Effect of Silver in Common Pb-Free Solder Alloys
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Effect of Silver in Common Pb-Free Solder Alloys
- 1. an Alent plc Company Effect of Silver in Common Pb-Free Solder Alloys
- 2. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Silver (Ag) in Electronic Solder • Used in electronic solder for over 50 years • Reduces Ag scavenging from Ag plated parts • Improves Thermal Fatigue Resistance • Good electrical and thermal conductivity properties • Wets well to most common PCB surface finishes
- 3. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Ag in Pb-free Solder • Key Advantages Low liquidus temperatures Lower overall operating temps Higher Ag results in lower surface tension Better overall soldering performance vs. low and no Ag alternatives Better thermal fatigue resistance vs. Ag-free bearing alloys (ie SN100C) • Disadvantages • Ag has the greatest impact on cost of the solder – Price fluctuations of 30%+ in ’08 • Higher Cu Dissolution rates • Higher Drossing rates • Reduced high strain or “drop shock” reliability – (critical to SMT devices)
- 4. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Melting and freezing characteristics of SnAgCu alloys • Study by NIST – SAC305 and SAC405 are near eutectic – 217ºC eutectic reported for SN3.7Ag0.9Cu
- 5. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Melting and freezing characteristics of SnAgCu alloys • DSC plots of SAC305, SAC105 and SAC0307 indicate single high peak for SAC305 but lower “transition” peaks for lower silver Pb-free alloys • This is verified by a phase diagram for an isothermal section set between the two transition peaks (223ºC) showing a small liquid only window at lower Ag levels and a higher presence of solids 200 210 220 230 240 250 10 20 30 40 50 60 70 80 SAC305 SAC105 SAC0307 HeatFlow Temperature ( o C)
- 6. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Melting and freezing characteristics of SnAgCu alloys • Among the Ag bearing Pb-free alloys there is a significant differences in microstructure of solid soldered joints even though alloy compositions are not that different • Coarse Sn dendrites for SAC305 are different from fine Sn dendrites for Sn3.9Ag0.6Cu • Sn3.7Ag0.9Cu shows an Sn dendrite pattern similar to SnAg eutectic while Sn3.6Ag1.0Cu microstructure does not appear dendritic at all.
- 7. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company 0.2oC/min 1.2oC/min 3.0oC/min Sn3.8Ag0.7CuSn2.5Ag0.9Cu 0.2oC/min 1.2oC/min 3.0oC/min Sn3.8Ag0.7CuSn2.5Ag0.9Cu Below are micrographs of cross-sectioned solder joints formed with two SAC alloys. Reflow temp was 240ºC. 3 different cooling rates were used A combination of high silver content and slow cooling rate results in the growth of large size Ag3Sn platelets Melting and freezing characteristics of SnAgCu alloys
- 8. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Melting and freezing characteristics of SnAgCu alloys • Ag3Sn has a higher melting temperature, thus these platelets start precipitating and growing while solder is still in the liquid state. Sometimes the Ag3Sn platelets can grow so large in the liquid stage that when solder shrinks during freezing, the Ag3Sn platelet protrudes outwards severely deforming the solder interconnection.
- 9. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Wetting Behavior of Pb-free Alloys • An optimized balance of Ag and other additives helps to lower the surface tension of solder alloys • Ag-free alloys have a higher surface tension at standard operating temperatures Lower surface tension results in faster wetting contributing to better soldering performance
- 10. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Wetting Behavior of Pb-free Alloys • There is a significant difference in the wetting times of alloys with different levels of Ag content when the test is carried out at a 250oC pot temperature • The difference in wetting times of these alloys at 260ºC is small in absolute values but the trend is the same Another wetting balance test was run with SAC alloys with the Ag level varying from 0% to 1%. The test was run at two pot temperatures, 250ºC and 260ºC.
