Thanks Bob,
am sure it has nothing to do with solder mask, cross section showed that
gold had only co vered outer ends of vias, remaining was Nickel. Seems as
something clogged in the via, liquidous as it is (looks like orange blossom
honey from California)the capillary forces gave typical rounded shape some
fractions of a millimeter from each via's opening. My very question was
this: do you believe that any stuff like this could stop the flux activation
and following solder wetting? Some here have a theory, that this wet stuff
creeps under flux and, moreover, stops the 220 Celcius hot solder from
wetting, and you have a melted ball, but no electrical connection. In my
opinion, this seems  unlikely to happen. It's a complicated matter of
adhesion vs. cohesion forces, not easy to predict of course, but among the
hundreds of bright people on the net, someone may have had similar single
ball disconnection problem. To get 100% yield when BGA
soldering...yee..that's not  obtained by a single coffee break
brainstorming...not here at least. Seems as I have to dig in IBM's eminent
reports to learn more about sir Murphy's different ways of make us crazy.

Can send you a cross section view if you like, Bob.

Ingemar Hernefjord
Ericsson Microwave Systems

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: den 20 november 2000 15:00
To: [log in to unmask]; [log in to unmask]
Subject: Re: [TN] BGA Soldering Problem


Ingemar:

Consider what could reasonably be in the via holes:

1. Residual solder mask.
2. Residual nomenclature ink.
3. Misc. cleaning solutions.

1. SOLDERMASK
Presuming the solder mask is an LPI of the common type, you're looking a
polymers. Vias are the toughest part of the PWB to remove unwanted solder
mask due to their relatively small size. The "bubbling", however, bothers me

-- even if it were uncured solder mask (these vias aren't plugged on one
side
by any chance?). Time for a micro-section view.

2. NOMENCLATURE
Presuming your PWB has nomenclature, there is a solvent-based ink that may
be
in the holes as a residual. The ink could be in the vias due to original
design placement, or the PWB vendor may have had a "wash-down", whereby the
first attempt to screen the ink failed and the panels were washed in a
solvent to remove the uncured ink, but perhaps the panels were not
thoroughly
cleaned and dried (again, the vias are most difficult to cleanse).

3. CLEANING SOLUTIONS
Alcohol is a common cleaner, but again I wouldn't expect it to "bubble".
There's a whole world of specialty cleaners on the market so that I won't
even begin to examine those.

=The Big Guess=
Because you mentioned the PWB finish is gold we can discount HASL, OSP and
other sources of mischief. Because you described it as polymeric (I'm
thinking solder mask) -- and because you said there was too little of it to
analyze (I'm thinking residual), my first suspect is Residual Solder Mask
(if
the plated hole integrity is sound).

Please tell us what you ultimately discover!

Bob Lazzara
Circuit Connect, Inc.

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