch reflow oven.

vii

Conclusions

From the analysis in the report body, it was concluded that:

The ramp-to-spike profile is the optimum profile for studying voiding within solder paste

while altering an additional variable, such as water concentration.

For small water concentrations, equal to or less than 0.10%, the solder paste water

concentration does not significantly affect the number of voids within the solder.

For the same small water concentrations, the solder paste water concentration does not

significantly affect the percentage of voiding within the solder.

For large water concentrations, greater than 0.50%, the solder paste water concentration

greatly affects the stability of the solder paste, often resulting in bursting of the solder and the

creation of large, non-circular voids.

For all concentrations of water, the addition of water to solder paste increases the presence of

small voids within the solder.

The position of voids within the solder is random due to a migration effect as a result of the

strong, forced air of the batch reflow oven.

viii

Recommendations

Based on the analysis and conclusions in this report, it is recommended that:

More research is made on methods to increase solder voiding with the addition of high-

boiling solvents in solder paste.

Further studies are made on the differences between the assembly-line reflow oven and the

batch reflow oven from the experiment, specifically with regard to the location of voids on

the test coupons.

Additional research is spent on solder voiding for different types of test coupons, especially

with coupons that can control the spread and flow of solder during the reflow process.

Supplementary investigations are made on voiding with actual electronic components, such

as ball grid arrays.

More tests are made on solder voiding using alternate reflow profiles, such as ramp-soak-

spike and long-soak profiles.

ix

Table of Contents

Page

Contributions .................................................................................................................... iii

Summary .......................................................................................................................... vi

Conclusions ..................................................................................................................... vii

Recommendations .......................................................................................................... viii

List of Figures ................................................................................................................... x

List of Tables .................................................................................................................... xi

1. Introduction ......................................................................................................................1

2. SMT Assembly ..................................................................................................................2 2.1. Leaded and Lead-free Solders .............................................................................. 2

2.2. Common Reflow Processes and Profiles .............................................................. 3

2.3. RSS, RTS and LSP Reflow Profiles ..................................................................... 4

2.4. Voiding and its Presence in Electronics ................................................................ 4

3. Experiment Purpose and Design .....................................................................................6 3.1. Basic Experiment Procedure ................................................................................. 6

3.2. The Batch Reflow Oven versus the Conventional Reflow Oven ......................... 7

3.3. X-Ray and Image Analysis ................................................................................... 8

4. Reflow Profile Development and Results .....................................................................10

5. Experimentation with Moisture in Solder Paste .........................................................14 5.1. Project Data Analysis – Part One ........................................................................ 14

5.2. Project Data Analysis – Part Two ....................................................................... 15

5.3. Project Data Analysis – Part Three ..................................................................... 16

5.4. Project Data Analysis – Comprehensive Notes .................................................. 17

6. Concluding Notes and Recommended Actions ............................................................19 References ....................................................................................................................... 20

x

List of Figures

Page

Figure 1 - A Blank Test Coupon for the Voiding Experiment ................................................. 7

Figure 2 - Basic Solder Sample with Voiding .......................................................................... 8

Figure 3 - Example Solder Void Mask ..................................................................................... 9

Figure 4 - Sample Reflow Profile with Reflow Oven Parameters .......................................... 11

Figure 5 - RTS Reflow Profile - Attempt One ........................................................................ 12

Figure 6 - RTS Reflow Profile - Attempt Two ....................................................................... 13

Figure 7 - RTS Reflow Profile - Attempt Three ..................................................................... 13

Figure 8 - Unusually Low Solder Voiding for Solder with 0.50% Water Concentration ...... 15

Figure 9 – Solder with Erratic Voiding due to High Water Concentrations ........................... 15

xi

List of Tables

Page

Table 1 - Criteria and Requirements for the RTS Profile ....................................................... 11

Table 2 - Oven Parameters for Reflow Profile Development Attempts ................................. 12

Table 3 - X-Ray Image Analysis - Part One ........................................................................... 14

Table 4 - X-Ray Image Analysis - Part Two .......................................................................... 15

Table 5 - X-Ray Image Analysis - Part Three ........................................................................ 16

Table 6 - Summary of X-Ray Data Analysis .......................................................................... 17

1

1. Introduction

The purpose of this report is to investigate methods to increase and control solder voiding in solder on

a printed circuit board (PCB). In order to manipulate solder voiding throughout the experiments,

various concentrations of water were added to samples of solder paste. The solder paste was then

applied to test boards and reflowed. Due to the relatively low boiling point of water and the

possibility of bubbles within the solder paste, the water concentration within solder paste is usually

kept at very low values. As a result, this experiment is a relatively novel idea. Water is deliberately

added to the solder paste in an attempt to increase solder voiding, in order to form voids on demand

for study.

