r.p.j. denteneer mt07 · here the solders (sn40bi60)0.9174 + (in)0.0826 (100382) and...

Wettability study &

bump height measurements of solder material

R.P.J. Denteneer

MT07.27

Wettability study & bump height measurements of solder material R.P.J.Denteneer 0489159

Table of contents Summary .................................................................................................................................... 2 Introduction ................................................................................................................................ 3 Part I ........................................................................................................................................... 4 1 Method ............................................................................................................................... 4

1.1 Wetting balance.......................................................................................................... 4 1.2 Test samples ............................................................................................................... 5 1.3 Solder alloys............................................................................................................... 6

2 Results and discussion........................................................................................................ 7 2.1 Copper ........................................................................................................................ 7 2.2 Aluminum................................................................................................................. 10

Part II........................................................................................................................................ 14 3 Method ............................................................................................................................. 14

3.1 Optical microscope, focusing and micrometer......................................................... 14 3.2 Test samples ............................................................................................................. 14

4 Results and discussion...................................................................................................... 16 4.1 Solder alloy composition.......................................................................................... 16 4.2 Bumping treatment ................................................................................................... 19 4.3 Addition of X1 and X3 .............................................................................................. 20 4.4 Summary .................................................................................................................. 21 4.5 Cross sections........................................................................................................... 22

Conclusions & recommendations............................................................................................. 23 Appendix A - MUST SYSTEM II software manual................................................................ 24 Appendix B - test sample IP4041CX25/LF ............................................................................. 25 Appendix C - References ......................................................................................................... 26

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Summary In part I of this report a wetting balance (MUST SYSTEM II) is used to study the wetting behavior of low-melting solder alloys to aluminum substrates. In a first stage the wetting balance is evaluated by experiments on copper samples. A spin-off is the estimation of the activation energy of 0.32 eV in the range 120-180°C. The obtained process settings were subsequently applied to aluminum samples, but no proper interface reactions could be observed. In part II of this report solder alloys that are developed for soldering to aluminum are tested in the form of solder bumps. These bumps are applied to pieces of NiAu-finished wafers with ICs that have an array of 5x5 bumps. By means of optical microscopy the height of these solder bumps is determined. The effects of additives to the alloy and the bumping process (additional reflow step, use of flux) are studied.

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Introduction

In the SALT-project one aims at developing solder material and processes for soldering of ICs to aluminum terminals. One of the areas to be served by the project is RF-ID tags for high volume applications such as tracking of goods in warehouses and airports. Therefore these tags must be very cheap. They consist of a RF-ID chip assembled on substrates carrying an antenna. To further reduce manufacturing costs unpackaged chips are to be used equipped with solder bumps.

One of the crucial items in this project, as in all activities to develop solder, is the reaction of the solder alloy with the substrate material. The most direct observation of this reaction is recording of the wetting behavior. In part I of this note the wetting balance used for this purpose is evaluated and some process variations were studied.

A common process to produce ICs with solder bumps is to immerse wafers with a suitable metallization in liquid solder. The solder alloys with the metallized solder land and – depending on the diameter of the land, the immersion time, et cetera – a solder bump of certain shape and height is made. For conventional types of solder this is a mature process. As for the newly developed low-melting solder alloys, this has to be established yet.

In the second phase of the SALT-project the solder alloys have to be made fit for immersion solder bumping. Wafers were obtained from NXP, containing area array ICs and processed up to metallization with NiAu. The results of measuring the height of the solder bumps and other analyses such as their microstructure are laid down in part II of this note.

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Part I

1 Method 1.1 WETTING BALANCE

To test the solderability or wetting of the soldering process the MUST SYSTEM II has been used, see figure 1.1. This system is based on the principle of the wetting balance. In addition, with MUST SYSTEM II one has the option to use a solder globule instead of an entire solder bath. Thus small solder pellets can be used, so that it is not necessary to produce a big volume of solder material (figure 1.2).

With a measuring head the vertical forces of surface tension acting on the sample are measured which will be monitored in a force-time diagram [1].

