Improved Reliability and Mechanical Performance of Sn58Bi Solder Alloys

Microstructural and mechanical properties of the eutectic Sn58Bi and micro-alloyed Sn57.6Bi0.4Ag solder alloys were compared. With the addition of Ag micro-alloy, the tensile strength was improved and this is attributed to a combination of microstructure refinement and an Ag3Sn precipitation hardening mechanism. However, ductility is slightly deteriorated due to the brittle nature of the Ag3Sn intermetallic compounds (IMCs). Additionally, a board level reliability study of Ag micro-alloyed Sn58Bi solder joints produced utilising a surface-mount technology (SMT) process, were assessed under accelerated temperature cycling (ATC) conditions. Results reveal that micro-alloyed Sn57.6Bi0.4Ag has a higher characteristic lifetime with a narrower failure distribution. This enhanced reliability corresponds with improved bulk mechanical properties. It is postulated that Ag3Sn IMCs are located at the Sn-Bi phase boundaries and suppress the solder microstructure from coarsening during the temperature cycling, hereby extending the time to failure.

Effects of Static Heat and Dynamic Current on Al/Zn∙Cu/Sn Solder/Ag Interfaces of Sn Photovoltaic Al-Ribbon Modules

This present study applied Cu∙Zn/Al ribbon in place of a traditional Cu ribbon to a photovoltaic (PV) ribbon. A hot-dipped and an electroplated Sn PV ribbon reflowed onto an Ag electrode on a Si solar cell and estimated the feasibility of the tested module (Ag/Solder/Cu∙Zn/Al). After bias-aging, a bias-induced thermal diffusion and an electromigration promoted the growth of intermetallic compounds (IMCs) (Cu6Sn5, Ag3Sn). To simulate a photo-generated current in the series connection of solar cells, an electron with Ag-direction (electron flows from Ag to Al) and Al-direction (electron flows from Al to Ag) was passed through the Al/Zn∙Cu/Solder/Ag structure to clarify the growth mechanism of IMCs. An increase in resistance of the Ag-direction-biased module was higher than that of the Al-direction biased one due to the intense growth of Cu6Sn5 and Ag3Sn IMCs. The coated solder of the electroplated PV ribbon was less than that of the hot-dipped one, and thus decreased the growth reaction of IMCs and the cost of PV ribbon.

Optimization of Ag Composition in Cu Pillar Bumps with Sn-xAg Solders

In order to minimize the large plate growth of Ag3Sn intermetallic compounds (IMCs) adversely affecting the mechanical behavior and reducing the reliability of solder joints, the Ag content of less than 3wt% has been suggested in ball grid array (BGA) Sn-Ag solders. The suggestion (< 3wt% Ag in the solders) is invalid in small solder bumps, and moreover there is no available guideline of the Ag content in the small Sn-Ag solders. In this study, the optimum level of Ag content in small Sn-Ag Cu pillar bumps (CPBs) of less than 50um in diameter was investigated to suppress the large plate growth of Ag3Sn. Since the undercooling of Sn-Ag solders increases with a decrease of solder ball size, the Ag3Sn IMCs are more prone to become largely plate-like in small solders. Actually, in small Sn-Ag CPBs, the large plate growth of Ag3Sn was observed on the top surface of solder bumps even though the Ag content was less than 3.0wt%. For suppression of Ag3Sn plates in Sn-Ag CPBs, two ways were suggested; increasing the cooling rate of the reflow profile and reducing the Ag content in the solders. Reducing the Ag content was effective on reducing the large plate growth of Ag3Sn, while a high cooling rate was not effective. Through the thermal analysis and thermodynamic calculations, the optimized Ag content in Sn-Ag CPBs of less than 50um was suggested, and the large plate growth of Ag3Sn was effectively reduced within the suggested Ag content.