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Date: | Mon, 6 Oct 2008 18:21:07 -0700 |
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You should use belville washers in the bolt /board/regular washer assembly,
these will keep the assy from becoming loose over time/thermal cycling.
John Burke
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of - bogert
Sent: Monday, October 06, 2008 4:19 PM
To: [log in to unmask]
Subject: [TN] Seeking information on relaxation of FR4 under load over time
October 6, 2008
We have an OEM who designed a snubber printed wiring board using FR4
material with a core of heavy copper. This is a small board that is bolted
to the terminals on top of a power SCR component. The snubber board
provides for the power connection as well as includes PWB traces that
connect to some capacitors and resistors.
The board design uses a center layer of heavy copper (0.168 thick) that is
captivated between two layers of FR4 board each of which is .017 thick. The
final pad-to-pad board thickness is .235 inches.
Electrical connection is made from the the power device terminals through
plated holes to the internal heavy copper. The PTH has plated copper 40 oz
(20 once each side; starting hole diameter is 0.345 inch; finished diameter
after copper plating is 0.286 inch). The PTH pads that connect to the SCR
start as 2 oz copper that is plated up to 10 oz. The FR4 material is isola
FR406 (IPC-4101/24, 170C rated).
The power device is bolted to the snubber PWB and the bolts are torqued to a
specified value (40 inch oz recommended by the SCR manufacturer). The side
of the board that directly contacts the SCR is free of any FR4 material.
However, the backside of the board has a .017 inch layer of FR4 material
(basically this is a standard multilayer PWB that makes the resistor and
capacitor connections). This arrangement puts one layer of the FR4 material
in the torque path so that when the fastener is torqued to the specified
value, torque is applied to the FR4 epoxy.
This is the first time we have experienced this type of design. Normally
when we connect to power devices such as this, the connection is made via a
rigid boss that is brazed to internal copper layers in a laminated bus or
power-plane assembly.
My question is, is there any data available that shows how much, if any, FR4
material will relax over time under load. Our concern is will the power
connection loosen over time because of the FR4 material relaxing causing
problems. Also, has anyone used this type of desaign approach to connect to
power devices, and if so, are there any concernswe need to consider.
Any help folks can offer will be appreciated.
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Search the archives of previous posts at: http://listserv.ipc.org/archives
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