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December 1998

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Subject:
From:
Test Engineering <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 7 Dec 1998 12:43:47 -0500
Content-Type:
text/plain
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Frank,

As Test Engineering Manager at our PCB facility, I would like to share with
you our experiences of micro BGAs.

These minute devices are playing havoc in test arenas all over the globe.
The density of these devices are such that they physically exhaust a test
systems grid area in terms of points/Sq. in. Also in order to design a test
fixture that will perform well, the test pin diameter is of large concern.
This alone can relate to breakout in the guide plates that will most likely
cause false shorts and erratic continuity readings. We presently use an 8
plate fixture with a .016" pin. We are faced many times with the "split
net" process, where the primary fixture will test 100% for isolation and a
percentage of nets for continuity. The balance of the nets are then split
to a secondary fixture(s) to resume the test for opens. This operation has
been viable and 100% effective for us. The key is to accommodate all nets
within one cycle for isolation. For continuity, which tests the integrity
of a net, you could have as many fixtures as there are nets. We have had to
build as many as three fixtures for a part to ensure 100% test but you need
to have solid software tools to accomplish this. Also we have done "hybrid"
testing, where our flying probe system picks up the remainder instead of
building a secondary fixture.

As far as test equipment is concerned, the progression of PCB design has
far advanced what ATEs are capable of. We have 9 grid testers, one of which
is on 80 mil grid, the others are 100 mil, and one flying probe unit. The
focus for the present and near future has been on fixturing techniques. I
have seen some of the most incredible fixturing schemes that would set your
Calculus book on fire if it were near it. Many go beyond what the
properties of physics allows which makes me believe there must be life on
other planets. The fixtures that I've seen that work are mechanically
computer aided assemblies.

Bottom line is the bare board test world is in trouble and swift changes in
fixture concepts are what we are developing. Your letter on the TechNet
caught my attention and deserves more inquiries than I can give.

Blaine R. Shelkey
Test Engineering
407.752.8843 (Ph)
407.752.8868 (Fax)
[log in to unmask]

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