Solder voiding has always been a concern in the electronics industry, especially with regard to surface

mount technology (SMT). The concern is becoming even more prevalent, as lead-free solders are

becoming more common in electronic assembly. Lead-free solders tend to have larger, more

frequent voids in components on PCBs [1]. It is believed that voids have a significant effect on the

time-to-failure for solder joints under thermal and mechanical loading conditions [2]. As a result,

there is a strong motivation to reduce the number and size of voids present in the more fragile solder

joints of components, such as those in ball grid arrays (BGAs).

This report was written as a summary of a research interest for the Materials Interconnect Lab at

Research In Motion (RIM). The research interest was investigated through a project that was

completed over a two-month period of time. The intended audience of this report includes any of the

manufacturing staff or personnel at RIM; however, any individual with a basic knowledge of solder

defects and the reflow process may read this report. A comprehension of the solder and flux

chemistry involved in electronic manufacturing is required to fully understand this report.

This report introduces the reader to more advanced aspects of SMT assembly, with topics in reflow

processes and profiles, solder defects, and solder voiding. Using this knowledge as a base, the report

describes the methods and procedures used throughout the aforementioned project. The final portions

of this report provide analysis with respect to both the reflow profile designed for the project, and

solder voiding as a function of water percentage in the solder paste. As the primary topic of this

report, solder voiding as a function of solder paste water concentration is examined extensively using

images obtained through x-ray analysis.

2

2. SMT Assembly

SMT assembly is an industry-standard approach to creating PCBs for electronics. The basic process

for creating a PCB using SMT involves:

- the application of solder paste to various areas on a circuit board with the use of a stencil

- the placement of individual components onto specific positions on the solder paste on the

circuit board

- the reflow process, in which the circuit board is baked in a reflow oven using a specific

temperature profile, thereby melting the solder, which on cooling holds the components on

the board

This section of the report will discuss the recent change from leaded to lead-free solder in SMT

assembly, and provide a discussion regarding reflow processes and profiles. These aspects of SMT

are particularly important with regard to controlling voiding within solder, as will be detailed in the

final portion of this section. These discussions will provide a strong basis of understanding for the

completion of the project described in section 3 of the report.

2.1. Leaded and Lead-free Solders

In most modern electronic manufacturing plants producing consumer products, the solder in the

solder paste is lead-free. One of the most common lead-free solders is composed of silver, copper

and tin [3]. The recent lead-free movement is a result of the Restriction of Hazardous Substances

(RoHS) for electric and electronic equipment enforced by the European Union. The RoHS initiative

prohibits certain concentrations of cadmium, lead, mercury, hexavalent chromium, polybrominate

biphenyls and polybrominate diphenyl ethers within electronic parts, components and solders [1]. As

mentioned in section 1, these lead-free solders often exhibit more extensive voiding than leaded

solders. Additionally, lead-free solders have higher melting temperatures and narrower process

temperature ranges [3]. Consequently, they must adhere to stricter reflow temperature profiles in

order to provide functional and defect-free solder joints.

3

2.2. Common Reflow Processes and Profiles

The overall reflow process effectively attaches components onto the circuit board, creating numerous

electrical connections, in order to make a functional PCB. The notion behind reflow is relatively

simple: one must use solder paste as glue between the pads on the circuit board and the terminations

of the components. In order to achieve wetting among the pad, the component, and the solder paste,

it is necessary to melt the metal beads within the solder paste at an appropriate temperature. Reflow

ovens use forced convection air to spread heat evenly over PCBs, allowing the solder paste to reach

required temperatures.

