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August 2001

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Subject:
From:
Tony Steinke <[log in to unmask]>
Reply To:
Tony Steinke <[log in to unmask]>
Date:
Wed, 1 Aug 2001 15:49:24 -0400
Content-Type:
text/plain
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Phil,
As far as holding small dams-we have just recently qualified an LPI from
Taiyo
that we were repeatedly capable of holding .003in. solder dam.
Tony Steinke
Circuit Technologies Inc.
----- Original Message -----
From: phil bavaro <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, August 01, 2001 12:15 PM
Subject: [TN] 0.5 mm pitch components and soldermask relief


> We are just starting our development of 0.5 mm pitch BGAs and I am hearing
> conflicting reports as to the approach we should take with respect to the
> soldermask relief around the solderball pads.
>
> First of all, the 0.5mm BGA is made up of pairs of pads in a perimeter
> layout, along with a larger perimeter also made up of pairs of pads.
There
> is also a ground block in the center.  Basically it looks like two
> concentric windowframes with a large gap between the frames for routing.
>
> Historically we have always made our soldermask 1:1 with the pad size and
> then the pwb fabricators would swell the opening so that the soldermask
did
> not encroach upon the pad and make it a mask defined pad.  That works fine
> for 1.0 and 1.27 mm pitch devices because there is still enough soldermask
> left to actually stick to the board.
>
> When we reduce the pitch down to 0.5 mm, the gap between and the
> registrations swelling, basically eliminates the thicker stripe of
> soldermask down to a very thin (and poor yielding) line which may or may
> not stick.
>
> This is the exact same problem we have always had with 0.5 mm QFPs and the
> solution was always to permit the pwb fabricator to window out the  whole
> area between the fine pitch pads.
>
> So here is the question:
>
> If we open up the soldermask aperture so that it is something like .065mm
> away from the actual etched pad, doesn't that just reduce the
producibility
> of the board at the pwb house.   The reason for specifying this tight
> requirement is speculated to be related to reliability but I'm not
> convinced yet.
>
> In reality very few of the pads are strictly etch defined round pads
> because many have surface conductors coming off of them which makes the
> pads a hybrid of etch and mask defined.  The greater the relief of the
> soldermask, the more non-uniform the exposed metal pad will become.
>
> Because there is more surface to wet to, this has the effect of reducing
> solderball volume and we expect to see the diameter not the height change
> accordingly.
>
> My suggestion is to eliminate the soldermask wherever the tight pad to pad
> areas are so that for two round pads, the aperture would be a very wide
> oval shape.  This also means that the routing away from the pads has to go
> to the outside away from the open soldermasked area.
>
> The intention here is minimize the enlargement of the pads with respect to
> wettable area and at the same time make the soldermask application process
> easier.
>
> Ironically it seems that 0201 pads can also follow this logic.
> Hopefully I've explained this in detail enough for everyone to understand.
>
> What does your land patterns guidelines when it comes to soldermask for
> fine pitch QFPs, BGAs, and 0201s?
>
> All inputs are appreciated.
>
> Phillip A. Bavaro
> QUALCO/\/\/\/\  Incorporated
> Engineer, Staff
> [log in to unmask]
> Pager (858) 271-3640
> Tel  (858) 658-2542 Voice mail
> Cell (858) 845-9968 (workday)
> Cell (619) 602-8644 (offsite, after hours)
> Fax (858) 658-1584
>
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