Thickness Critical to Immersion Tin Success

As with all PCB surface finish choices, immersion tin has both positive and negative aspects. It is a flat surface that is widely available and has performed well in mixed flowing gas corrosion testing.  For these reasons this coating is an alternative to HASL coating for fine pitch circuit features.  There are some negative features that must also be considered including shelf life.  An immersion tin PCB plating uses a thin tin plating directly over the PCB copper pad.  The tin protects the copper pad from oxidation and therefore the pad remains solderable for component assembly soldering. Common industry standards specify the thickness of the tin plating has a minimum thickness of only 1.0 micron. This may have been fine for Sn/Pb soldering however a 1.0 micron coating proves marginal for an extended shelf life and a Pb-free double reflow process. The reason for this is that with time and temperature the tin and underlying copper will alloy together and form a tin-copper intermetallic compound (IMC) and in this way the pure tin plating is reduced.  This pure tin plating on top of the copper and IMC is critical to good solderability.  Studies have shown that there must be at least 0.2 to 0.3 micron of pure tin going into the second pass of a double reflow process to be successful. Therefore the initial tin plating must be thick enough so that even if some (or most) of the tin plating is alloyed in to an IMC layer there is a minimum amount of pure tin plating remaining.  The depletion of tin and the growth of the IMC occurs naturally as the PCBs sit on the shelf as well as during any soldering processes.  

For these reasons, controlling the shelf life of immersion tin PCBs is important.  Generally speaking PCB suppliers state that the shelf life of immersion tin PCBs is six months which is a short time within a high mix electronic assembly facility. Can a slightly thicker tin coating improve the situation? In a study I performed in 2009 I showed that the shelf life is highly sensitive to the plating thickness.  An increased thickness of only 1.1 microns extends the shelf life to 1 year and a plating thickness of 1.2 microns extends the shelf life to 2 years.  This being the case, one may ask why not increase the specification to 1.2 microns?  This idea has been met with resistance from the PCB suppliers since the time in the plating bath would need to be extended to achieve this. The immersion tin plating process is a substitution plating process where the tin is plated on the surface and copper is driven into the plating solution according to the chemical equation Sn2+ + Cu -> Cu2+ + Sn.  As the tin plating builds up it begins to prevent the copper from transferring into solution and therefore the plating process is self-limiting.  However it is possible to achieve a 1.2 micron plating thickness by increased time the plating bath.  Atotech, a major immersion tin supplier, actually recommends 1.2 microns of tin for success in a Pb-free double reflow process.  However the suppliers are unwilling to agree to an increased specification for fear of process variability and lot rejections. For these reasons careful attention must be given to control the shelf life of immersion tin PCBs.  

References

1.  Investigation of the Recommended Immersion Tin Thickness for Lead Free Soldering, Atotech Technical Information, January 2005