In addition to heating the solder paste, metal oxides on the metal beads must be removed in order for

proper wetting to occur. The flux within the solder paste plays the crucial role in removing oxides

from the metal beads. As the solder paste is heated, the flux effectively removes oxides; however, the

flux is activated at a significantly lower temperature than the melting temperature of the solder beads.

If the solder paste is kept at elevated temperatures for too long, prior to the melting of the solder

beads, the flux will prematurely evaporate [4]. This could result in poor solder wetting, as a new

layer of oxide may have opportunity to accumulate on the solder beads.

In order to combat the issues related to flux activity and a number of solder defects including solder

voiding, specific reflow temperature profiles have been developed for baking PCBs. There are an

infinite number of variations of temperature profiles for reflow ovens. Two particular profiles are

commonly used in electronic assembly: the ramp-soak-spike (RSS) profile and the ramp-to-spike

(RTS) profile. An additional profile developed by AIM Solder, named the long-soak profile (LSP),

will also be considered for the purpose of this report [5].

Reflow profiles are created using a number of zones. In the simplest profiles, there are four zones to

consider: the ramp or preheat zone, the soak zone, the spike or reflow zone, and the cooling zone [6].

The ramp zone involves a gradual heating of the board, while the soak zone keeps the board at a

constant, elevated temperature. The purpose of the soak zone is to allow the different-shaped and

different-sized components on the board to reach the same temperature [4]. The spike zone heats the

board to a peak temperature, and is followed immediately by the cooling zone. In the spike and

cooling zones, the solder is melted and a metallic bond is formed between the pad on the board and

the termination of the component. On a manufacturing assembly line, these zones are created using a

conveyor belt and a reflow oven with a number of temperature-controlled areas. Based on the speed

4

of the conveyor belt, the board spends a specific amount of time in each area in order to create the

reflow profile.

2.3. RSS, RTS and LSP Reflow Profiles

The RSS profile is one of the most basic profiles used in SMT assembly. In the RSS profile, the

circuit board is heated in a ramp zone to a mid-range temperature. The circuit board soaks at this

mid-range temperature in order to allow all components on the board to reach the same temperature.

The temperature then spikes to a specific reflow temperature, and the board is immediately cooled to

allow the solder joint to form. This method of reflow is commonly used in older manufacturing

facilities due to the inability of reflow ovens to keep similar temperatures among components. In

newer facilities, the difference in temperature among components is usually negligible, as the reflow

ovens adequately force hot air to all parts of the board [4].

As an adaptation to the newer reflow ovens, the RTS profile was designed to remove the soak zone.

The RTS profile exhibits a less-steep, constant ramp to a temperature close to the melting temperature

of the solder. At this point, the temperature spikes rapidly to the required reflow temperature and the

board is then cooled. The RTS profile is used as a regular reflow profile for electronic assembly by

many industry leaders in SMT, including RIM. This is the profile chosen for experiments that will be

described in section 3.

The LSP profile is a reflow profile designed by AIM Solder specifically to reduce voiding in solder.

The profile is very similar to the RSS profile; however, it spends an extended amount of time in the

soak zone. In this case, the soak zone is not only used to allow components to reach the same

temperature. In addition, the soak zone is used to dry the flux and solvents within the solder paste

more than the traditional RSS profile [5]. The importance of drying the solder paste will be discussed

further in section 2.4.

2.4. Voiding and its Presence in Electronics

Although flux plays an important role in removing oxides from the metal beads in solder paste, it also

has the potential to cause voiding within solder joints because of flux entrapment. Additional voids

can also form within solder joints because of air and moisture entrapment. These voids generally

5

occur as a result of outgassing due to moisture or contaminants in the laminate. Larger voids, in

particular, may reduce the fracture strength of solder joints [7]. Specifically regarding BGAs, where

voiding is very critical, there are three recognized classes of voids: small, medium and large. These

classes are categorized by the percentage of cross-sectional area that the voids occupy with respect to

the full solder joints. Small voids occupy less than 9% of the total area, whereas medium voids

occupy 9 to 20.25% of the total area, and large voids occupy 20.25% to 36% of the total area [8].

In addition to void size, void position also has a large role in the integrity of solder joints. Voids in

high-stress locations can reduce the time-to-failure under thermal and mechanical loading conditions.

