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Surface Finishes 3: Soldering to Immersion Silver (ImAg)

Category:
  • Voiding
  • Tin Silver
  • Soldering
  • Next on the soldering to list is soldering to immersion silver (ImAg). ImAg is probably the only solder finish that is sensitive to the profile used for reflow. Usually, the reflow profile can be changed to optimize for voiding, wetting or component limitations. But, ImAg has a track record of causing "champagne" voids.
     
    Champagne voiding is a term used to describe small voids (bubbles) that form along either the intermetallic layer or ImAg surface. These voids are known to be caused by the volatization (out gassing) of the organic co-deposit that is put down with the silver during the plating process, or the out gassing of the tarnish layer after the flux has cleaned it from the surface.
     
    On the other hand, being a metal surface, the in-circuit testing (ICT) is facilitated by the lack of a non-conductive layer (like Organic Solderability Protectant (OSP) over copper). So this means that printing of solder paste is not needed to make the contact points able to be probed. Another advantage is that the shelf life of the bare boards is longer than that of OSP copper and immersion tin (ImSn).
     
    But, there are some reflow profile tricks that can be used to get a good, solid solder joint without the champagne voids. Typically, shorter profile work better, as they prevent the tarnishing of the silver as well as artificially increase the slumping of the paste to increase the spread. The solder spread is usually reduced on ImAg, as well, so the slumping of the paste to increase the spread of the solder helps 2-fold.
     
    Take it from experience; soaking the paste to remove the voiding on ImAg boards does not work. Short, fast and cool is the way to go.