pcb carolina – 2017

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© Copyright 2017errari

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PCB Carolina – 2017

Lead - Free Impact on

Design for High Reliability

Gary Ferrari MIT

Ferrari Technical [email protected]

(860) 350-9300


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Agenda

• Why Lead Free

• Material Selection

• Lead Free Impact on Board Fabrication

• Land Characteristics

• Surface Finishes

• Copper Dissolution

• Quality Assurance

• Marking Standardization

• Lead Free Assembly Issues

• Component Selection

• Standard Notes

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Why Lead Free

Concern over:ExposureLandfill4% of landfill is ElectronicsShorter lifetime of products

Issues:No full life cycle analysisWhat are the alternatives?Cost of substitutes1% of lead used is Electronics


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RoHS Directive

RoHS Directive number 2002/95/EC

• Restriction of Hazardous Substances in Electrical and Electronic Equipment.

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RoHS Requirements

From July 1st 2006 new electrical and Electronic equipment should not contain any of the following:

• Lead

• Mercury

• Hexavalent Chromium

• Cadmium

• Polybrominated Biphenyls (PBB)

• Polybrominated Diphenyls (PBDE)

RoHS Directive


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Scope

Includes all equipment dependent upon electrical currents or electromagnetic fields falling into the following categories:

1. Large Household Appliances

2. Small Household Appliances

3. IT/Telecommunications

4. Consumer Equipment

5. Lighting Household (including electric light bulbs and household luminaires)

6. Electrical and Electronic Tools

7. Toys, Leisure & Sports

8. Medical devices (exemption removed in July 2011)

9. Monitoring & Control Instruments (exemption removed in July 2011)

10. Automatic Dispensers

RoHS Directive

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RoHS - Product Types Covered

• Consumer, Office, IT, telecom etc.• Some cases not clearly defined

• In areas of overlap, uncertainty exists related to exact scope– E.g. car radio, computing for aerospace– Depends on purpose equipment designed for......


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Product Types NOT within RoHS

• Automotive, Aerospace, Military− Exclusions have never been within the legislative scope but

being evaluated for inclusion at a later date

• Intended to protect national security and/or for military• Where electricity is not main power source

− e.g. electronic control of gas heating

• Where electronic products are not needed to fulfil the primary function

− e.g. musical birthday card

• That are part of another type of equipment − do not have a direct function outside that equipment− e.g. vehicle engine management

• Batteries − Covered under battery regulations

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Product Types NOT within RoHS

• Network Infrastructure (solder only)

• Servers and Storage Arrays

• Equipment rated above 1000 VAC or

1500 VDC


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Military Exemption??

MIL-PRF-31032

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Military Exemption??

MIL-PRF-55110 “SUPERCEDED FOR FUTURE DESIGN”??


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Established Levels

Current levels are as follows:

• Lead (Pb), Mercury (Hg) and Hexavalent Chromium (Cr6+) <0.1% component weight = <1000 PPM

• Cadmium (Cd) <0.01% component weight = <100PPM

• PBB’s and PBDE’s <0.1% component weight = <1000PPM

“ By weight in homogeneous materials …… shall be tolerated”

What is the definition of ‘homogeneous materials’?

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Homogeneous material

• Homogeneous means of uniform composition throughout’

• Examples are individual types of plastics, ceramics, metals, alloys, paper, board, resins and coatings

• Mechanically disjointed means that materials can be, in principle, separated by mechanical actions such as unscrewing, cutting, crushing, grinding and abrasive processes


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• EU examples describe a plastic cover as a homogeneous material, but an electric cable as two homogeneous materials (the wire and the insulation)

• EU examples also describe a semi-conductor package as containing many homogeneous materials

– i.e. the impurity limit does NOT apply to entire components

Homogeneous Material

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lead-frame

coating

solder

copper board finish

board

surface finish

copper

glassepoxy



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• Lead-frame and coating can clearly be separated as homogeneous materials

• Mechanical separation of board materials (glass and polymer) may not be possible ..? but probably do not contain banned substances..?


