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August 2000

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Subject:
From:
Sherry Goodell <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 11 Aug 2007 13:54:27 -0700
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text/plain (113 lines)
Hi,

You did not say what the thickness difference was that you are seeing.  By
the description of 'mounded" I will assume, that we are talking thickness in
the 1 mil (1000 microinch) range.

HAL is capable of producing Thickness' on QFP sites that are well within any
assembly uniformity (coplanarity) requirements that I am aware of for 20 and
25-mil pitch QFPs.  There is no industry standard that I am aware of on HAL
thickness and all HAL is often considered equal.  This creates many issues
when we discuss capability.

Two common specifications called out are 80-800 microinches and 100 - 1000
(or 100 - 1500) microinches.  I have been processing PWBs to the 80 - 800
specifications for 15 years and have never had a reject for solderability or
coplanarity on QFPs.  We typically hold a mean around 300 microinches with a
standard deviation of around 50 on QFP
sites.

Having said that, thickness control will be dependent on the HAL process
that they were run on. Not all HAL is equal. Vertical HAL typically has a
much higher difference in the pad height than horizontal HAL for QFP sites.
It is very important to process panels at an angle to achieve uniformity.
All HAL processes are not capable of this.  This is not needed for some
technologies, but as you get into QFP surface mount features,
it becomes critical.  A paper called " Benefits of Angle" is downloadable
in the tech library at www.huggroup.org .   A HAL Specification Guideline is
downloadable at www.tet-halco.com.

I hope this helps. If I can be of further assistance please feel free to
contact me off line for details of studies and related questions.

Sherry Goodell
Mgr. Applications Engineering
TET Halco, 8 Delta Drive, Londonderry, NH 03053
Phone: (603) 437-8653
Fax:  (603) 434-4156


----- Original Message -----
From: Hamilton, Richard CLE 4454 <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, August 09, 2000 4:52 PM
Subject: [TN] QFP Pad Height Uniformity


> 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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