We have experienced the "black pad" type failure with some nickel /
immersion gold boards. HASL can work, but it varies significantly depending
on your supplier. Our experience with OSP's have in general been pretty
good, but when there is a problem it isn't detected until a lot of parts
have been placed. The coating that I presently prefer is immersion silver.
We started testing this finish several years ago and have been using it
successfully in production for the last two years. We have produced over a
million assemblies with good results. There has been a very limited supplier
base for this finish, but several of our board suppliers are gearing up to
provide it.

Thanks,
Robert Furrow
SMT Process Engineer
Lucent Technologies
978-960-3224    [log in to unmask]

> -----Original Message-----
> From: Clive ffitch [SMTP:[log in to unmask]]
> Sent: Friday, August 11, 2000 3:56 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] QFP Pad Height Uniformity
>
> Richard,
>
> May not be an immediate solution for you, but a change to Ni/Au finish
> at minimal (if any) cost penalty could provide a better way forward. We
> spec Ni/Au in preference to HASL for anything with 25thou pitch or
> smaller for the very reason/problem you state. HASL cannot give us the
> level finish we need for fine pitch SMT.
>
> Hope helps,
>
> Clive ffitch
> MBUK
> Stevenage, England
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]
> Sent: 10 August 2000 00:55
> To: [log in to unmask]
> Cc: [log in to unmask]
> Subject: [TN] QFP Pad Height Uniformity
>
>
>                    *** WARNING ***
>
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>      Keep this in mind if you answer this message.
>
>
> Hello all,
>
> I hope I can relate what is happening here so all of you experts can
> lead me
> down a path! (Am I asking for it or what?)
>
> 25mil and 20 mil QFP's on a double sided PWB. We are seeing variances
> in the
> total height of the pad and resulting HASL finish. Specifically maybe
> 5-10
> pads out of the 84 on the QFP exhibit an increase in HASL result in
> that the
> solder is mounded on top of the copper. The rest of the pads all are
> flat in
> nature with good HASL finish observed. We are being told that the
> resultant
> solder height is not controllable. Is this true? Is it unexpected to
> see all
> 84 pads looking the same?
>
> The situation came to light at the post printing process where a visual
> inspection detected abnormal paste placement. We ended up with either
> more
> solder per pad (potential shorts) or less solder than normal (potential
> reduced reflow) per pad. When I queried TechNet some time ago about
> stencil
> aperture sizes, the bulk of the results came back that apertures were
> ordered at some percent smaller than the actual pad size. In the
> neighborhood of 5-20% reduction.
>
> If apertures are smaller than the pad size wont this add to problems of
> uneven pad heights, assuming pad heights are not very well controlled?
>
> As usual, Thanks in advance.
>
> Richard Hamilton
> Clemar Mfg. / Rain Bird
> [log in to unmask]
>
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