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• Mechanical fixings –may be Zn plated with hexavalent Cr used as a passivation layer

• Plastics – may contain Pb, Cd or PBDE

• Components – may have Pb on surface finish or as internal connections

• Etc !

Hidden Banned Substances?


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Additional EU Directives

ELV Directive

(End of Life Vehicle)

Design Compliance

� Enact in 2007

WEEE Directive(Waste Electrical and Electronic Equipment Directive)

Electronic waste recycling & disposal

� Waste collection system established Aug., 2005

� Pay for system & take back waste Jan.,2006

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Material Selection


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Material Selection

� Resin Formula� Flame Resistance� Thermal Stability� Mechanical Strength� Electrical Properties

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Material Selection Cont'd

� Flexural Strength• Reinforcing Sheet Material• Maximum Operating Temperature� CAF Resistant

� Halogen Free� Decomposition Temperature - NEW


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Glass Transition Temperature - Tg

Definition:

The temperature at which an amorphous polymer changes from being in a hard and relatively brittle condition to being in a viscous or rubbery condition.

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170 oCTg Epoxy

250 oCTg Polyimide

Therm

al

Expansio

nIn

ches/Inch

SolderTemp: 260 oC

Increased layer count and thermal reliability have prompted specifying minimum Tg’s

Is that enough??


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Test Methods

Dynamic Mechanical Analysis (DMA)

Measures modulus

Differential Scanning Calorimetry (DSC)

Measures rate of heat absorption

Thermal Mechanical Analysis (TMA)

Measures expansion rate


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Post-Tg CTEs are higher than pre-Tg CTEs.

• “A” exhibits more total Z axis expansion than “B” because of a lower-Tg.

• But “C”, even with a higher Tg, exhibits more total Z-axis expansion than “A” because its post- Tg CTE is so high.

Z-Axis Expansions


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Typical Supplier Data Sheets

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IPC-4101/129

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IPC-4101 Slash Sheet


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Lead Free Impacton

Board Fabrication

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Typical Soldering Profile


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Lead Free Impact on Board Fabrication

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Mis-conceptions

• “Lead-free assembly will have only a minor effect on laminates.”

• “Just switch to existing high-Tg materials.”

• “Most existing materials can be used in Lead-free assembly without a significant problem.”



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Problems

• Through hole reliability

• Delamination

• Measle

• Blister resistance

Mechanisms

• Z Axis expansion

• Thermal stability

Key Base Material Properties

• Degradation temp/Tg

• Time at temperatureperformance

Challenges as a result of Higher Temperature Processing


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Lead Free Impact on Board FabricationWorse Case Results

A Common High Tg FR-4 After Lead-Free Assembly


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Lead Free Impact on Board FabricationDecomposition Temperature - Td

Definition:

The Temperature at which a 5% weight loss occurs by thermal gravimetric analysis (TGA)

using Test Method ASTMD3850

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Lead Free Impact on Board FabricationDecomposition Temperature - Td

What happens as the decomposition

temperature is exceeded?

Irreversible degradation and damage of material due to breakage of chemical bonds

Even 2-3% loss, especially when exposed to multiple thermal cycles, can significantly degrade reliability

• The point at which this level of decomposition occurs is critical.


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Lead Free Impact on Board FabricationTime at Temperature Performance

Definition:

A number of ways to evaluate -

T260/T288 measures time to delamination at specific temperature (i.e. 260°C/288°C)

Test Method: IPC-TM-650

–T260 = 30 minutes minimum

–T288 = 10 minutes minimum

–T300 = 1 minute minimum

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Lead Free Impact on Board FabricationTime at Temperature Performance

What happens and what does it mean?

• At the test temperature after some period

of time the sample will delaminate

• Longer T260/T288 times indicate better

delamination/measle/blister resistance

• Results are dependent on decomposition

temperature, Tg and other factors


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Lead Free Impact on Board FabricationSupplier Material Studies

Park Electrochemical Corp.