High-stress void locations include crack propagation paths and the intermetallic layers of solder

joints. Furthermore, voids along the board-component interface reduce the bonding area of the solder

joint, which can effectively lower joint strength [2].

There are several methods to significantly reduce and alter voiding within solder joints. In general,

changing the reflow profile to increase both the peak reflow temperature and the time above the

melting temperature of the solder can decrease voiding [4]. Bubbles of air, contaminants and flux can

more easily bubble out of the solder at these elevated temperatures. The LSP profile, as described in

section 2.3, removes excess flux and solvents in order to reduce the number of entrapped chemicals in

the solder. The remainder of this report will examine a method to purposefully increase voiding

within solder joints, by controlling the amount of moisture within the solder paste prior to applying it

to the circuit board.

6

3. Experiment Purpose and Design

Solder joint voiding can be caused by flux, air and water entrapment. The experiment detailed

throughout this section provides additional moisture to solder paste. The intended purpose of this

experiment is to increase the amount of voiding in a controlled, predictable manner. Additional water

within solder paste should result in more water entrapment. However, an additional hypothesis exists

that the moisture will outgas from the solder paste more readily than entrapped flux or air. This

outgassing would potentially remove additional voids within the solder joint in the reflow process of

the reflow profile. As a result, it is possible that the percentage of voiding within the solder joint will

decrease as additional moisture is added to the solder paste.

In addition to testing the aforementioned hypotheses, the experiment detailed in this section will

provide further research on other effects of moisture within solder paste. This section will also

provide recommendations regarding the overall experimental process. Specific recommendations will

address the reflow oven used throughout the experiment and the methods of analysis for the reflowed

samples.

3.1. Basic Experiment Procedure

The following procedure was used to conduct the voiding experiment:

- An RTS reflow profile was designed using the batch reflow oven, type-k thermocouples, and

a temperature profiling device. The Advanced Techniques PRO 1600 Batch Reflow Oven

was used as the reflow oven for this experiment [9]. The SuperMOLE Gold Data

Temperature Profiler was used to create the reflow profile used in this experiment [10]. The

profile design process will be discussed further in section 4.

- Approximately 100 grams of solder paste were added into several separate containers.

Alpha® OM-338T Solder Paste was used for this experiment [11].

- Shortly prior to running the samples in the reflow oven, varying amounts of deionized (DI)

water were added to the containers of solder paste. The DI water was added using a micro-

pipette to achieve the following concentrations of moisture by weight: 0%, 0.050%, 0.10%,

0.50% and 1.0%.

- The various samples of solder paste were applied to specially-designed test coupons. An

example of a blank coupon pair can be seen in figure 1 below.

7

Figure 1 - A Blank Test Coupon for the Voiding Experiment

The blank coupons were cut in pairs and cleaned using water and isopropyl alcohol. The

coupons were then dried completely using a compressed air gun and a dry box. This cleaning

process was performed shortly before the application of solder paste to the coupons.

- Solder paste was applied using a stencil of 0.78 mm thickness and a squeegee in order to

ensure that the same amount of solder paste was provided to each sample. For further testing,

it is recommended that various different types of coupons with various different components

are reflowed, in addition to these blank coupons. It would be of particular interest to test the

attachment of BGAs to test coupons using solder paste with varying water concentrations.

- Each coupon pair was placed individually onto the rack inside the reflow oven. Due to the

small size of the coupons, the coupon pairs were attached to the rack using a small amount of

kapton tape. This was done to prevent the coupons from sliding within the oven as a result of

the forced, convection air. In order to ensure that the kapton tape did not affect the heating of

the board, reflow profiles were analyzed using similarly-sized test boards taped to the rack in

the same manner. These test boards showed negligible differences in their reflow profiles

when compared to boards that were not attached to the oven rack.

- Each coupon pair was baked alone in the reflow oven using the RTS reflow profile.

- After completing the reflow process for all of the desired samples, x-ray images were taken

for all of the coupons.

The above experiment was performed initially using all of the previously-mentioned solder paste

water concentrations. After the initial trial, the experiment was performed an additional two times

using only the three lowest concentrations of water.

