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Reliability of Pb-free Solder Alloy Study

NEPP FY02

Harry Shaw at NASA-GSFC

Jong Kadesch at Orbital Science Corp./GSFC

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Objectives

• Identify a candidate of Pb-free alloys based on a literature search

• Validate Hand-soldering assembly process: soldering process, usage of flux, selection of soldering guns temp.

• Compare the solder joints between Sn/Pb solder vs. Pb-free

solder alloys: optical solder joint comparison after cross-

section.

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Schedule and Cost• Q1-FY02 $10,000

– Identifying Pb-free candidates– Procurement of test boards, components, solder alloy

• Q2-FY02 $10,000– Board assembly

• Q3-FY02 $10,000– Visual and X-ray inspection– Cross-section analysis

• Q4-FY02 $10,000– Final report

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Alloy Melting Temp. (C) Study BySn/Ag3.4//Cu1.0/Bi3.3 205-214 NCMSSn/Ag4//Cu1.6/Sb1/Bi1 214-220 NCMSSn/Ag2.5/Cu0.8/Sb0.5 ITRISn/Ag4/Cu0.5Sn95.8/Ag3.5/Cu0.7Sn/Ag4/Cu1.0

216-219 NCMS, NEMI, ITRIJPLNCMS, NEMI, ITRI

Sn/Ag3.5/In1.5 218-223 NCMSSn/Ag2.5/Cu0.8/Sb0.5 210-216 NCMS, JPLSn96.5/Ag3.5 221 NCMS, NEMI, ITRI, JPLSn/Cu0.7 NEMI, ITRISn77.2/In20.0/Ag2.8 NCMS, JPLSn/Bi/Ag ITRISn/Bi/Zn ITRI

Pb-free Alloy Evaluated by the Organization

Sn96.5/Ag3.5Sn96.5/Ag3.5GSFC Selection for Hand-Soldering

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Core WireCore Wire

Solid WireSolid Wire

No Clean FluxNo Clean Flux

Water Soluble FluxWater Soluble Flux

Rosin Base FluxRosin Base Flux

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Cost Comparison

Note: cost generally driven by the metal market, not by Pb-free solder alloy demand

Vendor1

2

3

4

5

Wire Diameter Sn/Pb Sn/Ag Minimum Order Lead Time Comments0.010" $360.93/lb 1lb0.020" $360.93/lb 1lb0.030" $360.93/lb 1lb0.010" $185.79 lb. 5lb0.020" $105.47 lb. 5lb0.030" $100.11 lb. 5lb0.010" $35.75/lb $38.92/lb $5000.020" $17.15/lb $20.32/lb $5000.032" $15.10/lb $18.27/lb $5000.010" N/A $1000.020" $1000.030" $1000.010"0.020"0.030"

3 - 4 weeks

1 - 2 weeks

Core wire

Core wire

Core wire

Core wire

Solid wire only1 week

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Ultimate CTE Thermal Electrical Density Melting

Tension Conductivity Resistivity gm/cc Temperature

Mpa (W/m.K) (mW -cm) °C (°F)

63Sn/37Pb eutectic 52 24.7 50.9 14.5 8.40 183 (361)

96.5Sn/3.5Ag eutectic 38.7 (5.61) 30 33 11 7.36 221 (430)

• Sn/Pb– cost: relatively low– rosin base flux– easily available – lead time: within 2-3 days– core wire available

• Sn/Ag– 2.2 to 2.7 times higher*– rosin base flux (2%)– easily available– 8 - 10 days– core wire available

Solder Alloy Comparison

* [source from Kester]

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Flux Type

No CleanNo Clean• Convenient• Corrosion

problems in room temp.

Rosin BaseRosin Base– RA

– RMA

• Reliable solder joints if cleaned well.

• CFC cleaning solvent phase-out.

• Corrosion problem in high temp exposure if not cleaned well.

Water SolubleWater Soluble• No solvent

needed for cleaning.

• Additional time required for bake out

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Test Boards

• Used existing board design - High Voltage Power (HVP) Board requires two component types

• Dimensions: 4” x 5.5”• Board surface finish

– Eutectic Sn/Pb

– Electroless Ni/Immersion Gold (ENIG)

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Component Finish (as available)

• Diodes (SGB10UFSMS)– Pb/In finish (not available

in Pb-free alloys)

• Capacitors (1515)– Pt/Ag finish

• Predicted failure*– 1206 chip resistors: 2P

mean life is 2014 cycles from -55C to 160 C

– 0805 chip resistors: 2P mean life is 4725 cycles from -55C to 160 C

– 1515 chip capacitors: no data available

*Data from NCMS

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Component Mounting Matrix

Additional two (2) boards are sent to JPL for their evaluation using reflow soldering technique.

SN# Sn/37Pb ENIG Pb/In Pt/Ag Sn/37Pb Sn/3.5Ag1 a a a a

2 a a a a

3 a a a a

4 a a a a

5 a a a a

6 a a a a

7 a a a a

8 a a a a

9 a a a a

10 a a a a

11 a a a a

12 a a a a

Board Finish Component Lead Finish Solder Alloy

Current Work Future Work

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Hand Soldering Process

• Use MetCal soldering gun• 600 series: soldering iron temperature at 350 °C

– the actual temperature at the board is less than 350 °C and the exact temperature is not measurable. It is therefore, critical to control the time of heat exposure to the components

• Observations:– Sn/Pb soldering process is easier and quicker than Sn/Ag

– Sn/Pb has well established core solder wire and various diameters It is always not the case for Pb-free solder alloys

– wetting of Sn/Ag is more difficult than Sn/Pb, flux definitely required to improve the wetting

– longer time to work on the components with Sn/Ag

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Inspection

• Visual inspection using optical microscope– solder appearance: color or brightness/dullness, graininess– wetting appearance: shape of solder– burning or melting of components

• X-ray inspection– voids

• Cross-section inspection– voids– intermetallic formation

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Visual Inspection of Solder Joints

• no difference in coloration between Sn/Pb and Sn/Ag• most Sn/Pb solder joints are shiny, some Sn/Ag

solder joints are shiny and some are appear to be dull

• smooth surface of Sn/Pb versus rough surface of Sn/Ag

• more irregular shape of solder joints in Sn/Ag solder, perhaps due to difficulty in wetting

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Sn/Pb Solder Joints

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Sn/Pb vs. Sn/Ag

Sn/Pb Sn/Ag

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Sn/Ag Solder Joints

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Sn/Ag Solder Joints

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X-ray Inspection

• No voids detected in solder joints both Sn/Pb & Sn/Ag

• Some solder joints shows poor workmanship– missing solder on one termination of the capacitor (02A1-

C14)– spot solder on one termination of the capacitor (04A1-Cxx)

• Insufficient solder fillet due to poor pad design of the board– poor placement of the component (03A1-C12, & 04A1-Cxxx)

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X-ray Images of Missing Solder

02A1-C1404A1-Cxx

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X-ray Images of Poor Placement

04A1-Cxxx03A1-C12

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Cross-section of Sn/Pb

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Sn/Pb

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Cross-section of Sn/Ag

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Sn/Ag

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Conclusions

• Sn/Ag has marginal wetting– Sn/Ag requires flux to improve wetting– Sn/Ag may require longer process time (x2)

• Sn/Ag solder joints appears to be somewhat different than Sn/Pb solder joints– Sn/Ag appear to be less shinier due to grainy surface

• Over all, the same soldering process for Sn/Pb can be adopted for Sn/Ag without major difficulties.