Hi Bill -
     
     I would suggest you track down what the exact pin alloy and finish 
     details - that would help diagnose the problem better. If the pin is 
     really a tin coated brass there could be a couple of factors that are 
     causing the nonsolderable condition. Brass is a mixture  of copper and 
     zinc - past studies have shown that the zinc can diffuse to the 
     surface of the alloy and interfere with the solderability (i.e. make 
     it nonsolderable). A second factor is possibly diffusion has occurred 
     with the copper and tin finish resulting in a copper/tin intermetallic 
     which is definitely going to make soldering an interesting endeavor! 
     Most fabricators of brass based pins use an underplate of copper or 
     nickel between the final tin finish and the brass to prevent diffusion 
     problems. One additional note, if a copper underplate is used the same 
     copper/tin intermetallic problem can "surface" if the tin plate isn't 
     thick enough. Good luck.
     
     Dave Hillman
     Rockwell Collins
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: GEN: Help / solderability
Author:  [log in to unmask] at ccmgw1
Date:    4/16/97 1:35 PM


Hello TechNetters,
     
I am a bare board fabricator and writing on behalf of one of our customers. 
They are having a solderability problem with one of the components, a press-fit 
pin, and are seeking any advice I can collect.
     
These pins are .028" diameter and are pushed through .062" thick PC-75 laminate 
until just .010" protrudes out the other side, and held in place by friction. 
The .010" stub of the pin is surrounded by a pad to which solder paste is 
conventionally applied and flowed. After soldering, inspection shows excellent 
wetting of the pad surface, but *no bond to the pin*. Specifics of the pin 
materials are not available at this time, but visually they appear to be tin- 
coated brass. Mechanical scraping/brushing of the pin and hand soldering fixes 
the problem, they obviously want to avoid this in production.
     
Personally, I think that the pin is being coated with resin as it is press fit 
through the hole wall (this is not a plated-through-hole board). Can anyone 
shed some light on this problem?
     
Thanks!
     
Bill Parlin
     
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