Steve,
        I was going to voice the same opinion in response to Bogdan Gabi's
question.  We measure solder paste height on every screen printer, every
hour, and even keep SPC charts on the height.  Surprise, surprise, the chart
is always good!
        Sometimes when operators set up the screen printer, they find a
particular board has too much height (because solder mask is holding up the
stencil), and I'll be danged, there is nothing we can do about it. (except
pick the magic spot on the board to measure).  They will even go so far as
to increase the squeegee down force that they start to coin the stencil.
        Type 3 paste solder balls are roughly 0.001" in dia., so a 5 mill
stencil will stack balls 5 high.  If I want to decrease paste height by half
a mil, I can't cut the solder balls in half, so I'm SOL.  I also can't press
down hard enough to compress the stencil.  If I want to increase paste
height, I can't suspend the stencil above the PCB.
        Conclusion, paste height is important for engineering purposes in
order to know what is going on, but from a production aspect, the critical
issue is paste registration and coverage, not paste height.

Ryan G.

> -----Original Message-----
> From: Stephen R. Gregory [SMTP:[log in to unmask]]
> Sent: Monday, January 17, 2000 12:48 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] Solder paste height gages
>
> In a message dated 01/17/2000 12:16:13 PM Central Standard Time,
> [log in to unmask] writes:
>
> << Technetters, who makes solder past height gages?  Thanks for any
> comments...Lou Hart >>
>
> Hi Lou!
>
> There's a bunch out there:
>
> Cyberoptics (http://www.cyberoptics.com/products/solder.htm)
> ASC International (http://www.ascinternational.com/page3.html)
> Malcom Instruments (http://malcom.org/td3.html)
> I Systems Inc. (http://www.cyberenet.net/~spimpis/ISYS/3d.html)
>
> They're all laser based big buck systems that have got more "bells and
> whistles" than you can shake a stick at. They'll all set you back AT LEAST
> $10,000.
>
> There used to be a depth measuring system called a "SMarT Measure" made by
> a
> Jim Vaugh of Screen Manufacturing Technologies (who is now known as Alpha
> Metals). It was great! Accuracy of plus or minus half-a-mil (the same
> accuracy as the big buck systems) for only $1,600...unfortunately, since
> Alpha bought out Screen Manufacturing Technologies, Jim can no longer
> market
> his system (conflict of interest or something along those lines).
>
> However, Start International (I think all of us gets mail from Start
> International) has a system that's just like Jim's.
>
> Go
> to:http://www.startinternational.com/pdf_files/microscopes_specialty.pdf
>
> It's their Depth Measuring Table Top Microscope (it's a few pages into the
> *.PDF document) Looks a lot like Jim's set-up, but they want $2,695 for
> theirs...cheaper than a laser system that's for sure!
>
> It's a shame you've got to spend so much money on solder paste height
> measuring equipment ain't it? I gotta a question to pose though. Since the
> advent of metal squeegee blades (which I use exclusively), and I've pretty
> much got down what stencil thickness I'm going to depending on the pitch
> and
> solder mask type is on the PCB, I've found that when I do measure paste
> height, it's always exactly what I expected it to be. Now that wasn't true
> back when I was using plastic squeegee blades, you could easily vary the
> height of the paste by squeegee pressure, but metal blades won't "squish"
> down into the apertures like the plastic ones did.
>
> So my question is; "Why measure something that you know will be good?"
>
> What is ya'lls opinion of my opinion? When using metal blades and a known
> stencil thickness, is there a REAL reason to measure solderpaste height?
> Shouldn't inspecting for registration, coverage, and definition be enough?
>
> -Steve Gregory-
>
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