Roger,
I agree largely with Bill and Valerie.
This is an old chestnut to-ing and fro-ing between fabricators who wish to
match to their preferred process and material supply, and design houses/end
users. For end users, the pain of having a working design change PCB
supplier which results in re-engineering to get it working again should not
be underestimated when contemplating a change of supplier.
Having worked in military, commercial telecoms & RF, I have experienced this
but in GHz designs or safety related areas (particularly air carry) it is
critical that designs proven and certified with specified materials, builds
and topologies are not tampered with. I have even experienced one design
supplied by two different fabricators which were compliant with drawing
specification and limits but one supplier's boards worked first time every
time, the other supplier's boards unfortunately, did not (it cost many $s in
post-mortem to find out why).
On less sensitive designs (a very grey area to define), I would agree with
Jeffrey that the PCB Fab House should be allowed freedom in final build
subject to technical appraisal - but not just commercial $cost criteria.
So responding to your question in that TechNet time honoured cliché of "it
depends", it really does come down to a case by case evaluation of
certification level, sensitivity of design, application, environment and
working with your supplier base to target critical designs with
Purchasing/Supply Chain Management awareness. The key is to be aware of
design options early in the process, but having certified a design, stick
with the build specified.
My humble two-bobs worth.
Regards,
Phil.
-----Original Message-----
From: Kasprzak, Bill (sys) USX [mailto:[log in to unmask]]
Sent: 13 July 2004 15:35
To: [log in to unmask]
Subject: Re: [TN] Board Stack geometry
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Roger,
I'm thinking that purchasing can't be allowed to change board houses unless
the new board house can build the board with the same stack up. I know that
in qualified military hardware this change could not be allowed because the
new board is not the same as the one that was qualified. We would have to do
a "delta" qual test that would be agreeable to all parties involved.
When changing board stackups with a qualified design, then the cost of a
"delta" qual test must be subtracted from the cost savings incurred in the
first place. Then only a true measure of savings would be realized. You'll
generally find that you'll be ahead of the game if you stick with the
original stack up.
My 2 cents worth.
Bill Kasprzak
Moog Inc., Systems Group, Process Engineer
-----Original Message-----
From: Roger Stoops [mailto:[log in to unmask]]
Sent: Tuesday, July 13, 2004 9:05 AM
To: [log in to unmask]
Subject: [TN] Board Stack geometry
This brings up a question I have had bumping around in my head for a while,
for example:
A pcb has been designed, assembled, tested, and now goes through FCC and CE
emissions and susceptibility testing. The board stack was determined by the
pcb vendor. The assembly passes first time. Purchasing or the EMS finds a
less expensive yet reliable board house and buys production boards from this
new house. The new house changes the board stack to their preferred lay-up.
Would not this board need re-testing for FCC/CE emissions/susceptibility?
For boards requiring this type of testing, would not a board stack specified
on the print be required?
TIA and much appreciated,
Roger Stoops
-----Original Message-----
From: Jeffrey Bush [mailto:[log in to unmask]]
Sent: Monday, July 12, 2004 8:40 AM
Subject: Re: 7628 prepreg?
Common type, stable, low resin content and likely predicable thickness
is this is important. Otherwise PCB house should be left to determine
the optimal stack for the board geometry and end use.
Jeffrey Bush
Director, Quality Assurance and Technical Support
76 Technology Drive - POB 1890
Brattleboro, Vermont 05302-1890
Tel. 802.257.4571.21 Fax. 802.257.0011
[log in to unmask]
http://www.vtcircuits.com
-----Original Message-----
From: Brooks,Bill [mailto:[log in to unmask]]
Sent: Friday, July 09, 2004 5:08 PM
To: [log in to unmask]
Subject: [TN] 7628 prepreg?
Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510 http://pcbwizards.com
Why would a designer specify 7628 prepreg in the construction of a
board? What characteristics are unique to it? Bill Brooks, CID
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