As shown in Figure 6, the solderability of a tin-lead coated surface gradually deteriorates with time, so the question becomes one of whether Stage II can be extended to three years. Accelerated tests, either directly using steam ageing, or indirectly using SERA, can be used to predict life. As with all these tests, comparisons with other parts, for which one has extended experience in the field, would be advisable.
A ‘quick and dirty’ approach to assess potential long-term solderability is to measure initial solderability – parts which are easily wettable tend to remain so – and to section the lead to determine the coating thickness and assess its porosity.
Some improvement to life expectancy may be possible by reflowing or dipping the components, depending on the type, in order to build up the solderable layer and ensure its surface has been fused. One should also take the precaution of storing the products in a dry environment at a moderate temperature.