Protection from Tarnishing of Tin Layers

Post Dip 270

Harsh conditions of lead-free soldering are posing a challenge for all surface finishes not only towards the accelerated formation of an intermetallic layer but also towards the increased reactivity of the surface with the environment due to a more aggressive temperature profile. Also an immersion tin surface reacts easily with oxygen of the air causing a yellow tarnish.

When soldering is performed in a chemically inert nitrogen environment this discoloration can be avoided.
Some end users are not willing to apply nitrogen during reflow soldering due to the infliction of extra cost. On the other hand they are not ready to accept a discolouration even when there should not be an obvious impact to the wetting performance.

One way to avoid a chemical interaction of tin with the environment could be a protective coating but naturally no negative side effects towards wetting behaviour are permitted.

Tin PostDip 270 fulfils the above requirements. The tarnishing of the tin layer is suppressed by an inorganic protection layer. Tin PostDip 270 is an optional process step for Stannatech Horizon. It is positioned between the Ionix rinses and the final rinses of the immersion tin process. The post dip can easily be employed in an existing rinse tank of the Horizon line.  
Thousands square meters of boards produced by PCB manufacturers and successfully assembled at end users show that Tin PostDip 270 is an established approach to avoid yellowing of immersion tin when exposed to air during reflow soldering. It is gainining more and more consideration with OEMs.