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July 2008

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Subject:
From:
"Lamote, Jan" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Lamote, Jan
Date:
Fri, 4 Jul 2008 11:11:39 +0200
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text/plain
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text/plain (53 lines)
Dear Technetters,
For a specific application we have some specific tolerances on the PCB.
The allowed tolerance on one dimension (between fiducials - 433.480) is
75um.

PCB characteristics:
approx 450mm long, 50mm wide, 1.7mm thick, 12 layers, 17u copper inner
layers, Isola IS420 material, boards are produced with longest dimension
in warp direction in PCB production panel.

When measuring the boards at the PCB fab we're perfectly within spec,
Ppk 1.95
When we received the boards from our assembly house, so after reflow
(only 1 time, leadfree profile (SAC), only components on top side) all
boards are out of spec. (mean shift from 433.502mm to 433.359mm).

Boards were again measured on the system at the PCB fab and measurements
were confirmed.
So the boards shrink with almost 0.15mm. 

Differences between first measurement and last measurement are
- boards dried before reflow
- components mounted
- 1x leadfree reflow
- depanelize
- testing of boards

Is this what should be expected ?
Anyone seen similar issues ?
Anyone have experience with similar requirements and a way to make sure
the final product is within spec ? (boards ressemble drivers for LCD
panels but have only some basic passives and some A/D converters
mounted).
Any insight the mechanism behind this phenomenon ?


Thanks in advance,
Jan.


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