Harwin - Discoloration in Pure Tin Plating Finishes

Tin discoloration is a natural phenomenon that happens to pure tin plating finishes during soldering at certain temperatures and in certain processes. This color change will not adversely affect the solder or the long-term reliability of the product – no effect on fit, form or function.

Many electroplating solutions (including Nickel and Tin) include organic chemical additives to create a high-quality bright finish as expected by the electronics industry. During normal use, organic brighteners and wetting agents break down and start to build up in the electroplating solution. These organics are very difficult to analyze so levels are unknown. Visual and functional inspection of the plated components is a good measure of the integrity of a plating solution. Functional inspection includes solderability & hotplate tests of all plated components. Before and after ageing is also carried out during normal inspection processes.

The discoloration is Tin Oxide – just 0.5% oxygen present can cause the development of oxide in molten Tin. Tests have confirmed that this surface oxidization and discoloration are related to time and temperature but, importantly, a naturally occurring phenomenon with pure Tin. This is why plating solutions include oxide resisting additives (also an organic). Extensive testing performed by Harwin has proved that the surface oxide discoloration does not affect solderability or electrical conductivity and is purely cosmetic.

Extensive investigations have shown that discoloration is not evident until soldering temperatures exceed 260°C. The discoloration is Tin oxide, which forms in the presence of oxygen, and generally presents as a straw color on the surface. This effect can be avoided above 260°C by soldering under pure Nitrogen.

See also:
Source - Harwin Tin Discoloration pdf
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