Figure 1.1. MUST SYSTEM II. Figure 1.2. Sample fixture in the MUST SYSTEM II. Theoretical background – Some specific theories behind the test will be discussed briefly. The surface tension is related to force as follows:

0=++ LALSSA γγγ (1.1) θγγγ cos⋅+= LALSSA (Young’s relation) (1.2)

434214434421buoyancywetting FF

LA VgPF ⋅⋅−⋅⋅= ρθγ cos (Laplace law) (1.3)

The symbols have the following meaning (see figure 1.3 for a schematic representation):

γSA: Solid-air (flux) surface tension γLS: Liquid-solid surface tension γLA: Liquid-air (flux) surface tension θ: Contact angle F: Capillary forces P: Component wettable perimeter ρ: Specific mass of solder g: Gravitational acceleration (9.81 m/s²) V: Volume of the component part immersed in the molten alloy

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0° < θ < 30° Very good wetting

30° < θ < 40° Good wetting 40° < θ < 55° Acceptable wetting 55° < θ < 70° Poor wetting 70° < θ < 90° Very poor wetting 90° < θ No wetting

Figure 1.3. Surface tensions. Figure 1.4. Dipping steps a to f during one passed test.

These relations are true only if the sample is immersed perpendicular to the surface of the solder material and the dimensions are constant. γSA should be high and is influenced by type of flux, γLS should be low and is influenced by the type of termination and γLA should be low and is influenced by the type of flux, solder and temperature. Perpendicularity, tip shape and amount of flux influences the diagram.

In figure 1.4 it can be seen step by step how the solder and sample behave during one passed test and in figure 1.5 a force-time diagram standard is shown. The steeper the rising part between b and d, the better the wettability [1].

Figure 1.5. Force-time diagram standard of one passed test. Operation – The MUST SYSTEM II has its own software. While testing a stepwise program has to be followed. This program can be found in Appendix A. After following the main menu properly good results will follow [1].

1.2 TEST SAMPLES

In this project the wettability of aluminum is the topic of investigation. Prior to this, the MUST SYSTEM II itself was tested using copper, because of its well-known solder behavior. The test sample was copper in the shape of a wire and the solder alloy consists of

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tin, bismuth and indium (SnBiIn) in the shape of a pellet. The fluxes used to improve the wetting process are Actiec2 and ND 52R.

1.3 SOLDER ALLOYS

During the wettability tests different solder alloys have been tested. In table 1.1 the composition and density properties of these solder alloys are described.

Table 1.1. Properties of solder alloys. Material Composition S-bond 220 SnTiAg 100288 ((Sn40Bi60)0.9 + (In)0.1)0.9 + (S-Bond 220)0.1 100369 SAC405 (95.5% Sn / 4.0% Ag / 0.5% Cu) [2] 100382 (Sn40Bi60)0.9174 + (In)0.0826 100383 (Sn40Bi60)0.9091 + (In)0.0909 100477 (Sn40Bi60)0.9091 + (In)0.0909 100478 (Sn40Bi60)0.9086 + (In)0.0909 + 0.05 wt% X3100479 (Sn40Bi60)0.9046 + (In)0.0904 + 0.50 wt% X3100480 (Sn40Bi60)0.9001 + (In)0.0900 + 0.99 wt% X3100485 (Sn40Bi60)0.8913 + (In)0.0891 + 0.99 wt% X1 + 0.98 wt% X3

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2 Results and discussion 2.1 COPPER

The MUST SYSTEM II has the option to vary many parameters. These are very influential to the behavior and results of the experiments. The parameters are the temperature of the solder alloy, the speed of the sample while entering the solder alloy, the immersion depth of the sample into the solder alloy, and the time from the start of immersion until the release of the sample from the solder alloy.

To determine the best combination of parameters such as speed, depth and process time, trial runs were done with copper wire. From these try outs a speed of 1.0 mm per second, a depth of 1 mm and a process time of 10 seconds were chosen.

