For the solder surface electroless tin (chem. Sn) or ISn (Immersion Tin), a 0.8 to 1.4 µm thin tin layer is deposited on the copper surfaces in a wet chemical process. According to IPC-4554, this is ≥ 0.6 µm pure tin (usable tin). The tin layer forms a planar soldering surface and thus provides the prerequisite for very good solderability of Finepitch components. After deposition, an intermetallic compound (IMC) zone is formed. When stored properly, the PCB manufacturer provides a 9-month solderability warranty. Wetting of solder pads is evaluated in accordance with J-STD-003. Lack of wetting in the soldering process can be easily detected visually and by X-ray.
Immersion tin is RoHS compliant and suitable for press-fit technology as well as special micro press-fit technology. On the other hand, this surface is unsuitable for any wire bonding processes. As a solder surface, electroless tin can be combined with an electroplated nickel/gold surface for connector contacts. Immersion tin and OSP (Organic Solderability Preservative) are the only solder surfaces that can be refreshed on an industrial scale in case of overlay or poor solderability.
Addition of silver prevents whisker formation
One possible defect with pure tin is whisker formation. Whiskers are very fine single crystals, with a diameter of about 0.3 to about 10 µm and a length of up to several millimeters, which form on the tin surface and can cause short circuits. The whisker formation of the tin can be reliably prevented by the addition of silver. As an alternative to silver, so-called organic metals are also used. Immersion Tin is also described as a self-limiting process because the deposition process of tin ions is active only as long as free copper surface is available. Short process times at around 70 °C are advantageous, with only minimal stress on the base material and the solder resist of the printed circuit board.
Tin diffusion in copper can be accelerated by temperature exposure. Therefore, the chemical tin surface is sensitive to thermal processes, e.g. the curing of lacquers applied after the soldering surface or drying processes during the processing of rigid-flex printed circuit boards. For this reason, there are special drying recommendations and limited solderability. The layer thicknesses of the tin are determined non-destructively using X-ray fluorescence analysis. The assumed density factor for X-ray fluorescence measurement is 7.3 g/cm³. A pure tin coating thickness measurement, on the other hand, is only possible with a destructive coulometric measurement. Immersion tin is significantly less expensive than immersion nickel/gold in a direct comparison of process costs. Among KSG’s customers, chemical tin is the third most commonly used solder surface.