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Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I'm working on a project that needs some DIY circuit boards to be extremely flat, and my stock has a bit of warp to it. I've found industry documentation saying that a 50psi press at 350F heating for a few hours would do the trick, but I do not have access to such a tool. Has anyone tried doing this in an oven with some flat metal plates?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
is there anything stiff and flat you can epoxy to the un-populated side of the board? i dont know of any relible method for flattening boards that are not presently flat enough. epoxy generally doest like to alter its shape from mere heat and coercion.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Attaching something to one side of the board is a receipe for problems, an unbalanced board will be guarranteed thermally unstable.
FR4 can be persuaded to change its shape with a bit of heat and bending. Just increase the oven temperature steadily and see what happens. You may benefit from manually bending the board back a bit too far, then letting it relax between flat plates.
However, first questions
1) how big is the board? 2) is it balanced, in terms of copper layers? 3) How flat is extremely? FR4 is considered flat if it's within a tolerance. Check with suppliers for their specs. You can't expect long term stability better than that.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
The board is 100% copper covered, single sided, so it is perfectly balanced. Size is about 4" x 5". It has several thousand holes drilled in it. I was hoping to do better than the stock tolerance, but I guess if it won't have long term stability, I'll just live with what I've got which really isn't that bad.
Can't really put a number on the flatness I need, just the flatter the better!
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
dude_500 wrote ...
The board is 100% copper covered, single sided, so it is perfectly balanced. Size is about 4" x 5".
If it's single-sided, it's as unbalanced as it can possibly be. Double-sided would be balanced. The thermal coefficient of expansion for copper and epoxy glass are different, so copper on one side and not on the other is a recipe for changing curvature with temperature.
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Flattening FR4