Here the solders (Sn40Bi60)0.9174 + (In)0.0826 (100382) and (Sn40Bi60)0.9091 + (In)0.0909 (100383) and flux type ND 52R were used. Between these solder types no big differences were observed. 2.1.1 Temperature dependence

As can be found from the averages of a large number of tests, the temperature influences the wetting time, wetting force and immersion force. The lower the temperature the poorer the wetting, the higher the wetting time and the higher the immersion force. These relations can be observed in figure 2.1. Here the results out of the tests with solder 100383 are shown. Also the wetting activation energy of 0.32 ± 0.02 eV is determined assuming the validity of Arrhenius’ law, see figure 2.2. The wetting time is the point labeled d in figure 1.5. In figure 2.3 images of tested samples are shown.

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Figure 2.1. Average of test results with solder 100383 (left). Results obtained at T = 165 ºC (right).

T = 165 ºC

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Table 2.1 Wetting time - temperature relation

T (ºC) twetting (s) σ (s) σ (%) 120 7.00 1.16 17 125 5.17 1.84 36 130 3.77 2.43 64 135 3.00 1.41 38 150 3.02 1.10 36 165 2.73 1.05 38 180 1.46 0.68 47

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Figure 2.2. Arrhenius’ law for wetting time: average

data points ± 1σ. Regression lines are drawn.

Figure 2.3a. Copper wire (100383) at T = 120 ºC. Figure 2.3b. Copper wire (100383) at T = 130 ºC.

Figure 2.3c. Copper wire (100383) at T = 150 ºC. Figure 2.3d. Copper wire (100383) at T = 180 ºC. 2.1.2 Cross sections

After the tests were done, the samples were prepared for cross section images, figure 2.4. These images give a good insight about the wettability, microstructure, intermetallic phase and temperature dependence. In these pictures the continuous matrix is Sn, the inclusions are of Bi and BiSn.

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Figure 2.4.d. Copper wire (100383) at T = 120 ºC (holes, solder resist and thin intermetallic phase).

Figure 2.4c. Copper wire (100383) at T = 130 ºC (less solder resist and thicker intermetallic phase).

Figure 2.4b. Copper wire (100383) at T = 150 ºC (less solder resist).

Figure 2.4a. Copper (100383) at T = 180 ºC (less solder resist).

Intermetallic layerSolder

Copper

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2.1.3 Conclusions As found in figure 2.4 intermetallic phases between copper and solder are observed.

Referring to figure 1.2, at a temperature of 120 ºC wetting is poor while at 130 ºC, 150 ºC and 180 ºC wetting is good to very good. As the temperature is higher the wetting time becomes shorter and the wetting force is higher. From these observations it can be assumed that the tests were fairly reproducible.

2.2 ALUMINUM 2.2.1 Bare foil substrates

After the copper wire tests were done, the test conditions were applied to test the wettability of aluminum. First of all bare aluminum foil was used as a substrate. This foil, supplied by BESI, is a stack of 17 µm thick pure aluminum with a PET layer as carrier (see figure 2.5). In these tests the solder (Sn40Bi60)0.90 + (In)0.09 + 0.99 wt% X3 (100480) was used. The MUST SYSTEM II settings applied were a speed of 2.0 mm per second, a depth of 1.0 mm, and a process time of 10 seconds.

The wetting curves of figure 2.6 – which should be compared to those of figure 2.1 for copper – indicate that the solder does not react with the aluminum. Still, the initial drop of the force followed by an increase is an indication for the onset of wetting, which does not proceed. Examples are shown in figures 2.7. Also the use of flux type ND 52R didn’t improve or deteriorate the results and no wetting occurred.

Figure 2.5a. Bare foil substrate (Al side). Figure 2.5b. Bare foil substrate (PET side).

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Figure 2.6. Results of bare foil substrate tests at different temperatures. Solder 100480.

Figure 2.7. Results from bent bare foil substrate tests at different temperatures. Solder 100480.

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Figure 2.8a. Bare foil substrate (100480) at T = 150 °C. Left Al-side, right PET-side.

