A study into the re‐processing of pure tin termination finishes into tin‐lead

Abstract

Purpose

The purpose of this paper is to present a process (chemical stripping) that can be used to remove pure tin plating from component leads and propose this activity as a mitigation strategy for tin whisker risk reduction associated with spacecraft hardware. The problems associated with the nucleation and growth of tin whiskers are stated. Present contractual requirements, having the purpose of avoiding the selection of pure tin plating, are highlighted to those not familiar with spacecraft standards.

Design/methodology/approach

The key issue here is that the process must remove pure tin from the entire length of terminations present on a wide range of component types. A resilient component can be dipped into liquid solder directly to the component's body in order to remove pure tin by alloying it with tin‐lead. However, delicate heat‐sensitive devices and those with glass‐to‐metal seals can suffer from the effect of thermal shock associated with immersion of their leads/terminations into molten solder. Solder dipping was performed on chemically stripped terminations but, in the specific case of the delicate components, the solder was prevented from wetting along the complete length of the exposed base metal or barrier layer metal. Any negative effect of the chemical stripping solution was assessed by inspection of each components' markings, their construction materials and, after tin‐dipping, a wide range of standard tests were performed on re‐processed devices in order to check that they had not degraded either physically or electrically.

Findings

The majority of components that were re‐processed by chemical stripping, and subsequently solder‐dipped, suffered no degradation from either of these processes. In order to avoid the removal of identification markings it was found that the bodies of certain components should not be completely immersed into the chemical stripping solution. Devices sensitive to heat or thermal shock could be stripped and their leads partially re‐tinned by immersion into liquid solder to a predetermined depth. A short length of exposed bare termination substrate adjacent to a seal is considered, for the purpose of this study, to be acceptable.

Originality/value

The European space industry has not previously utilized chemical stripping as a viable method for removing pure tin from components and other space hardware. The complete procedure for stripping and solder‐dipping will be of benefit to companies manufacturing products for industries that have exemption from the RoHS lead‐free directives.

Citation

Turner, I.C., Dunn, B.D. and Barnes, C. (2013), "A study into the re‐processing of pure tin termination finishes into tin‐lead", Soldering & Surface Mount Technology, Vol. 25 No. 4, pp. 218-228. https://doi.org/10.1108/SSMT-05-2013-0013