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Lead Free Impact on Board Fabrication Supplier Material Studies

Polyclad Laminates

Thermal cycles @ 235ºC


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Lead Free Impact on Board Fabrication Processing Realities of T288 Materials

• More difficult to drill

• Lower tool life

• Lower chip load required

• Under cut drill bit geometry, high helix angle

• Fracturing at array break-off

This equates to $$$ someplace

along the line!

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Tg and Td are not the only issues to consider CTE must be considered as well

The following formula takes all three into consideration. A minimum index of 215 is recommended for the lead free environment:

Soldering Temperature Impact Index (STII)

STII = (Tg + Td)/2 – (% thermal expansion from 50 – 260oC)X10


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Lead Free Impact on Board Fabrication CAF (Conductive Anodic Filaments)

Definition:

Conductive Anodic Filaments are copper corrosion by-products that emanate from the anode of a circuit and “grow” subsurface toward the cathode, frequently along separated fiber-epoxy interfaces.

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Physical Influences:

• High Humidity (~80%RH anytime during product life)

• High voltage gradient between anode and cathode (~3-8 V/mil)

• Hole drilling

• Multiple thermal Cycles during processing

• Certain soldering flux ingredients (polyglycol)


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Design Influences

• High Density Circuits

− Hole-to-hole

− Hole-to-feature

− Feature-to-feature, in plane

− Feature to feature, out of plane

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Catastrophic Field Failure of Military Hardware,

Flux Enhanced Degradation

+20V

Power plane

Component Through Hole

CAF


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Material Selection Summary:

• Specifying Tg may be necessary, but it is insufficient. Higher Tg is

not always better.

• Decomposition temperature is a critical property to understand when

specifying materials for Pb free assembly.

• CTE values should be considered also. Calculate STII !

• Time to delamination tests are increasingly relevant, but multilayer

PCBs can be affected by other variables than just materials.

• If switching materials, verify other performance characteristics also,

e.g. Dk and Df values. Don’t forget the electricals!


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Stress Testing:

• Typical fabricator thermal testing is in the range of 1 – 3 thermal cycles (solder floats)

• Revised requirement for Lead Free; 5-6 thermal cycles @ 260oC

• IST testing at lead free soldering temperature


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Land Characteristics

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IPC-2221

9.1.2 Annular Ring Requirements

The minimum annular ring for unsupported and supported holes shall be in accordance with Table 9-2 and Figures 9-2 and 9-3.

Table 9-2 Annular rings (Minimum)

Annual Ring Class 1,2,3Internal Supported 0.03 mm

External Supported 0.05 mm

External Unsupported 0.15 mm


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IPC-2221 Figure 9-2 External Annular Ring

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IPC-2221Figure 9-3 Internal Annular Ring


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Minimum Standard Fabrication Allowance For

Interconnection Lands

Level A

0.4 mm

[0.016]

Level B

0.25 mm

[0.010]

Level C

0.2 mm

[0.008]

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Land Size Calculation

Maximum Hole Size = 0.041

Annular Ring (0.005) X 2 = 0.010

Fabrication Allowance = 0.010

Minimum Land Size 0.041 + 0.010 + 0.010 =0.061 Diameter


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What about PB Free Impact?

• Surface Adhesion− increased annular rings?

• Plated Barrel Thickness− ductility of the copper

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SURFACE FINISHES


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Surface Finishes

The “Ideal” PCB surface finish would have these characteristics:

• No-clean and water soluble paste & flux compatibility

• Strong intermetallic joint - BGA - tensile stress

• Three reflow sequences spread over 1 week

• Storage for 1 year

• Press fit connector Compatibility

• Visually inspection - quality assurance

• Lead-free - environmental initiatives

• Minimize total cost

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Surface Finishes

Popular Lead Free Surface Finishes:

• PCB Surface Finishes

• OSP

• Tin (Sn)

• ENIG (NiAu)

• Silver (Ag)

• Palladium / Gold (PdAu)

• Pb Free HASL

• Gold (Au) on copper


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Surface Finishes

Lead free Solder Characteristics:

• Eutectic Solder (tin/lead)– 183° C Melt for Sn/Pb Solder– 210°-235° C peak reflow range– “sharp” liquid transition, melts and solidifies rapidly

when temp is reached

• Sn/Ag/C Solder (tin/silver/copper)– 217°-221° C Melt for Sn/Ag/Cu Solder.– 235°-260° C peak reflow range.– Sn/Pb SAC “pasty” range requires 15-30 seconds

longer liquidus

• Aggregate difference 40°C hotter, 15-30 seconds longer

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Surface FinishesTin Whiskers

Whiskers grow because of compressive stress in the plating which is caused by irregular growth of intermetallics

Problems with Tin Whiskers

A number of government and industry alerts have been issued relating to whiskers for aerospace, defense and medical industries. Tin Whiskers can contribute to a number of potentialproblems in electronic hardware. These problems include:

• Permanent short circuits• Transient short circuits• Metal vapor (plasma arc in a vacuum)• Debris / contamination


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Copper Dissolution

Copper dissolution is a natural metallurgical reaction through which copper dissolves in a tin-rich liquid.

It happens during all types of soldering. But more pronounced in Lead Free soldering due to higher tin content, higher soldering temperatures, and longer dwell time.

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Copper Dissolution – cont’d


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• Isolate the tin rich solution from the copper

• Be careful in the selection of a surface finish. Know what soldering process will be used


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Quality Assurance

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IPC Lead-Free Alloy Testing


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Raw PWB Materials Declaration (Sample)

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Raw PWB Materials Declaration (Sample)


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Used to evaluate the effects of high temperature soldering PCB structure

Types:

• Air–to-Air

• Interconnect Stress Test (IST)

• Highly Accelerated Thermal Shock (HATS)

• Reflow Oven

Stress Testing

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Interconnect Stress Test (IST)The method uses the copper

circuits (both traces and vias)

integrated into the DUT

as direct-current heating elements,

and is cooled to

ambient temperature with circulated

air.


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Highly Accelerated Thermal Shock (HATS)

The test uses a single chamber in which high volume hot and cold air pass stationary samples.

Chamber

Wired Modules

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HATS IST

Range: -60 to 160oC Range: 25oC minimum

Heating Method: air Heating Method: DC current

36 coupons 6 coupons

4 nets each 2 nets each

144 nets total 12 nets total

Coupon emulates PCB design

Coupons with special heating nets

Typical cycle time:

3.5 min. from 25 to 150oC

6 min. from -45 to 150oC

Typical cycle time:

3 min. from 25 to 150oC

Stress Testing


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Preconditioning using reflow oven the most accurate assessment of survivability!

Now an indirect requirement per IPC-6012

Stress Testing

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Marking Standardization


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Suggested marking per IPC-609

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Lead Free Assembly Issues

The final solder joint alloy could be very unpredictable.Who pays for the failures could end up with a lot of finger pointing

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Reflow Process Window

With Tin-Lead: melting point = 183Clower temp limit for reflow = 200Cupper temp limit for reflow = 235Cprocess window = over 30C

With 95.6Sn-3.7Ag-0.7Cu: melting point = 217Clower temp limit for reflow = 235Cupper temp limit for reflow = 260Cprocess window = 25C

High temperatures can lead to many new problems: componentcracking and "popcorning", laminate measling & delamination, stresson thru-holes, second-side oxidation and non-wetting andintermetallic growths. The lower the temperature, the better!


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Typical Soldering Profile

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• Wetability

• Joint appearance

• Tombstone failures

• Voiding

• Flux tarnishing

• Component damage

• Dross Issues

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Wetability

•Lead-free solders do not spread well during reflow

•Dependent upon Surface Finish



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Wetability


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Joint Quality and Appearance

• Fillet appearance not the same

• Surface finish not the same



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Tombstone failures more prevalent


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Possible reasoning behind tombstone failures:

Composition of SAC alloys affects tombstoning and voiding at soldering, mainly through physical effects.

In general, composition deviating from the ternary eutectic SnAgCu in Ag content, particularly with a Ag content lower than 3.5Ag, exhibits a greater solid fraction at onset of melting thus results in a lower tombstoning rate, presumably due to a slower wetting speed.

