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Date: | Fri, 29 Sep 95 13:21:02 EST |
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The other side of the coin is conductor or trace thickness
tolerance. For commercial applications the applicable criteria
can be found in IPC-RB-276 (page 13). It takes into account the
starting base foil and sets criteria for minimum finished
conductor thickness. Also plating/coating thickness of exposed
copper on non-solderable surfaces is permitted to be 1% for
class 3 (Microwave).
I understand your concerns regarding possible degraded
electrical performance. You might want to try your worst case
scenario and see what effect electrically these surface anomalies
may have. If nothing is effected and it meets the other thickness
criteria than the minor exposed copper spots should not effect
your long term reliability any more than the possibility of
any exposed copper edges along traces. (Provided the surface
finish is that type that has exposed copper along edges.)
Hadco Printed Circuits
Tech Center Two / Watsonville
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Dave Hoover
(408) 728-6677 Senior Process Engineer
(408) 728-1728 Fax [log in to unmask]
______________________________ Reply Separator _________________________________
Subject: Pitting Condition on PTFE PWB's
Author: [log in to unmask] at SMTPLINK-HADCO
Date: 9/27/95 9:59 PM
To someone familiar with PTFE Microwave Application PWB's:
We are looking at some very small pits (1-5 microinches) located
sporadically on conductor traces. The pits were obviously created by
some minute contamination on the plated copper surface which left small
voids in the tin-lead electroplate. In most cases, copper is exposed
at the bottom of the pit. The etching process may have attacked the
copper to varying degrees but we are unable to determine the depth of
these pits into the copper conductor.
Specifications allow +/- 10% conductor width variation. Nominal
conductor widths are .088" on traces that exhibit these pits. I mention
this fact as it relates to controlled impedance. The amount of copper
loss due to the pitting condition could in no way add up the amount
allowed by this specification (10% .088 = .0088 x conductor length) when
on the low side of the tolerance.
What effect will these pits have on performance and long term
reliability?
Will it create difficulties in 'tuning' the board for it's end use
application?
Will the small amount of exposed copper pose any long term reliability
problems?
In advance, Thanks
Ralph Malloch - Process Enginnering
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