3.2. The Batch Reflow Oven versus the Conventional Reflow Oven

The assembly-line reflow oven described in section 2.2 is composed of a number of zones, created

using a conveyor belt and temperature-controlled areas. In the experiment described in section 3.1, a

8

batch reflow oven instead is used to bake the coupons. A batch reflow oven uses a single heating area

instead of the multiple areas described for an assembly-line reflow oven. In order to achieve the

various zones of the reflow profile, the batch reflow oven uses heating coils to heat and cool the oven

as the board is baked.

It is believed that poorer resolution in reflow profiles is witnessed in batch reflow ovens than in

assembly-line reflow ovens. For batch reflow ovens, the profile must take into account both the

heating of the board and the time to heat the oven itself. It is more difficult to achieve higher,

consistent rates of heating for the board, as the oven must first heat to the required temperature

needed to achieve the specific ramp. The reflow profile, in particular, has a large impact on solder

voiding, especially with regard to the peak temperature and time spent above the melting temperature

of the solder. Consequently, it is recommended that further studies are completed on the differences

between reflow profiles and solder samples for an assembly-line reflow oven and the batch reflow

oven used for this experiment.

3.3. X-Ray and Image Analysis

In the final portion of the experiment, it was necessary to analyze the images taken through x-ray

imaging in order to determine the percentage of voiding within the solder. This process involved the

selection of solder voids using the image analysis software ImageJ, and the creation of mask images

of the solder voids [12]. In order to achieve consistent results, a 900X900 pixel selection was made

in the middle of the solder for all samples. This procedure is illustrated below in figures 2 and 3.

Figure 2 - Basic Solder Sample with Voiding

9

Figure 3 - Example Solder Void Mask

Using the mask image, area calculations were performed and used to determine a number of

important data, including the total percentage of voiding. There are currently methods to do these

selections and area calculations automatically; however, they are inefficient, due to the shading

gradient often produced by the x-ray machine. Automatic void calculations rely on differences in

shading between the voids and the solder in order to determine void percentage. In this experiment,

where the solder had the freedom to wet outward from the center of the pad, the outer edges of the

solder were thinner than the center of the solder. As a result, the edges of the solder appeared lighter

in x-ray examination, and had the same shading as small voids in the middle of the solder pad.

Consequently, for automatic void calculations, it was necessary to either regard the thin edges of the

solder as voiding, which is false, or completely disregard the voiding within the center of the pad.

In order to avoid the negative aspects of automatic void calculations, manual selections and

calculations were performed. This method, however, also has a negative aspect in the form of bias.

One individual may interpret a void size to be slightly larger or smaller than another individual. In

order to minimize this variation, only one operator made the selections for all of the images. In the

ideal case, this variation would be avoided entirely by removing the human factor from the void

calculations. It is recommended that future studies on voiding use modified test coupons that prohibit

the solder from spreading beyond the pads of the coupons. These modified coupons would ensure

uniform thickness of solder, which would make automatic voiding calculations more viable.

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4. Reflow Profile Development and Results

One of the most critical issues that affects the presence of voids within solder is the temperature

reflow profile used to bake the solder. As mentioned in previous sections of the report, the peak

temperature and the time above the melting temperature of the solder are critical aspects of a reflow

profile that significantly affect the amount of solder voiding. In order to achieve valid results for the

solder voiding experiment described in section 3, it is necessary to develop a stable reflow profile that

provides a consistent amount of voiding. Using a stable profile, one may easily differentiate between

the voiding caused by the addition of water to the solder paste and the normal voiding within the

solder. This section describes the reflow profile development process, and offers recommendations

for future projects in solder voiding.

The first decision in the reflow profile development was to use an RTS profile over an RSS or an LSP

profile. This decision was made using a number of key points:

- The RSS profile is one of the most commonly used profiles in electronics assembly, and

would provide valid results specifically for RIM‟s current SMT methods.

- The PRO 1600 Reflow Oven exhibits difficulty in ramping the board quickly and soaking the

board immediately afterward. This behaviour in a reflow profile is required for both the RSS

and the LSP profiles. The RTS profile exhibits a constant ramp up to a peak temperature, so

this profile was more appropriate given the oven‟s capabilities.

- The soak zones in both the LSP and the RSS profiles may be too long, such that any added

moisture to the solder paste would completely evaporate prior to the melting of the solder

paste. As a result, there would be no difference between moisture-free solder paste and the

samples of solder paste with significant water concentrations.