Figure 2.8b. Bare foil substrate (100480) at T = 180 ºC. Left Al-side, right PET-side.

Figure 2.8c. Bare foil substrate (100480) at T = 240 °C (melted PET). Left Al-side, right PET-side.

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Also a trial was done with a bent foil (figure 2.9). As can be seen in figure 2.7 no wetting appeared although the force increases stronger than in the straight samples of figure 2.6. The soldered tip can be tapped off very easily, which means that no reaction occurred.

Figure 2.9. Bent bare foil substrate wetted (100480) at T = 180°C (left) and T = 210°C (right). 2.2.2 Coated foil substrates

After observing the results of the bare aluminum foil substrates, another way of testing was introduced by adding a coating of solder to the aluminum side of the foil. This layer was added ultrasonically at a temperature of 140ºC. Although it was expected that this layer was firmly connected to the aluminum, it released quite easily before or during the wettability tests. Figure 2.10 shows some images of this phenomenon.

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Figure 2.10. Coated foil substrates fracture. Solder 100480.

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Part II

3 Method 3.1 OPTICAL MICROSCOPE, FOCUSING AND MICROMETER

The height of a bump is determined by subsequent focusing on top of one bump and on the metallization right next to the bump. A Tesa Modul micrometer connected to the microscope measures the height of the microscope table with a resolution of ~ 1 µm. By subtracting the value measured on top and the value on the metallization, the height of one bump was calculated.

An optical Olympus BH2-MJL Microscope with a “4WLD MSPlan 80 0,75 ∞/0 f=180”-lens in was used during measurements.

3.2 TEST SAMPLES

During the measurements test samples were used to compare differences and influences of various solder alloys compositions and bump process settings. To this end un-bumped 8”-wafers of IP4041CX25/LF were obtained from NXP-Hamburg (see appendix). These samples were made with different solder alloy compositions and different reflow and flux properties. All samples are cut pieces of bumped wafers with geometry of 6 by 20 ICs as shown in figure 3.1.

Figure 3.1. Test sample; IC IP4041CX25/LF.

One IC has 25 bumps as shown in the schematic figure 3.2a and in figure 3.2b. From randomly picked ICs the height of the four corner bumps and the center bump was measured.

This implies measuring the height of bumps 1, 5, 13, 21 and 25 as in the schematic figure 3.2a.

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Figure 3.2a. Schematic IC Figure 3.2b. Optical image of an IC

Also the height of all 25 bumps from randomly picked ICs on one wafer sample is

measured. Out of these measurements a 3-D diagram can be obtained, so remarkable errors can be found and analyzed.

With this information statistic height analyses will be made and the stability of the bumping process can be determined.

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4 Results and discussion 4.1 SOLDER ALLOY COMPOSITION

First of all the differences between different solder alloy compositions were measured. The compositions of these alloys are:

(Sn40Bi60)0.9091 + (In)0.0909 (Sn40Bi60)0.9086 + (In)0.0909 + 0.05 wt% X3(Sn40Bi60)0.9046 + (In)0.0904 + 0.50 wt% X3(Sn40Bi60)0.9001 + (In)0.0900 + 0.99 wt% X3

These will be referred to further on as: SnBiIn + 0 wt% X3SnBiIn + 0.05 wt% X3SnBiIn + 0.5 wt% X3SnBiIn + 1 wt% X3 The supposed role of the additive X3 in this solder alloy composition is schematically

shown in figure 4.1. In this figure contact is established between the solder SnBiIn + 1 wt% X3 and the aluminum substrate. While the SnBiIn part takes care of the low melting point, the X3 addition breaks through the Al2O3 layer and connects to the aluminum.

Solid Al Al2O3

X3

X3

X3

Liquid SnBiIn + X3

Figure 4.1. Schematic representation of the role of X3 in the solder alloy composition.

The bump height of all four sample types was measured. In figure 4.2 the average height of 8 ICs randomly picked out of one sample per type is shown and in figure 4.3 histograms of the bump height distribution are shown. Here tails in the distribution are marked which means that some bumps are much higher than the average value.

