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I've just finished to solder this little circuit on perfboard and now I realize that the board is bent:

bent perfboard

I am pretty sure that the board was flat when I purchased it. What may cause this bending ?

  • excessive heat ?
  • there are some long solder bridges in that part of the board; is it possible that the solder shrinks when getting solid/cold ?

Edit: Those boards are sold as "bakelite" boards at a local shop. I'd say they are low quality boards (eg. the solder pads tend to come off easily)

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  • \$\begingroup\$ I have no idea what to tag this. Ideas? \$\endgroup\$ – antony.trupe May 17 '11 at 13:10
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I have noticed that perfboard does all sorts of bad things: corrosion, bending, peeling, etc. It even bends when stacked neatly in a pile and is not in use.

So I think the best answer to your question is "it just does", or perhaps "it uses cheaper, less stable laminate methods, and all laminates are at risk of bending." (Plywood for example often warps when one side is exposed to air and the other is not.)

My recollection is that it is made of FR-2 (synthetic resin) and not FR-4 (bonded epoxy) like commercial circuit boards, and is not as stable a base.

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I think you actually hit the nail on the head at the end of your question:

it possible that the solder shrinks when getting solid/cold

The coefficient of thermal expansion of solder is around 20 ppm/C, meaning that when you heat it up 200 degrees C to melt it, its length increases by a factor of 200 * 20 * 10^-6 = 0.004. When it cools, it grabs onto the perfboard, and then the length decreases that much.

You could probably calculate the radius of curvature of the board, knowing the length of contraction and assuming it will bend into a circle.

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It's not very stable stuff, because of all those holes. I think it's caused by having all the components on the same side, and all the connections on the other side. I've had large four-layer PCBs warp during manufacture for similar reasons - unbalanced copper areas on the outer layers. Boards made by another supplier were OK, though.

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These boards are fiberglass and epoxy laminate.

When heated above their glass transistion temperature, the epoxy will become less rigid and more 'rubbery' resulting in warpage and permanent deformation.

Perf boards which are usually made with cheaper and thinner materials than most other PCB's will warp with heat greater than 140 deg C.

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  • \$\begingroup\$ That perfboard looks more like phenolic than fiberglass. I don't think I've ever seen any fiberglass perfboard, with the exception of proto boards that featured drilled plated through holes and were made by a full-fledged double-sided circuit-board making process. Phenolic is soft enough to allow holes to be punched rather than drilled. Such holes are too uneven to be plated, so phenolic boards are only used in applications where a single-sided board will suffice. \$\endgroup\$ – supercat May 20 '11 at 19:44
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It could also be the way you had the board clamped when you soldered the components. If the board had a bend when it was clamped it may not be able to "spring back" after the components are soldered. Not clamping the board could also cause the problem.

What type of board are you using? If you are using epoxy-paper I would switch to epoxy-glass. It should spring back a lot better. It also is less likely to crack and break. I like Vector 169P84WE. The parts I use for breadboarding are listed at the bottom of http://wiblocks.luciani.org/FAQ/faq-tools.html

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  • \$\begingroup\$ Those are likely "epoxy-paper" boards. I did not clamp the board while populating it. \$\endgroup\$ – JonathanD May 27 '10 at 20:37
  • \$\begingroup\$ They kind of look like epoxy-paper. If you can hold the board flat with a vice or clamp while you solder it may bend less when the clamp is removed. \$\endgroup\$ – jluciani May 27 '10 at 22:14

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