- The LSP profile, in particular, is effective in removing most or all voiding from the solder

[5]. In order to test the effect of moisture in the solder paste, a degree of voiding within the

solder is necessary.

For further testing in solder voiding, it is recommended that variations of the RTS, RSS and LSP

profiles are used to bake the solder paste on the test coupons. Additional data on the ability to

augment and control solder voiding through profiling would be particularly useful.

A well-designed RTS profile must satisfy various criteria in order for the solder paste to bake with

adequate wetting and minimal defects. These criteria ensure that sufficient time is spent in each zone

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and specific temperatures are reached. The criteria chosen for this project are detailed in table 1

below.

Table 1 - Criteria and Requirements for the RTS Profile

Criteria Description Criteria Requirements

Time below 180 ºC 2/3 of total profile time

Rate of ramping 0.7 – 1.5 ºC/s

Time above the liquid state of solder 60 +/- 15 seconds

Peak temperature 235 – 255 ºC

Total profile length 210-240 seconds

The PRO 1600 Reflow oven has 7 parameters that can be adjusted in order to achieve the RTS criteria

in table 1. The parameters control the various zones of the reflow profile by altering the oven‟s rate

of heating, soak temperatures and time spent soaking. The parameters are shown below in figure 4.

Figure 4 - Sample Reflow Profile with Reflow Oven Parameters

It is important to note that the oven‟s parameters are controlled by the oven‟s internal thermocouples.

In order to negate the temperature difference between the oven‟s thermocouples and the board‟s

thermocouples, the programmed temperatures must be set much higher than required for the actual

profile. Numerous attempts were made to effectively satisfy the criteria for an effective RTS profile.

The oven parameters for three of the most noteworthy attempts are summarized in table 2 below.

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Table 2 - Oven Parameters for Reflow Profile Development Attempts

Attempt Ramp 1

(ºC/s)

Ramp 2

(ºC/s)

Temp. 1

(ºC)

Temp. 2

(ºC) Time 2 (s) Time 4 (s) Time 5 (s)

1 3 1.3 220 270 20 40 150

2 1.8 2.8 235 290 20 40 150

3 1.8 2.8 235 310 20 50 150

The first RTS profile attempt was far too conservative, with the oven temperature only slightly above

the desired board temperature. Additionally, the early large ramp created a significant difference in

the temperature between the oven‟s thermocouples and the board‟s thermocouples. Furthermore,

instead of spiking at the end of the profile, the second ramp had a slower rate of heating than the

initial ramp. The profile for the first attempt is available below in figure 5.

Figure 5 - RTS Reflow Profile - Attempt One

The above reflow profile problems were corrected in the second RTS profile attempt, by decreasing

the „ramp 1‟ parameter, increasing the „ramp 2‟ parameter, and increasing the „temp 1‟ and „temp 2‟

parameters. The second RTS profile attempt was a substantial improvement over the first attempt.

However, the peak temperature was still too low and the time above the melting temperature was too

small. The profile for the second attempt is available below in figure 6.

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Figure 6 - RTS Reflow Profile - Attempt Two

The errors from the second profile attempt were addressed in the third RTS profile attempt, by

adjusting the „time 4‟ parameter to 55 seconds and the „temp 2‟ parameter to 310 ºC. The third RTS

profile satisfied all of the initial criteria detailed in table 1. This profile was run several times to

confirm its stability, and was chosen for the water concentration solder voiding experiment. The

profile for the final attempt is available below in figure 7.

Figure 7 - RTS Reflow Profile - Attempt Three

14

5. Experimentation with Moisture in Solder Paste

This section contains the data taken from the x-ray images of the reflowed samples described in

section 3. The data analysis is separated into the three different trials to complete the project

guidelines. After the individual analysis of each trial, a combined analysis is provided to examine

general data trends.

5.1. Project Data Analysis – Part One

Three key variables were examined in the x-ray image analysis. The voiding percent was calculated

by dividing the total voiding area by the mask image area. Large void percent is the percentage of

larger voids, greater than 250 square pixels, present in the total voiding area. The number of voids

reflects the number of individual voids present in the mask image. This data for part one of the

experiment is available in table 3 below.

