The necessity of corrosion protection for solderable pure tin deposits on IC outer leads

Due to its ease of application pure tin has been selected by the industry as the main alternative to solderable tin/lead deposits following RoHS legislature. After several years of industry experience several issues have developed which are due to the corrosion of the top layer during storage. The most serious of effects has been the growth of whiskers during heat/humidity (55^degC/85% r.h.) storage. In the vicinity of exposed Copper, at the dam bar or toe cut, whiskers appear after an induction period, whose lengths may exceed the tolerable limit after 4000 hour storage. Another deficiency of equal concern is the deterioration of solderability after long term storage or, as an equivalent, after steam ageing. An improvement of these defects will be achieved by applying a corrosion resistant post-treatment, which has to combine corrosion preventive properties with being able to withstand the reflow procedure, while still being present on the surface afterwards to protect the deposit from oxidation. Whisker and solderability studies show the effectiveness of these new coatings. Solderability after heat/humidity storage can be considerably improved and even at low temperatures excellent wetting is obtained. TOF-SIMS (time-of-flight secondary ion mass spectroscopy) is being used to clearly demonstrate that these coatings will withstand multiple reflow operations. In the search for effective corrosion protection, the hydrophobicity of the final tin layer is being investigated. Furthermore this water-repellent property has to be maintained throughout prolonged contact with water. Thus a clear indicator for a compound to be suitable as a protective layer is its property to avoid discoloration during steam ageing.