Hello Daan,
One area to look is the dull pad itself. You said it looks dull and barely
has any thickness of solder. If you can, have a x-section done to verify the
copper plated thickness. Reason being BGA pads are a
b*&%$ to develop off at image. Consequences can be adhesion promoters,
developer residues and/or anti-foam residues coating the pad and preventing
electrolytic copper plating (leaving a dull electroless copper pad). The pad
may or may not accept further platings. But the look will always be dull or
matte.
This mechanism is applicable to all final finishes. Unfortunately, I've
witnessed this personally on HASL,ENIG and Electrolytic tin finishes. At my
current employ, copper panels are removed from a hand acid copper plating
bath after 20minutes to assess complete coverage. If not, panels are re-run
through the pre-clean line. For lack of applied research, this resolves the
issue when it appears. With the help of a colleague, we found the
image/develop guys were using an excessive amount of undiluted anti-foam in
the developer (hard to dissolve effectively, and difficult to rinse).
Another area is the solder pot.
"Solder is replaced every 6 months, but except for copper contamination no
composition-check is ever done."
The composition must be done at least every week, in general tin will need
to added to maintain eutectic composition. Lab should be able to analyse for
tin or lead on AA machine.
"Throughput of the machine is about 700 panels per day."
My experience has been to skim after 350 panels to maintain copper control.
Controlling the air pressure and maintaining the optimum gap on the air
knives can be a challenge. But that is how the flatness of the solder on
each side is achieved.
I hope this helps.
Russell Burdick
Process Engineer
Prototron Circuits, Tuscon
>From: "d. terstegge" <[log in to unmask]>
>Reply-To: "TechNet E-Mail Forum." <[log in to unmask]>, "d.
>terstegge" <[log in to unmask]>
>To: [log in to unmask]
>Subject: [TN] HASL process OK or not ?
>Date: Thu, 31 Oct 2002 13:00:21 +0100
>
>Hi Technet,
>
>Yesterday I visited one of our bare board manufacturers and had a close
>look at their HASL-process. The reason for the visit was solderability
>problems on BGA-pads, which sometimes look thin and dull. Occasionally
>copper seems to be visible through the tinlead.
>In general I think this is a very professional boardshop, but (being
>not an expert) I don't know what to think about their HASL-process. I
>hope someone on this great forum is willing to give me some input on my
>findings, in terms of good or bad, or possibilities for improvement:
>
>Machine: Quicksilver SM, vertical HASL, 250 kg solderpot, no preheat
>
>Time between microetch (drying) and fluxing can be up to two hours
>
>Solder pot temperature set to 255 °C
>
>Leveling is done is such a way that the solder is as flat as possible,
>although the design of the airknives causes a big difference betweeen
>the two boardsides. With XRF we measured a thickness of only 0.4 microns
>on BGA-pads, with about 3 microns on similar pads on the other
>boardside.
>
>Water-based flux from an open tank, without any composition control,
>but tank is replaced every 24 hours.
>Solder is replaced every 6 months, but except for copper contamination
>no composition-check is ever done.
>When copper is above 0.3 % the dross is skimmed off to assure that
>copper content stays below 0.35 %.
>Throughput of the machine is about 700 panels per day.
>
>Daan Terstegge
>Thales Communications
>Unclassified mail
>Personal Website: http://www.smtinfo.net
>
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