Hello Peter,

 

I have not looked at the suspect assemblies. However, this defect can
occur when the Nickel under the gold is oxidized prior to the TH
component assembly process. The electroless nickel is not pure and it
contains 10 to 14 percent phosphorus by weight. The nichel-phosphorus is
a good diffusion barrier to avoid corrosion of the solderable surface.
If the phosphor content is low i.e. 3 to 5% by weight then the
solderable surface will oxidize by each reflow heat application. The
oxidize nickel will then prevent soldering to the surface. The link
below is a good read on the ENIG surface finish.

 

http://www.pfonline.com/articles/129901.html

 

The assemblies should be reworkable with aggressive water soluble
fluxes, some mechanical action and a suitable soldering iron tip
temperature.

 

Hope this helps,

Paymon

 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Peter Wong
Sent: March 21, 2007 4:18 PM
To: [log in to unmask]
Subject: [TN] Solderability problems on through holes only.

 

Hi Technetters,

 

 

 

We are having a solderability problem confined to only through hole

locations where we need some help or advice.  

 

 

 

We are soldering a no-clean SAC305 alloy to an ENIG finished board.  All

SMT solder joints (both sides) are fine.  During hand soldering in our

dedicated lead free area, we are having problems with solder wetting to

some of the annular rings and barrels of through hole locations, not all

holes.  The assembly is an 0.062" thick 4 layer 1"x1" board so it is not

thermally challenging.

 

 

 

We've built thousands of these in the past without this particular

problem.

 

 

 

The time lag between smt and through hole assembly is 1-2 days.

 

 

 

We've tried using a more aggressive flux such as OA flux with no

improvement.

 

The only way that we can get solder to wet is to mechanically scrape the

annular rings with a pick.  

 

 

 

Has anyone seen this before?  Why would this only occur on some of the

through hole locations and not on smt pads?

 

 

 

We have isolated this to a specific lot code of boards and are waiting

for some advice from our board vendor as well.

 

 

 

Thanks

 

Peter

 

 

 

 

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