Lead-free Doped Solder Joint Reliability under Harsh Temperature Cycling Environment to study the Long Term Isothermal Aging Effects of Heat Sinks, Solder Paste Volume, Board Substrate Material, Component Substrate Material and Component Sizes

Abstract

Restriction of Hazardous Substances (RoHS), Waste Electrical and Electronic Equipment Directive (WEEE) organizations have restricted the use of lead in solder materials for consumer electronics. The electronic industry have drifted apart from eutectic 63Sn-37Pb (Tin-Lead) solder to the near eutectic lead-free [Sn-1.0Ag-0.5Cu (SAC105)] and [Sn-3.0Ag-0.5Cu (SAC305)] solder material in past decade. Knowledge about microstructure, mechanical properties, and failure behavior of lead-free solder joints in electronic assemblies are dynamically changing when exposed to isothermal aging and temperature cycling environments. Researchers and scientists in the semiconductor industry and academic world have experimentally demonstrated that the long-term performance of the lead-free solder joint material is dependent on inter metallic composition formed during the reflow of surface mount assembly of electronic packages. The inter metallic composition for the binary eutectic Tin-Lead (63Sn-37Pb) and its degradation properties over long-term isothermal aging are very much different from the Ternary non eutectic Tin-Silver-Copper [Sn-1.0Ag-0.5Cu (SAC105), Sn-3.0Ag-0.5Cu (SAC305)]. However, the initial performance of the ternary Tin-Silver-Copper lead-free solder material is much better than the binary Tin-Lead solder materials reliability. The reliability performance degrade up to 70% for lead-free solder joint materials SAC105 and SAC305 that are exposed to long- term isothermal aging conditions in elevated temperatures (75°C for 24 months).The major contribution to this degradation in lead-free solder joint material is the accumulation of the low cyclic stress induced on the bulk solder near the intermetallic formation over a long period (>10 months) at elevated temperature(>50°C). This phenomenon observed by many researchers and scientists in the field, demonstrated as the intermetallic composition thickness growth and recrystallization of grain boundaries. There are many investigations targeted to resist this change in material property over time and temperature effects. One such possible investigation is to develop a new solder alloy composition with additional elements in smaller composition as dopants mixed with ternary lead-free (SAC105, SAC305) solder materials. The top materials that qualify for such metallurgical composition are Antimony (Sb), Magnesium (Mg), Nickel (Ni), Cobalt (Co), Indium (In) and Bismuth (Bi). Material researchers and scientists previously evaluated these additives. The results showed finer grain boundaries and reduced the intermetallic formations of the tin with silver or copper during the reflow process of surface mount assembly. The resulting intermetallic formation had a more uniform grain formation in the lead-free bulk solder alloy. This process of adding metallurgical elements in small quantities to the lead-free solder composition is defined “solder doping”. This improves the mechanical properties of the lead-free solder materials initially, but there are limited research results on the effects of the long-term isothermal aging performance of such doped alloys. Lead-free solders have reliability concerns related to elevated process temperatures and the formation of Ag3Sn intermetallic. Tin-Lead reliability performance compared to that of SAC105, SAC305, and SAC-Bi containing low melting temperature (Tm) alloys show deleterious effects of long-term isothermal aging time and temperature conditions.