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Tin plating on a component lead makes its soldering easier. Everybody knows that. Not so well known is that tin plating has shelf life -- its ability to be easily soldered degrades over time. The speed and severity of degradation depends both on storage conditions and on the plating itself.
Both the component manufacturer and the user share responsibility for the shelf life of the tin plating. The manufacturer must use platings and thicknesses less prone to rapid degradation. The user must store components in an environment that does not accelerate degradation. Principally, this means avoiding high temperatures. In addition, the user should understand shelf life sufficiently so that components are used before the soldering failure becomes likely.
Soldering Basics
Soldering involves bringing together liquid tin-lead solder and the componenets being soldered. The tin or tin-lead plating provides a surface that remains solderable for a long time. At the common soldering temperatures of 250 to 260 degrees C, the platings melt and alloy with the molten solder, which in turn wets the substrate. Wetting -- akin to the way water wets wood -- is essential. A nonwettable surface, such as glass is nonwettable by water, does not solder. Oxides are nonwettable.
At temperatures below the plating's melting point, the solder dissolves the tin plating. Bond integrity depends on how well the solder and the substrate/underplating bond. The interaction of the molten solder with the copper substrate or nickel underplating creates a copper-tin or nickel-tin intermetallic.
Solder bonds depend on the solder's tin being either soluble in or capable of forming a metallic bond with the material being soldered. Solderability demands that the plating surface be clean and remain clean and wettable by the solder and flux.
Intermediate growth begins at the plating-underplate/ substrate boundary and moves outward.
Loss of Solderability
The commonly accepted definition of solderability failure of 5% of the solder surface to be soldered. Loss of solderability in tin and tin alloy plating stems from three conditions:
- Excessive thickness of the intermetallic layer
- Excessive amounts of oxides and other surface contaminants on the plating
- High levels of codeposited carbon from the organic brighteners in bright tin coatings
Each of these conditions can become worse over time; storage conditions -- temperature in particular -- can influence the rate of degradation and therefore the shelf life of the plating.
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