- 11. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Effect of Ag on Copper Dissolution Time required for 50µm copper wire to dissolve in 260ºC molten alloy bath. • Ag increases Cu erosion rate • Cu, Ni and Co reduce the erosion rate
- 12. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Effect of Ag on Wave Soldering Temperatures • As mentioned earlier, Ag lowers the surface tension of Pb-free alloys – This allows faster wetting at lower operating temperatures Higher Ag alloy exhibits much better overall hole fill 0.3% Ag SAC alloy 3% Ag SAC alloy• 6 layer, 2.4mm thick board w/ OSP pad finish • 260º C pot temperature • 3.9 second contact time • 3 different alcohol based fluxes
- 13. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Ag bearing SAC alloys have lower liquidus temperatures than Ag-free alloys and require overall lower operating temperatures Higher operating temperatures can damage PCB laminates Weight loss due to thermal decomposition of laminate materials Source: Isola PCB’s absorb moisture and can be damaged during processing by the release of high pressure water vapor. Lower Operating Temperatures Reduce these Risks! Vapor pressure of water increases exponentially above 250ºC.
- 14. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
- 15. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
- 16. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company
- 17. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Effect of Adding Bismuth (Bi) to a Ag-bearing Pb-free Alloy • Bi contributes to a refinement of grain structure in SnAgCu alloys reducing stress at grain boundaries • This reduces stress build- up along grain boundaries during temperature cycling • It also reduces grain coarsening Both of these would result in improved thermal fatigue resistance
- 18. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company Summary of the Effects of Ag in Pb-free Alloys • High-silver SnAgCu alloys perform poorly in high- strain rate situations (i.e. drop shock) • High-silver SnAgCu alloys exhibit better thermal fatigue resistance than lower or no Ag alternatives. • Silver lowers the required operating temperature for a given soldering application • A small amount of silver shows noticeable improvement in wetting characteristics of the solder. • Addition of grain refining elements such as Bi changes the microstructure and improves the thermal fatigue resistance of low silver alloys.
- 19. HIGHLY CONFIDENTIAL AND PRIVILEGED INFORMATION an Alent plc Company For more information on ALPHA products click the link below: ALPHA Products
Editor's Notes
- SAC305 is our industries most popular Pb-free alloy. It provides generally excellent soldering performance compared to SnPb and good reliability, especially thermal fatigue resistance, for most assemblies. With it’s relatively high Ag level SAC305 is considered somewhat expensive compared to lower Ag alternatives. This is especially true when a lower Ag alternative alloy performs acceptably on a particular assembly. Technically it has become clear that higher silver levels, like the levels found in SAC305, contribute to higher copper dissolution and also reduced “drop shock” reliability performance. Drop shock reliability is generally more associated with SMT components than through hole parts.
- As stated earlier, higher Ag alloys exhibit higher rates of copper dissolution or erosion. This is due to the rapid formation and reformation of IMC’s at the solder / copper land interface. Along with simply lowering Ag levels, certain additives will also slow the IMC formation resulting in lower dissolution of the copper land. These additives include Copper, Nickel and Cobalt. These alloys also tend to raise the alloys liquidus temperature. Cookson has studied various combinations of these constituents and developed our SACX alloys to contain just the right amount of additives. The chart shows how the addition of Ni in our SACX alloy will slow Cu erosion in a standard wire break test.
- Higher operating temperatures can increase the potential for damage to the board laminate. According to the PCB board laminate industry when a board loses 5% or more in weight due to thermal decomposition the board’s reliability is compromised. Laminate decomposition occurs during high temperature exposures such as a single reflow or wave soldering cycle. The damage is accumulative, meaning each thermal excursion, or exposure, can potentially result in further damage. As an example, a typical double sided assembly with SMT and through hole components will have at least 4 different thermal excursions and possibly more if rework or other selective soldering is required. Also, PCB’s typically absorb moisture during processing, packaging, transport or storage. During the assembly process this moisture can cause board damage as it become vaporized and is released. As processing temperatures increase the amount of vapor pressure created goes up dramatically. Damage caused by the release of this pressure can lower process yields and / or decrease the boards reliability.