An investigation into the effect of dry bake on the solderability degradation of electrodeposited tin finishes

The solderability of component termination finishes degrades upon storage due to the combined effects of surface contamination, oxidation, corrosion and/or intermetallic compound (IMC) formation at the interface between substrate and finishes. Such solderability degradation impinges on the shelf-life of the electronics components and hence demands an in-depth understanding to ensure high reliability Pb-free soldering. This paper is concerned with the effect of the 155°C dry bake precondition on the accelerated ageing of bright Sn electrodeposited termination finishes over Cu substrates. Two coating thicknesses, 2.5 μm and 9 μm, were studied as-received and dry baked for 4 hr and 16 hr as per industrial standards and analysed with respect to their solderability, microstructural and compositional characteristics. It was found that, with increasing dry bake time, both the thickness variations experienced continued solderability degradation, with only the 2.5 μm thick coatings failed after 16 hr of dry heat. Such a loss of solderability was primarily contributed to the exposing of Sn-Cu IMCs. The thickness safety margin (i.e. maximum IMC thickness) for the dry bake condition (16 hr) of the given bright Sn samples was identified to be around 2.5 μm, which could provide a guidance for any future industrial initiative to further reduce coating thickness. Besides, the dry bake conditions have proven to be a consistent and reliable precondition approach for the Sn-based metal finishes.