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Bump height

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heig

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Figure 4.2. Average bump height of 8 ICs randomly picked out of 1 sample per type.

Figure 4.3a. Histogram of SnBiIn + 0 wt% X3.

Figure 4.3b. Histogram of SnBiIn + 0.05 wt% X3.

Figure 4.3c. Histogram of SnBiIn + 0.5 wt% X3. Figure 4.3d. Histogram of SnBiIn + 1 wt% X3.

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Figures 4.4 and 4.5 show the 3-D diagrams and optical images of SnBiIn + 1 wt% X3.

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Figure 4.4. 3-D diagram and optical image of SnBiIn + 0 wt% X3.

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Figure 4.5. 3-D diagram and optical images of SnBiIn + 1 wt% X3. Top-right: top view, bottom-right: side view.

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4.2 BUMPING TREATMENT

The differences between bumping processes were observed by measuring the bump height of test samples as made, reflowed and reflowed and fluxed. The exact composition of these alloys is (Sn40Bi60)0.9091 + (In)0.0909, further on called SnBiIn.

Figure 4.6 shows the average bump height of 32 ICs randomly picked out of four samples per bump process condition and figure 4.7 shows the bump height histograms per condition. Also here the so-called tails are marked which means some bumps are much higher than the average value. Figure 4.8 shows optical images of ICs with different process conditions.

Bump height

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Figure 4.6 Average bump height of 32 ICs randomly picked out of 4 samples per condition

Figure 4.7a. Histogram of SnBiIn as made.

Figure 4.7b. Histogram of SnBiIn reflowed. Figure 4.7c. Histogram of SnBiIn reflowed and fluxed.

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Figure 4.8. Optical images of SnBiIn as made (a), reflowed (b) and reflowed and fluxed (c).

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4.3 ADDITION OF X1 AND X3 Apart from the alloy composition SnBiIn + 1 wt% X3 another solder was made

containing two additives. This solder has a composition of (Sn40Bi60)0.8912 + (In)0.0891 + 0.99 wt% X1 + 0.98 wt% X3, further on called SnBiIn + 1 wt% X1 + 1 wt% X3. Here the difference between SnBiIn + 1 wt% X3 and SnBiIn + 1 wt% X1 + 1 wt% X3 will be discussed. The bump heights were measured in the same way as described earlier in this chapter.

Figure 4.9 shows the difference between the averaged bump height of each solder type and figure 4.10 shows the histograms of the measurements.

SnBiIn + 1 wt % X3

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Figure 4.9. Average bump height of 24 ICs randomly picked out of 3 samples per solder type.

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Figure 4.10. Histograms per solder type.

From figures 4.9 and 4.10 it can be concluded that the process conditions used while bumping with SnBiIn + 1 wt% X1 + 1 wt% X3 leads to better results than the conditions used while bumping with SnBiIn + 1 wt% X3. However, alternatively this might be caused by the composition itself. The only process difference was the number of immersions into the molten solder. The samples with SnBiIn + 1 wt% X3 were immersed once and the samples with SnBiIn + 1 wt% X1 + 1 wt% X3 were immersed twice while in between turning it 180 degrees upside down.

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4.4 SUMMARY

An overview of the bump height results is given in table 4.1.

Table 4.1. Bump height results. Solder alloy composition Treatment Height (µm) σ (µm) σ (%) SnBiIn + 0 wt% X3 Fluxed 44.67 10.05 22.51SnBiIn + 0.05 wt% X3 Fluxed 43.73 12.91 29.52SnBiIn + 0.5 wt% X3 Fluxed 50.53 14.63 28.96SnBiIn + 1 wt% X3 Fluxed 50.63 13.25 26.18 SnBiIn As made 52.57 12.41 23.60SnBiIn Reflowed 51.08 13.44 26.31SnBiIn Reflowed + fluxed 45.63 12.06 26.43 SnBiIn + 1 wt% X3 Fluxed 56.38 11.93 20.39SnBiIn + 1 wt% X1 + 1 wt% X3 Fluxed 48.67 7.71 15.85

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4.5 CROSS SECTIONS

To get a good insight of the solder alloy structure in a bump, cross sections were made. Figure 4.9 shows an image of one bump taken with a Scanning Electron Microscope (SEM) JSM 840A equipped with the Energy Dispersive Spectrometer (EDS) Noran.

