Being very interested in the use of nonfunctional pads in PCB designs,
from a manufacturability point of view, I concur with other
fabricators: Nonfunctional pads are undesirable Specifically in
regards to machineability in the drilling process. However, my main
concern is the increased propensity for tool breakage on small holes.
I have not found the Cu to induce additional tool wear, conversely,
that the glass substrate is more abrasive on the carbide. Also, I am
curious as to what interconnect-reliability issues are associated with
the presence of nailheading, in and of it's self.
Dan Buxton
_____
/____/\
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/________ /_| |< Continental Circuits Corp.
/ ______||_ </| 3502 E.Roeser Rd.
| / | /___| |/ Phoenix, AZ 85040
| | |___|_____/ Voice(602) 232-9133 Fax(602) 268-7386
| \/_____ /| E-Mail [log in to unmask]
\_________|/
______________________________ Reply Separator _________________________________
Subject: Re: Removal of non-functional pads
Author: [log in to unmask] at INTERNET
Date: 06/27/95 4:47 PM
The issue of the need for non-functional pads has been a matter of some
controversy for at least 20 years. AT&T originally asked designers to include
them on all layers. Studies of PTH failures following multiple solder shocks
(up to 100 or more) showed better life with non-fuctional lands. Apparently
replacing epoxy with copper reduces the Z-axis TCE and associated barrel
strain.
However, as plating technology has improved, the issue of PTH failures during
soldering has become a low level concern, and the need for reduced barrel
strain has dropped. At the same time, it was recognized that drilling through
copper generates significant heat. We have found that problems like
nail-heading, drill wear, pink ring, and defective hole walls are much worse
with non-functional lands. Consequently we have moved away from the use of
non-functional lands.
For at least 10 years the majority of our product has been made without
non-functional lands. Bellcore accepts these designs as reliable. I know of
no field problems attributed to the lack of non-functional lands. The only
problems, that ever occur are on the old designs that still have
non-functional lands. These designs occasionally cause process problems at
drill that lead to PTH quality issues.
In conclusion, I have no specific reliability studies to point to, but our
experience has shown that the omission of non-functional lands causes no
customer reliability problems and does lead to a more processable board with
better quality holes.
Bob Holmes
AT&T
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Subject: Removal of non-functional pads
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Bellcore TR-NWT-000078 in paragraph 6.2.1.2.4 recommends that
non-functional pads exist on every conductor layer. Yet all CAM systems
have a non-functional pad removal function. Some board manufacturers
use this functional automatically Most board shops, I think, would
like to use this function.
My question:
Does there exist a statistically significant study on the reliability
of boards that do not have the non-functional pads? If so, is this
study published?
Regards,
Jamie Baumgart, DSC Communications
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