Table 3 - X-Ray Image Analysis - Part One

Sample Water

Concentration Voiding Percent

Large Void Percent

Number of Voids

1a 0% 2.74 86.65 81

1b 0% 1.82 82.56 78

2a 0.050% 1.49 86.58 44

2b 0.050% 1.11 73.92 44

3a 0.10% 1.94 63.96 119

3b 0.10% 2.25 80.29 80

4a 0.50% 3.85 91.51 71

4b 0.50% 0.82 64.37 55

5a 1.0% 4.77 89.84 115

5b 1.0% 2.92 84.50 117

The following inferences have been made from table 3 and the x-ray images from part one of the

experiment:

- With the addition of 0.050% water by weight to the solder paste, the average number of voids

and average voiding percentage dropped by approximately 40 and 1%, respectively.

- The addition of more water increased the voiding percentage and the number of voids, but

decreased the number of large voids. This implies the presence of many, smaller-sized voids.

- Sample 4b was a statistical outlier with an unusually low voiding percentage and a small

number of large voids. This sample is displayed in figure 8 below.

15

Figure 8 - Unusually Low Solder Voiding for Solder with 0.50% Water Concentration

- Samples 4 and 5 exhibited unstable behaviour. More specifically, the solder seemed to have

exploded around the edges of the solder sample, leaving large, non-circular voids. As a

result, these high water concentration samples were excluded in further experiment trials.

This behaviour can be seen in figure 9 below.

Figure 9 – Solder with Erratic Voiding due to High Water Concentrations

5.2. Project Data Analysis – Part Two

Similar to section 5.1, the voiding percent, large void percent and number of voids were examined in

all x-ray images taken from part two of the experiment. This data is available in table 4 below.

Table 4 - X-Ray Image Analysis - Part Two

Sample Water

Concentration Voiding Percent

Large Void Percent

Number of Voids

1a 0% 1.24 85.36 46

1b 0% 2.03 69.70 102

2a 0.050% 1.38 66.42 72

2b 0.050% 1.40 63.94 66

3a 0.10% 1.76 76.73 60

3b 0.10% 1.38 65.96 64

16

The following conclusions can be made with the data in table 4 and the x-ray images from part two of

the experiment:

- With the addition of 0.050% water by weight to the solder paste, the average number of voids

and average voiding percentage dropped by approximately 5 and 0.25% respectively.

- In contrast to section 5.1, the voiding percent and number of void values from sample 1a

were found to be below the values found in samples 2a and 2b. However, sample 1b

observed the same data trend as found in part one of the experiment.

- The presence of small voids increased with an increased solder paste water concentration.

5.3. Project Data Analysis – Part Three

Similar to sections 5.1 and 5.2, three variables were examined in this portion of the experiment. The

data for part three of the experiment is available in table 5 below.

Table 5 - X-Ray Image Analysis - Part Three

Sample Water

Concentration

Voiding Percentage

Large Void %

#Voids

1a 0% 2.32 81.31 95

1b 0% 1.65 81.67 75

2a 0.050% 2.44 83.96 70

2b 0.050% 2.39 81.94 89

3a 0.10% 1.55 76.48 55

3b 0.10% 1.48 72.78 90

The following deductions can be made using the data from table 5 and the x-ray images from part

three of the experiment:

- With the addition of 0.050% water by weight to the solder paste, the average number of voids

decreased by approximately 5. This change can be considered negligible.

- Contrary to sections 5.1 and 5.2, the average percentage of voiding increased by

approximately 0.4% with the change from 0% to 0.050% water concentration.

- Further contrasting earlier tests, the average percentage of voiding decreased substantially

with the change from 0.050% to 0.10% water concentration.

- The presence of small voids increased with the addition of water to the solder paste.

17

5.4. Project Data Analysis – Comprehensive Notes

The data from the previous three sections is further averaged and summarized in table 6 below.

