Soldering surfaces for printed circuit boards, Part 3: Immersion tin

Printed circuit board surface chemical nickel-tin

For the Soldering surface chemical 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 the finepitch components. After deposition, an intermetallic connection zone (IMC) is created. With proper Storage the PCB manufacturer provides a 9-month solderability warranty. The wetting of solder pads is assessed according to J-STD-003. A lack of wetting during the soldering process can be easily detected visually and by X-ray.

Soldering surface chemical tin
Mounting of a connection pad with immersion tin surface

Immersion tin is RoHS-compliant and is suitable for press-fit technology and special micro press-fit technology. However, this surface is unsuitable for any wire bonding processes. As a soldering surface, immersion tin can be combined with an electroplated nickel/gold surface for plug contacts. Immersion tin and OSP (Organic Solderability Preservative) are the only soldering surfaces that can be refreshed on an industrial scale in the event of overlapping or poor solderability.

Addition of silver prevents whisker formation

Whisker formation is a possible defect in pure tin. Whiskers are very fine single crystals with a diameter of around 0.3 to around 10 µm and a length of up to several millimeters, which form on the tin surface and can cause short circuits. The formation of tin whiskers can be reliably prevented by the addition of silver. So-called organic metals are also used as an alternative to silver. Immersion tin is also described as a self-limiting process because the deposition process of the tin ions is only active for as long as free copper surface is available. The advantages are short process times at around 70 °C, which only minimally stress the base material and the solder resist of the printed circuit board.

Tin diffusion in copper can be accelerated by the effect of temperature. The chemical tin surface is therefore sensitive to thermal processes, e.g. the curing of lacquers that are applied after the soldering surface or drying processes when processing 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 the X-ray fluorescence measurement is 7.3 g/cm³. A pure tin layer thickness measurement, on the other hand, is only possible with a destructive coulometric measurement. In a direct comparison of process costs, immersion tin is significantly cheaper than electroless nickel/gold. Tin is the third most commonly used soldering surface for KSG customers.

3 Responses

  1. Hello Mr. Mandel,
    Thank you for your comment. The effect is known. At all those chemically tin-plated areas/positions that are not printed with solder paste and still undergo the thermal process (e.g. reflow soldering), the remaining pure tin layer thickness is reduced by the thermal diffusion of tin into copper and copper can come to the surface. This makes it look more yellowish.
    You are welcome to contact us directly if you have any further questions.

  2. How do you do?
    We are processors of your LP with chem. Sn surface and are currently experiencing the problem that some of the multiple panels tarnish yellow/gold in the reflow oven on the areas not wetted with solder paste. We do not see this phenomenon in our process, as the result with raw PCBs produces the same result and chem.Sn surfaces from other manufacturers do not exhibit these problems.
    Are you aware of this phenomenon? What do you think is the cause?
    Thank you for your comments.

    1. Hello,
      Why don't you take a look at where the DryPack is located in the packaging? There is a risk that the DryPack will contaminate the cards (outgassing despite the supposedly sulphur-free paper) and such yellowish/gold-colored phenomena may occur. It is also essential to check the solderability guarantee.

      Best regards

en_USEnglish