Figure 4.9. SEM image of a SnBiIn + 1 wt% X3 fluxed bump.

In these pictures the bright areas are pure-Bi, the light gray areas are BiIn intermetallic

compounds, and the darker gray matrix is of Sn. The bump is in intimate contact with the NiAu under-bump metallization. Apparently the microstructure is still rather course.

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Conclusions & recommendations

From the experiments and their analyses in the study to the wettability of low-melting SnBiIn solder to aluminum surfaces, it can be concluded that in the temperature range of 120°C to 180°C the solder wets very well to Cu-wires and an activation energy of 0.32 eV was estimated. Also the reproducibility of the wetting tests on the MUST SYSTEM II is good. Under the chosen conditions the SnBiIn-solder does not react with Al/PET-foil substrates. Possible explanations are the composition of the solder, which should contain elements such as Ag that react with Al, or the impossibility to combine the wetting balance with ultrasonic agitation.

From the measurements and analyses on test ICs it can be concluded that the scatter in bump height is in the range of 20-30% (10-15 µm). The average bump height ranges from 40 µm to 55 µm and the repeatability of the bumping process, based on two sets of data, is about 10%. Also was found that the bump heights and their standard deviation do not depend on the amount of additive, or on the after treatment (reflowing of the bumps, or fluxing). The only reduction of the scatter in the height of the solder bumps was observed after addition of two different additives (X1 and X3).

Based on these conclusions it is recommended to thoroughly investigate the composition of the solder alloys.1 Also the possibility needs to be explored to combine the wetting balance with an ultrasonic source. Looking at the bump height measurements it is recommended to introduce a (semi-)automatic bumping process to reduce operator dependent effects. Also possibilities have to be investigated to apply the new solder alloys in a large scale bumping process so as to process entire wafers in stead of small pieces.

1 During several follow-up meetings in the SALT-II project this was addressed and according investigations have been defined.

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Appendix A - MUST SYSTEM II software manual A brief overview of how to deal with the MUST SYTEM II software:

1. Receptacle change possible into 2mm (type 1) or 4mm (type 2) globule by Removing electrical connector on the left and unscrew the two hexagon head bolts

2. Temperature should be similar to parameter’s value 3. Select Component and Parameter (press F4) or add a new parameter (press F9) with

adding its own properties like globule type, speed, immersion depth, test duration, preheat time, test temperature and some parameters to compare different measurements to a standard

4. Align sample and solder pellet to a pre-heat distance of 2-3 mm using a globule 5. Perform wetting test

• Use clip type 6 for Al foil • Change solder at regular intervals • Change sample after each test

6. Analyse data • Search for result files • Type <Ins> to select file • Follow options • Type <F2> for a pass / fail overview

7. To quit program type <Alt-Esc> while located in main menu 8. To convert data files into txt or csv files type in DOS: “convtext”

• Select data files (folder: c:\data) • <Alt> ‘select output format’ (csv or txt) ‘save output file’ <Tab> and <Enter>

to change filename and destination ‘exit’ • Then copy the files to the a:\ drive to analyze them elsewhere by typing “copy

c:\***filename*** a:\”

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Appendix B - test sample IP4041CX25/LF

Package lay out of test sample IP4041CX25/LF.

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Appendix C - References [1] MUST SYSTEM II, Solderability test system for surface mount and conventional

components manual, Multicore Solders, 1996. [2] Practical components, Dummy components and mechanical samples,

http://www.intertronics.co.uk/resource/pci_cat.pdf. Practical components, 2006.

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r.p.j. denteneer mt07 · here the solders (sn40bi60)0.9174 + (in)0.0826 (100382) and...

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