Table 6 - Summary of X-Ray Data Analysis

Sample Water Percent

Voiding Percent Large Void

Percent Number of Voids

Mean Standard Deviation

Mean Standard Deviation

Mean Standard Deviation

1 0.00 1.96 0.52 81.21 6.02 80.00 19.46

2 0.05 1.70 0.57 76.13 9.51 64.00 17.49

3 0.10 1.73 0.33 72.70 6.48 78.00 23.99

The first two trials of the experiment implied a decrease in the voiding percent from 0% to 0.050%

solder paste water concentration. Due to the results of the third trial, these initial results became less-

significant. From table 6, it can be concluded with reasonable certainty that small water

concentrations within solder paste do not substantially affect the percentage of voiding in solder

joints. The 0.26 percent decrease from sample 1 to 2 can be considered negligible, and is likely due

to random variation within the voiding project.

The above conclusion only applies for small water concentrations, equal to or less than 0.10% by

weight. For samples of solder paste with larger water concentrations, the water drastically affects

both the stability of the solder paste and the amount of voiding within the solder joint. The test

samples with high water concentrations exhibited bursting effects in the solder, where the water

rapidly outgassed from the solder paste. This resulted in large, non-circular voids along the edges of

the solder joints.

The amount of large voids with respect to total voiding decreased with relative consistency

throughout all portions of the project. Combined with the analysis of the x-ray pictures, it has been

determined that the number of small voids increases as water is added to the solder paste. The

presence of small voids in the solder may provide additional support to the solder joint, as small voids

can change crack patterns, slowing crack propagation [13]. In this aspect, the addition of a small

concentration of water could be beneficial to the strength of the solder joint.

The position of voids was found to be random in all of the x-ray images taken in this experiment.

Previous voiding studies completed with these coupons exhibited large voids above the microvias on

the pads of the coupons. These previous studies were completed with the use of an assembly-line

18

reflow oven. For this experiment, it has been determined that the voids have migrated from their

initial positions above the microvias due to the strong, forced air from the batch reflow oven. This

migration effect warrants further research, specifically on the differences between the assembly-line

reflow oven and the batch reflow oven.

The number of voids throughout all samples in the project remains ambiguous. The averaged data

from table 6 implies a decrease and then increase in the number of voids with the addition of water to

the solder paste. However, the actual number of voids taken in each trial of the experiment varied

substantially from sample to sample. Given this large variation, it can be concluded that for small

concentrations of water, there is no significant correlation between the water concentration and the

number of voids in the solder.

19

6. Concluding Notes and Recommended Actions

The effect of water concentration on solder voiding did not satisfy either of the original hypotheses of

the solder voiding project. For small water concentrations, increasing the water concentration in the

solder paste did not increase the voiding percentage in the solder joint. Furthermore, the addition of a

small amount of water to solder paste did not aid in the removal of voids from the solder joint.

Instead, it was found that, for small water concentrations, the addition of water to solder paste does

not significantly affect the percentage of voiding. Additionally, for large water concentrations, the

addition of water to the solder paste lead to explosive outgassing, and the presence of large, non-

circular voids.

It was also determined that small water concentrations within solder paste did not significantly affect

the number of voids. However, the addition of water to solder paste was found to increase the

presence of small voids within solder joints, which could be useful in strengthening solder joints

against crack propagation. Finally, the position of the voids on the reflowed samples was found to be

random, due to a migration effect within the solder paste. This migration effect was likely caused by

the strong, forced air of the batch reflow oven. For future research, it is recommended that formal

studies are made on the differences between the conventional, assembly-line reflow oven and the

batch reflow oven used in this experiment.

Throughout the development of the project, it was determined that the RTS reflow profile is optimum

for solder voiding studies where additional variables, such as water concentration, are manipulated.

However, experimentation with various other reflow profiles, including the RSS and LSP profiles,

would also provide potential data for future research. In addition to altering the type of reflow

profile, further studies should also examine different types of components and different test coupons.

In particular, it is recommended that the tests involve the reflow of BGAs and special test coupons

that restrict the flow and movement of the solder in the reflow process.

The results of the experiments did not provide evidence that solder voiding can be increased with the

addition of water to solder paste. At high water concentrations, where more voiding may have been

possible, the solder instead outgassed in an explosive manner. A previous study completed in the lab

examined low-boiling solvents, and these were found to be unsuitable for the purpose of increasing

solder voiding. Consequently, for further research in increasing the degree of solder voiding in solder

joints, it is recommended that experiments be performed with solvents with higher boiling points.

20

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