Although a more balanced copper distribution will help you still have an
unbalanced stackup which is the other half of the problem.
A small improvement can be made with your present stackup by using a
High Tg core for the FR4 and a low Tg for the prepreg, perhaps it will
be enough of an improvement on this small board that no further changes
are needed.
Rather than using the 25 mil core a pair of 10 mil cores should have
been used with the FR4 cores sandwiched between, granted, this will
throw the impedance off drastically, but it would eliminate the problem
you are seeing.
Did the fabricator stress relieve the 6010 cores prior to use? With the
pressure to reduce costs, they may not have.
They can also stress relieve after fabrication, but I have heard this
only fixes the unpopulated boards, when the boards are heated again when
soldering, the warpage returns. Don't really know if that is true or
not.
The bottom line is: balance copper + balance stackup = minimum warpage
mark
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Harter, Robert
Sent: Thursday, September 30, 2004 9:34 AM
To: [log in to unmask]
Subject: [DC] Layer Stack Up Questions
Hi All,
I have questions about a board I am working on now and how best to keep
it as flat as possible. This board is part of a surface mount RF
pre-selector module that is long and narrow and unfortunately cannot be
changed since its shape was pre-determined by the amount of area left on
our customer's PCB.
Top Core is 25mil 6010 although any of the other materials will work as
long the Dk is close to 10.2. There are currently 2 other cores which
are 5mil FR-4 using 5mils of pre-preg for bonding. All edges of the
board will have plated half vias that serve as both mechanical fastening
and signal connections. I think another problem is that from top to
bottom the copper isn't balanced. This board has both RF and mixed
signal control circuitry onboard so a majority of the copper is on the
top layer with a solid ground plane layer directly below it. The other
4 signal layers have a minimum of copper on them and are used for
routing.
We have had this board built 4 times already strictly for proto-type and
theory proving, however this go round is for customer delivery and it's
warped in both the x and y directions. The shape and size of this board
add another twist (no pun intended) to this build, its dimension are
1.24 in. x 2.70 in. The finished stack height is 51mils and I can't add
much more to that for added stiffening so I'm hoping that someone can't
help me engineer what I have better.
Thanks,
Bob Harter, Jr.
PCB Designer
Paratek Microwave, Inc.
6935 G Oakland Mills Rd.
Columbia, MD 21045
PH# (443)259-0140 x278
CP# (443)739-8407
WWW.PARATEK.COM <http://www.paratek.com/>
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