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October 2008

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Subject:
From:
John Burke <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Burke <[log in to unmask]>
Date:
Mon, 6 Oct 2008 18:21:07 -0700
Content-Type:
text/plain
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text/plain (89 lines)
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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