Thank-you Ingemar,
My suspicions is that far to many variables on this problem are
undocumented.  The actual force required to pull the BGA off the board,
any pattern to the non-wetted pads, the dewetted pads and the lands pulled
of the laminate, the effect of a second reflow for the back-side BGAs, how
quick the switch-over from the HASL boards, which one of 6 SMT lines did
the defects show up on, no DOE experimentation on either the creation of
the 'Backward RoHS soldering process or looking for the factors on the
defect causes (the defects are at a 2% to 5% fallout, 95+ % are good
boards), how much handling while doing the 'cut & jumpers' on the bare
boards before assembly, how much 'optimization' to the paste and reflow
profile to handle a SAC305 balled FP BGA using tin-lead paste on an ENIG
board that has been reworked.
If I lined all the experts up, I would get a multitude of opinions.
Happy Holden




Hfjord <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
08/02/2006 03:42 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Hfjord <[log in to unmask]>


To
[log in to unmask]
cc

Subject
Re: [TN] SAC-BGAs on ENIG Using SnPb solder paste






Understand you are not happy, despite your forename.
Agree with other guys, not easy to help via email only. However, I just
wonder if your subcontractor is giving you all facts. I suspect (may be
wrong, of course) that they had a primary problem with the wetting, which
is
not so unusual. What to do then? Run the boards a second time, maybe in
combination with increased temperature to get better activation and
wetting.
The result may be better wetting, but also defective Cu-epoxy joints and
unexpected IMCs, as was mentioned by other Technetters. I have learnt
after
many years in the game, not to rely  too much on information given by
subcontractors. Not that people mislead on purpose, but rather lack of
disciplin and care. As your situation is also a economical disaster, I
would
doublecheck too see if there is some hidden facts before spending days and
nights with sample examinations.

Ingemar Hernefjord
Ericsson Microwave Systems



----- Original Message -----
From: "Happy Holden" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, July 31, 2006 8:18 PM
Subject: [TN] SAC-BGAs on ENIG Using SnPb solder paste


> Greetings Assembly Experts,
> This is the first time I have asked a question of this robust group of
> technologists, but I am way beyond my knowledge of what can happen.
>
> We have many thousands of ENIG multilayers (8 and 10 layer-0.062") that
an
> assembler has returned as "Contaminated or Defective BGA pads".  These
> boards are smaller (3.0" x 4.25") but they have 3 to 5 fine-pitch BGAs
> (0.8mm pitch with 0.3 mm lands) on the Primary side and some have an
> additional FP BGA on the Secondary side.
> The problem is that after reflow, if the BGAs are pulled, they come
right
> off the board.  Some BGA pads are still gold, some pads come off from
the
> laminate, some solder joints just break.  They returned the boards
because
> they said that the boards exhibited "non-wetting and de-wetting".
> We have finally collected all the facts that we can about the assembly
> process, and this is where I don't know what to expect, since the
assembly
> process seems to be a "Hybrid-RoHS Backward Compatible" assembly.  Here
is
> what we know about the assembly process:
>


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