What tin immersion -there also there is intermetallic of copper and tin? 
will tin immersion be a problem due to that? We were looking at gold, silver 
and osp coatings as alternate to HASL and tin immersion also emerged as one 
option. Will appreciate more light on tin immersion and intermetallic 
problems?thanks
 ----------
From: ddhillma
To: [log in to unmask]
Cc: [log in to unmask]
Subject: Re: GEN: Help / solderability
Date: Thursday, April 17, 1997 1:34PM


     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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