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Date: | Tue, 22 Oct 96 01:00:26 EST |
Content-Type: | text/plain |
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Gregg:
The simple answer to your question is that the holes
will be within .0085" DTP, or +/-.003" linearly from the
intended location. This is on finished product. Bringing
in the hole-to-pad issue is another animal. The
relationship of the hole to its pad involves multiple
disciplines:
drilling (hole location)
imaging (artwork location)
etching (pad size reduction)
material movement
tooling system
Since we can't assume that everything wrong will
happen to the same hole and pad, we use something akin to
the "root mean square" to calculate worst case scenarios.
In order to make 100% of your product fit our manufacturing
window in volume, the following formula is used at HADCO
to calculate the minimum pad size needed to sustain a
customer's annular ring requirements:
Drill Size* + Mfg. Allowance# + (2 X annular ring req'd)
* Drill size is nominal finished hole size + .005"
# Allowance for inner layers = .014"; outer layers = .012"
The mfg. allowance captures the multiple disciplines listed
above. For example:
.039" finished hole diameter; .001" annular ring
INNER LAYER: .044" + .014" + .002" = .060" min. pad size
OUTER LAYER: .044" + .012" + .002" = .058" min. pad size
Using these formulae, 100% of your boards will meet your
requirements in our volume environment. Not every hole will
be located in exactly the same spot on every pad. That is
why we need a manufacturing allowance.
Regards,
Tom Coyle
Field Services Engineer
HADCO Corporation
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