On the other hand, composition deviating from the ternary eutectic SnAgCu also exhibits a lower surface tension which results in an easier solder spread or solder wetting, and consequently results in a higher tombstoning rate and a lower voiding.



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Voids in BGA balls:

• Found with ALL lead-free solders

• Occur at the interface between solder and pad

• SAC305 gives lowest voiding (CE study)

• Flux design and process control are critical

• Not a reliability issue…..until 0.5mm pitch


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• Surface tension is higher

than Sn-Pb, flux residues

can not escape

• May be avoided by reflow

and paste control



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Flux tarnish increases with temperature cycles


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Placement Issues

• Decoupling caps placed next to large thermal masses, such as BGAs with SAC alloy balls and heat sinks, are subject to much greaterheat profiles !

• Becomes a major decoupling issue for high speed applications


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Component Damage

225°C 250°C


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Rework & Repair

• Need to avoid cross-contamination of solders > 0.5% Pb

will cause reliability issues

• Hand-soldering of lead-free alloys requires training

- Freeze behavior is different than eutectic

• Need to selectively apply heat to only the areas that need

rework (may need more robust lands)

• Sn-Ag is probably the universal rework material Available in wire (for parts lists)



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Dross issues:

• Lead Free alloys produce a continuous, thin layer of

dross across the surface of the wave under an air

environment

• Dross rates have been seen to be lowest for Sn-Cu

followed by SAC alloys


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Component Selection


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Component Selection

• Typical Component Lead Free finishes:

– Tin (Sn)

• 8 – 12 microns typical thickness

• Some manufacturers anneal right after plating

– Tin/Bismuth (SnBi)

– Nickel/Palladium/Gold (NiPdAu)

• Tested to Proposed NEMI and JEDEC Standards

– JESD22A121

• Tin Whiskers

– Some growth allowed under certain conditions

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Component Selection

Each component would need the following information:

• MSL testing for non-hermetic sealed plastic parts (J-STD-020E)

• Testing Data is needed to 2600C peak

• High temperature Storage data (JEDEC 22-A103-B)

• Tin whisker testing

• Maximum reflow temperature rating

• Solderability Test with tin-lead and lead free solder (J-STD-002D)

ROHS compliant:• Lead, Mercury, Hexavalent Chromium, PBB, PBDE < 0.1 wt%,

• Cadmium < 0.01wt%


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Component Selection

• Mechanical Hardware

– Screws, nuts, washers

– Mounting brackets

– Card extractors

– Roll pins

– Etc.

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Lead Free Solder Alloy Reflow

Nitrogen Inerting Atmosphere Soldering

• Solderabilities are improved considerably with the use ofnitrogen processing atmospheres.

• In addition, the required process temperatures for goodwetting has shown to be reduced (up to 30° C) with theuse of nitrogen, thereby reducing the potential damageto temperature-sensitive components.

• Therefore nitrogen atmospheres may be necessary,especially with complex boards with varying finishes andthermal requirements.

source: http//leadfree.ipc.org


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For a robust lead-free process consider the following

• solder used

• board surface finishes

• components and lead finishes

• printing process (stencil)

• reflow soldering

• wave soldering

• cleaning

• rework and repair

Lead Free Assembly Issues - Summary

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Considerations for

Master Drawing Notes


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MATERIAL:

LAMINATE GLASS FIBER EPOXY FLAME RETARDANT PER IPC-4101/126, 170OC MINIMUM, AS TESTED TO IPC-TM-650, 2.4.24C (TMA). DECOMPOSITION TEMPERATURE 340OC MINIMUM, AS TESTED TO IPC-TM-650, 2.4.24.6. A T288 DELAMINATION TIME OF 35 MINUTES MINIMUM, AS TESTED TO IPC-TM-650, 2.4.24.1C. A MAXIMUM THICKNESS EXPANSION OF 3% FROM 50 - 260OC. PREPREG MATERIALS PER IPC-4101/126 SHALL MEET THE SAME REQUIREMENTS AS BASE MATERIAL. CONSTRUCTION IN ACCORDANCE WITH FIGURE 1.

