Is it possible that you have an intermettalic growth on the end of the fine
pitch SMD pad? Is it possible that these boards have backprint (aka legend,
marking matrix, etc.)? If so, it may be a phenomenon of the baking after
HASL (assuming your supplier applies after solder mask). In my experience,
in fine pitch SMD applications with backprint, the registration of the ink
is super critical. This results in a lot of setup and perhaps misprints
requiring rework (this could also be a cause of this problem), forcing the
supplier to apply backprint after HASL to minimize ink on copper problems.
The additional thermal abuse that the SMD pads experience in order to cure
the ink applied can cause intermettalic growth. This would explain the
observances listed in your message. This intermettalic is explained by the
natural thin solder deposit resulting in the "leading edge" of the pad in
the HASL air knives. If so, the problem would be more pronounced on the
opposite ends of the SMD quad device pads, visually I would expect to see
this problem on the outside of one end of the fine pitch pads, while on the
inside end of the opposite pads of the same orientation (or vice versa). You
would not be able to see the inside problem due to the device itself. I
would also expect to see very little problem, if at all, on the other pads
that are perpendicular to the  pads exhibitig the problem.

I am curious to see if this makes sense.

Dave Rooke
__________ reply separator _______________

>Rudy,
>     Because it was confined to the fine pitch areas, there may be a 
>connection to residues but if the problem was solder mask residues, wouldn't 
>the result be "non-wetting" with some visible areas of exposed copper?
>     These boards were HASL'd by the supplier and they looked just fine. 
> After they went through our IR process, some of the fine pitch pads do not 
>stay fully wet with solder.  No exposed copper can be seen, but on some of 
>the finished solder connections ~20 - 30% of the pad surface refuses to 
>allow the new solder to flow onto it.  Generally these areas are from the 
>outside of the pads inward.  None were in the middle of the pad.
>      At this point, I'm open to suggestions.
>
>           Norm Dill
>           [log in to unmask]
> ----------
>From: TechNet-request
>To: TechNet
>Subject: Re: ASSY: Fine Pitch Dewetting?
>Date: Thursday, April 25, 1996 11:19AM
>
>Is it possible that the problem is coming from soldermask residues that the
>developer did not remove??  This would account for it not occuring in large
>pad areas, as the developer may have trouble getting into the very small
>areas.
>
>Rudy Sedlak
>RD Chemical
>
> ----------------------------- Original Post --------------------
>     Recently we have seen dewetting (~20 to 30%) that appears only on some
>of the fine pitch pads.  Larger pads with various components on the same
>boards are ok.  This has been seen on HASL boards from two different
>suppliers.  There have been no reports of problems with our regular daily
>production lots.
>     Any opinions would be welcomed.
>
>
>
D. Rooke
([log in to unmask])