An Investigation Concerning the Diffusion Of Elements in Sn-Ag3.8-Cu0.7 and Sn-Pb36-Ag2 on Electrolytic Ni-Au Pads during Pre-Pack Thermal Aging

Much literature has been written and debated on the effects of reflow profiles on IMC {Inter-metallic compound) formation for solder alloys and the diffusion of elements during this process. This data has been concerning primarily assembly operations. However, not much data has been acquired on the effects of thermal aging on solders at back end pre-pack operations. The thermal aging conducted at these operations is to dispose of any possible trapped moisture which could lead to severe failures at the SMT customer\´s site during board level reflow operations, or even on the field of usage due to overheating. The purpose of this study is to present a quantitative analysis to understand the effects of heat and time on the behavior and diffusion of elements in these solders just before they are shipped to the customers. By understanding the behavior of these elements, it would help to correlate towards brittle and ductile fractures much better through CBP tests {Cold ball pull). EDX spot analysis was performed at 3 main spots on cross-sectioned units, i.e. the bulk solder itself, the formed IMC layer and the subsequent layer just below the IMC\´s. Through this study, it has been conclusive that Copper diffuses more actively than Nickel. It has also been seen that both Copper and Nickel cannot co- exist, and the "cancelling effect" of either element is obvious. It has been conclusive that temperature and baking hours at the pre-pack operations do influence brittle failures in Sn-Pb36-Ag2 more acutely than Sn-Ag3.8-Cu0.7 due to Gold-embrittlement from the formation of AuSn₄ The precipitation of Ag₃Sn is restrained in Sn-Ag3.8-Cu0.7 due to the presence of Gold, thus no obvious Ag₃Sn IMC is observed. This study also confirmed the formation of Ni₃Sn₄ as a primary IMC layer for Sn-Pb36- Ag2. It was seen on the other hand that Sn-Ag3.8-Cu0.7 showcased the formation of two IMC layer\´s which were defined as (Cu, Ni)_{6<- /sub> Sn5 and (Ni, Cu)3 Sn4 where (Cu, Ni)6 Sn5 formed on top of the latter.}