I've never done a 4 layer board and am not familiar with the process of manufacture for them. Will the external layers isolate inner layers from water if say, I will be putting the board into water for long periods (months) of time?
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2 Answers
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No. Ordinary PCB technology does not protect against moisture ingress over the long term - that's actually a very, very difficult problem to solve where joints between different materials exist.
answered Mar 14, 2013 at 22:39
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\$\begingroup\$ Any references on that? \$\endgroup\$ Commented Mar 14, 2013 at 22:43
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\$\begingroup\$ Theory and experience. \$\endgroup\$ Commented Mar 14, 2013 at 23:08
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\$\begingroup\$ Conformal coating might make the board waterproof. I haven't done this kind of waterproofing. So, this is only a thought. \$\endgroup\$ Commented Mar 15, 2013 at 2:17
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\$\begingroup\$ @Nick -- well you're correct, so it's a good thought ;-) \$\endgroup\$ Commented Mar 15, 2013 at 2:20
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\$\begingroup\$ If you coat the entire wetted portion, perhaps. But beware of anything which pierces the coating in that area. \$\endgroup\$ Commented Mar 15, 2013 at 3:43
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The material most commonly used is FR-4 which is a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant. Because it's a woven cloth, the water can still go through the edges.
answered Mar 14, 2013 at 22:47
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\$\begingroup\$ For improved corrosion resistance, don't take the copper all the way to edge of the board, because conformal coating often doesn't cover the board edges. Tent the via's (that will cover one end), and conformal coat. This will reduce (but not eliminate) corrosion on all layers: the top layer will have the worst problems because of exposure through breaks in the conformal coating: problems with the inner layers are rare because the top layer fails first. \$\endgroup\$– davidCommented Mar 15, 2013 at 5:57