Control of storage parameters can profoundly impact solderability retention of stocked components, depending on the overall thickness and quality of the component termination finish. Components with marginal solderability due to thin or porous termination finishes can rapidly degrade if stored under environmental conditions such as high humidity, high temperature, and/or with contaminating sources such as gaseous atmospheres or containers.
WS-6536 and MIL-STD-2000 require solderability testing and control of storage parameters in an attempt to ensure that components are solderable when used on the manufacturing line. Using component tinning to enhance solderability for storage is also allowed.
The following environmental control is recommended to preserve the solderability of components:
(1) The storage environment should not deviate from the following requirements for a total of 90 days within the two-year storage period. Maximum RH 50%, temperature 77ºF and storage gasses not to exceed the following limits (in micro grams per cubic meters of air):
Reducible sulphur 0.2
Sulphur Dioxide 10.0
Chloride (acidic chlorine) 0.1
These storage controls are based on one contractor's research. While proven to be effective, there is insufficient research to determine the effects of deviating from these limits.
(2) Components and PWBs, regardless of the type of finish, may be stored without retesting for solderability for up to two years if they meet the original specified solderability test requirements, including a minimum of eight hours of artificial steam aging.
7.4 Future Investigations
Recommended areas for investigation include:
(1) Effect of environment on solderable surfaces.
(2) Establishing effects and levels for storage gasses.
(3) The effect of environment on the laminate materials of PWBs.