Immersion tin

For articles that require only a very thin film of tin, seldom exceeding 0.8 p.m immersion tin coatings are appHed. The process is based on chemical displacement by immersion in a solution of tin salts. Recently, a new autocatalytic tin-deposition process was developed at the research level. It promises to be useful to coat any base material including plastics, in addition to providing coatings of any thickness desired (21). 

Stannous Oxide Hydrate. Stannous oxide hydrate [12026-24-3] SnO H2O (sometimes erroneously called stannous hydroxide or stannous acid), mol wt 152.7, is obtained as a white amorphous crystalline product on treatment of stannous chloride solutions with alkaH. It dissolves in alkaH solutions, forming stannites. The stannite solutions, which decompose readily to alkaH-metal stannates and tin, have been used industrially for immersion tinning. 

With the growing global demand for immersion tin, the requirements placed on product and process technology are increasing significantly. Market leading OEMs require surface finishes with... 

When talking about immersion tin as a flnal surface finish for PCBs, whisker growth is a topic of concern. Ormecon has spent a significant amount of time and resources investigating the whisker formation phenomenon and its prevention. On the basis of this experience, the company has developed a whisker-reducing immersion tin product Ormecon CSN FF-W. All chemicals used are lead-free and halogen-free. 

FIGURE 1.36 Ormecon immersion tin deposit reaction (Reprinted from Arendt. N., Baron, F., Benz, V., Letterer, M., Merkle, H., Schroeder, S., and Wessling, B., OnBoard Technology, Wise Media, 30-32, With permission. Copyright 2004 OnBoard Technology.)... 

The silver-topped tin layer is relatively expensive, as it requires an additional process step and a significant amount of silver. Therefore, a new immersion tin product and process technology for whisker reduction is necessary to meet all the requirements of the market-leading OEMs and PCB manufacturers. As in Ormecon CSN and Ormecon CSN FF, the predip of the Ormecon CSN FF-W product contains... 

Sn can be found at a much greater depth in the Cu layer (up to 3 xm or more) compared to standard immersion tin products, where Sn can only be found up to about 1.5 p,m deep ... 

TABLE 1.2 Whisker Formation with Various Immersion Tin Surface Finishes... 

This new sandwich structure effectively reduces and even prevents whisker formation, as shown in Table 1.2, because less stress is induced due to the much lower diffusion of both Cu and Sn. This innovative process fulfills the requirements of several currently implemented OEM whisker-reduction specifications. The product was introduced in 2004 with some high volume PCB manufacturers. The PCB manufacturers choosing to apply Ormecon immersion tin technologies can easily change between the standard immersion tin product Ormecon CSN FF and the high-performance, whisker-reducing immersion tin product Ormecon CSN FF-W. The difference between two such products is only the predip step. 

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