Surface Finish
Chemical deposition of a thin tin layer directly onto the copper circuitry of a printed circuit board provides a flat surface for component placement and subsequent soldering. This chemical process, known as immersion tin, involves a displacement reaction that replaces surface copper atoms with tin atoms. It yields a coplanar surface finish that is highly suited for fine-pitch surface mount devices and backplane press-fit connectors.
Thickness Control
Maintaining the deposition thickness between one and one and a half microns prevents both solderability loss and excessive copper consumption. In thin coatings, the underlying copper can rapidly diffuse to the surface to form a thin oxidized layer, which hinders the solder wetting action. Conversely, excessive deposition of immersion tin requires longer bath exposure times and can lead to uneven coating thickness or aggressive copper dissolution.
Whiskering Risk
Formation of thin, conductive tin whiskers on the finished pads represents a major long-term reliability concern for tightly spaced circuit boards. This phenomenon is driven by compressive stresses within the tin layer, which are aggravated by the growth of copper-tin intermetallic compounds. To reduce the risk of short circuits, printed circuit boards using immersion tin should be assembled and soldered within the specified storage life of the board.
Proper storage in humidity-controlled environments helps limit the rate of this intermetallic growth, thereby extending the solderability window of the pads. This storage strategy ensures that the protective tin layer remains uncompromised by oxide formation until the assembly undergoes thermal reflow.
