Sherry,
Thanks for the reply and the insight.
In answer to your question, we are seeing a total thickness of up to .006"
at the peak of the mound. The good pads exhibit flat or level surfaces,
where the 'mound' is peaking in the middle length wise, and in most cases is
creating a dog-bone shape to the length of the mound where the center of the
pad has a smaller mound area than either ends.
I understand that the process that is being used is a vertical process and
not using an angled approach. From your feedback I would gather that this is
generally the worst way to perform HASL and get uniform solder placement.
Which means there are 'opportunities' here for alternate methods to help get
uniformity!
Thanks again, I appreciate it.
Richard Hamilton
Clemar Mfg. / Rain Bird
[log in to unmask]
> -----Original Message-----
> From: Sherry Goodell [SMTP:[log in to unmask]]
> Sent: Saturday, August 11, 2007 1:54 PM
> To: TechNet E-Mail Forum.; Hamilton, Richard CLE 4454
> Subject: Re: [TN] QFP Pad Height Uniformity
>
> 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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