Actually, the CTE of copper and CTE of FR4 are closely matched, this is by
design; with Copper about 15ppm/?C and FR4 about 13ppm/?C on the x & y axis.
The Z axis is another matter at 170ppm/?C or more. These are fairly linear
up to Tg. Then the epoxy laminate begins to expand at a much greater rate.
Copper stays linear as soldering temp is far from its melting point.

I would expect the side of the board contacting the wave should expand more
than the top, perhaps a lot more, as that temperature excursion is higher
than Tg of the laminate. Does this match your experience?

You may be able to increase topside preheat, reduce bottom side preheat,
reduce solder pot temp. and reduce warp. I have lowered our solder pot from
500?F to 475?F and improved soldering process performance (less dross,
better fluxing action). We did need to change preheat settings and belt
speed on some heavy boards.

If this is not your experience, here is another variable. The specific heat
of FR4 and copper are very different. So, heat transfer into the board is
influenced by the placement of the copper relative to the heat source. The
response can be counter intuitive, until you include this factor.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Blair K. Hogg
Sent: Thursday, December 15, 2005 9:07 AM
To: [log in to unmask]
Subject: [TN] PCB warping due to copper imbalance


We have a motherboard type PCB assembly that constantly warps during the
wave soldering process. From what I've learned on this forum, I believe it
is due to a copper imbalance from the top layer to the bottom layer - the
PCB is a 2 layer assembly, with traces on the bottom and ground planes on
the top.

My question is this - is it typical that the warping will occur towards the
side with more copper (the side with more copper will be bowed inward)?

Thanks,

Blair

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