This is the third time in my memory that the
non-functional pad debate has cropped up on the Technet. I
assume it is because of alot of new forum members. It's
beginning to feel like picking a scab.
I fall into the camp which advocates non-functional pad
removal. I also work for a board fabricator. Judging by
the tone of some of the comments, it looks like several
people are engaged in "one-sided partnerships" with their
customers/vendors. HADCO will request the removal of these
pads, but will only do so after reaching this conclusion
with the customer. We will encourage our customers to grant
a "global waiver", or place a statement in their board
specification allowing non-functional pad removal. We do
this for two reasons:
These pads may actually "float" during inner layer
lamination, ending up causing a short. As to someone's
comment to "get a better AOI system", HADCO has millions of
dollars invested in state-of-the-art AOI equipment.
Unfortunately, our equipment does not automatically remove
unwanted shorts; it can only detect them. I suppose we
could buy bigger trash barrels, or even hire an army of
short removers. We choose instead to monitor our processes,
and improve where improvement is possible.
More importantly, pads on every single layer of higher
layer count boards (8 layers and greater) cause excessive
wear on the drills, which if left unmonitored, can lead to
poorer hole wall quality. At the very least, it can add to
the expense of drilling a board.
I don't want to throw numbers at you for what savings
are available if these pads are removed. I can tell you
that drilling is the second biggest contributor to the cost
of a circuit board (about 10%, smaller only to material,
which is about 30%). Panels are stacked in multiples to a
certain stack height (typically less than .200" thick) under
each drill spindle. Our procedure calls for reducing the
stack height by one panel if the drills are drilling into
more than 20 ounces of copper/sq. ft. (about 7 ounces/sq.
ft. per panel in a typical 3 panel stack) and
non-functional pads are not removed. This will
significantly decrease drill through-put, and should be
reflected in the cost of the board. I might suggest that
board consumers submit a specific board to their vendor(s)
for quoting in both scenarios, one with a reduced drill
stack height. In that way you can get your hard numbers.
Someone referenced the Round Robin Small Hole
Reliability Study published by the IPC several years ago.
My take on this study is that it is outdated; at the very
least, a new study should be commissioned reflecting current
board chemistries, practices, and processes. Most reputable
board houses have fine-tuned and refined their copper
deposition processes to more ably handle microvias with
success. Our definition of microvias has even changed
greatly since then; 5:1 aspect ratios (board thickness to
hole size) are standard fare now that we are learning to
deal with aspect ratios in the 6:1 or even 7:1 range. They
were not at the time of the study. Please understand that I
am talking about boards in a volume environment.
Finally, let me say that we have to have more "win-win"
situations in this business. Designers and board
fabricators need to work more closely together, each working
for the other's success. Part of my job involves talking
about "Design For Manufacture" with our customers. I have
been very impressed with the hundreds of designers that I
have met with over the last year. Most of these folks are
as eager as I am to work together toward a common goal - to
produce the highest quality product at the most
cost-efficient price. We are all professionals, and need to
approach each other that way.
Best regards,
Tom Coyle
Field Services Engineer
HADCO Corporation
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