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0.062

LAYER 1

DIELECTRIC MATERIAL

DIELECTRIC MATERIAL

DIELECTRIC MATERIAL

LAYER 4

1 OZ (35 urn) LAYER 2

1 OZ (35 urn) LAYER 3

FIGURE 1


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COPPER PLATE:

A. SURFACE - TOTAL COPPER ON THE SURFACEINCLUDING ORIGINAL CLADDING PLUS PLATED COPPER SHALL BE 70 um [2 OUNCE],35 um [1 OUNCE] MINIMUM.

B. HOLES - COPPER PLATING ON WALL OF HOLESSHALL BE 20 um [0.0008] AVERAGE MINIMUM, NOVALUE LESS THAN 18 um [0.0007].

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SOLDER PLATE:

A. SURFACE - CONDUCTOR PATTERN, EXCLUDING CONTACT FINGERS, SHALL BETIN-LEAD (60% TIN, 40% LEAD) IN

ACCORDANCE WITH THE SOLDERABILITYREQUIREMENTS OF J-STD-003.

B. HOLES - SOLDER PLATE ON WALL OF PLATEDTHROUGH HOLES SHALL BE TIN-LEAD (60%TIN, 40% LEAD) IN ACCORDANCE WITH THESOLDERABILITY REQUIREMENTS OF J-STD-003.


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GOLD PLATE:

PRINTED CONTACT FINGERS SHALL BE PLATEDWITH 2.5 um [0.0001] MINIMUM THICKNESS OFLOW STRESS NICKEL FOLLOWED BY 0.8 um [0.00003] MINIMUM THICKNESS OF GOLD TO KNOOP HARDNESS SCALE. VENDOR CERTIFICATION REQUIRED.

OR

SOLDERABLE AREAS SHALL BE PLATED WITH 1.3 um [0.00005] MINIMUM THICKNESS OF LOW STRESS NICKEL FOLLOWED BY 0.8 um [0.00003] MINIMUM THICKNESS OF GOLD.

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CONDUCTOR WIDTH AND SPACING

THE NOMINAL CONDUCTOR WIDTH AND SPACING ON FINISHED BOARD IS:

WIDTH - 0.2 mm [0.008] +/- 0.02 mm [0.001]

SPACING - 0.2 mm [0.008] +/- 0.02 mm [0.001]

DESIGNED FABRICATION ALLOWANCE IS 0.4 mm [0.016]


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HOLE REQUIREMENTS:

A. MINIMUM ANNULAR RING - 0.05 mm [0.002].

B. HOLE LOCATIONS TO BE 0.15 mm [0.003] DTP.

C. HOLE SIZES APPLY AFTER SOLDER PLATING, REFLOW, OR DEPOSITION.

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SOLDERMASK:

SOLDERMASKING OF PRIMARY AND SECONDARY SIDES OF THE BOARD SHALL BE PER MASKING ARTWORK, OR CAD DATA, OVER BARE COPPER USING PHOTOIMAGEABLE PERMANENT POLYMERFILM PER IPC-SM-840, CLASS T, 0.001-0.002 THICK.

RESIZING FOR MINIMAL LAND TO MASK CLEARANCE PERMISSIBLE.


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MARKING:

MARKING SHALL BE PER MARKING ARTWORK USING WHITE NONCONDUCTIVE EPOXY INK. BOARD SHALL BE PURCHASED FROM U.L. RECOGNIZED MANUFACTURERS AND MARKED IN COPPER IN AREA SPECIFIED WITH VENDOR LOGO FOLLOWED BY 94V-0 AND DATE CODE OF MANUFACTURE CONSISTING OF A FOUR DIGIT NUMBER (FIRST TWO, WEEK; LAST TWO, YEAR.

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MANUFACTURE IN ACCORDANCE WITHIPC-6011 & 6012, CLASS 3.

NO MODIFICATIONS OR REPAIRS WITHOUT PRIOR APPROVAL.

MODIFICATIONS OR REPAIRS SHALL BE IN ACCORDANCE WITH IPC-7721.

pcb carolina – 2017

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