The global transition to Pb-free electronics has led component and equipment manufacturers to transform their tin-lead (SnPb) processes to lead-free (Pb-free). At the same time, Pb-free legislation has granted exemptions for some products whose applications require high, long-term reliability. However, due to a reduction in the availability of SnPb components, compatibility concerns can arise if Pb-free components have to be utilized in a SnPb assembly. This compatibility situation of attaching a Pb-free component in a SnPb assembly is generally termed "backward compatibility".

This paper presents the results of microstructural analysis of mixed solder joints, formed by attaching Pb-free solder balls (SnAgCu) of a ball grid array (BGA) component using SnPb paste. The experiment evaluates Pb phase coarsening in bulk solder microstructure and, the study of intermetallic compounds (IMCs) formed at the interface between the solder and copper pad.

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