potting material for high environmental pressure?

Hi, we have this subsea project where one of the challenges is to install a circuit board on the outside of a retrievable subsea
installation (20" pipe), rated for 350 bar (5000 PSI) environmental pressure, so the pressure load will be somewhat cyclic between 0 and 350 bar.
Some other equipment installed on this pipe is embedded in a hard, polyurethane (PUR) potting. So I am thinking to also embed the circuit board into this PUR potting. Since the surrounding potting material is quite hard, ShA 80, and that it is significant differences in the compressibility or bulk modulus of the PCB laminate (FR4) and the potting material, we are concerned that the compression differences between the PCB and the potting material will lead to high stress levels on the components mounted on the PCB. The components on the PCB is a few D2Packs and some 0603 resistors. The size of the PCB is 50x50 mm.
Two obvious alternatives is to put the PCB inside a 1-atm container. or submerge the PCB in some pressure compensated oil container. The reason not to choose one of these is to keep the size and complexity low.
So I am now thinking that this circuit board should first be potted in a form of softer potting material before it is embedded in the harder PUR potting surrounding the 20" pipe. So now I am wondering where to find, or what kind of soft potting material to chose for this purpose? Anny thoughts / experiences / suggestions, anyone?
Thanks..
Br, Rune
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Rune wrote:

Softer materials do have greater compressibility due to their looser molecular structure, but the difference is pretty small. I'd be way more concerned about voids in your system. Like, are there any bubbles between the packages and the circuit board? Even within a plastic-packaged device, there can be bubbles. At the least, I'd pull a vacuum on potted assemblies before they cure. Some volatiles can be expected from the potting material, and these will contaminate the chamber and pump. They are especially nasty if they polymerize on contaminated surfaces, which they probably will.
Shore A 80 polyurethane sounds like a pretty good potting material to me. I don't see any justification for going to a two-layer potting system. Problems from incompatible materials could make the cure worse than the disease. For example, polyurethane may poison the catalyst of a platinum-catalyzed addition-cure silicone. This can cause uncured silicone in contact with the cure inhibitor and the formation of hydrogen gas bubbles. Don't want that! Simpler is safer!
Worries about stress from compressibility mismatch should be addressed in the physical design, i.e. cut slots in the PCB wherever you can. Anticipate flexure and distortion of your assembly and design stress relief such as S-shaped curves in conductors.
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On 1/31/2014 5:18 PM, Mark Thorson wrote:

I think you're forgetting about the compressibility difference between the potting compound and the FR4. That will very likely lead to serious amounts of shear stress in the solder joints. Shore 80 polyurethane has roughly the stiffness of a super ball.
I agree about making sure there are no voids under the parts, though I'd be more worried about ceramic components than ICs.
Cheers
Phil Hobbs
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Dr Philip C D Hobbs
Principal Consultant
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Phil Hobbs wrote:

That wound be a reason for using a flip-chip underfill, which is designed to mitigate solder ball stress, even if this isn't flip-chip or BGA and there are no solder balls. A stiff material like a flip-chip underfill would also be protective for J-lead and gull-wing connections.
A flip-chip epoxy underfill around the packages and potting in polyurethane might be a good solution.
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On 31. jan. 2014 23:18, Mark Thorson wrote:

Thanks, I am aware of those potential compatibility issues and that a circuit could be adapted somehow to cope with the bulk modulus differences. Due to other design constraints this circuit can not be adapted this way so I am still in need of that two stage potting solution.
So far I have been suggested to try a inner potting of soft silicone which I will run a pressure test on. If anyone have any other sugestions I migth do a test on those to.
Br, Rune
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Rune wrote:

I think you might be better off going in the other direction, like a Shore D20 or D30 material. I believe Dexter offers some epoxies in that range, intended to be flowed under unpackaged chips used for flip-chip on PCB.
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On 04. feb. 2014 22:59, Mark Thorson wrote:

Maybe, if that material has a bulk modulus equal or maybe grater than that of the PCB, FR4, which is about 9GPa parallel to the PCB surface, (according to some other tread I found, so might not be very accurate).
Do you have any examples of such materials?
Br, Rune
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On 10. feb. 2014 09:34, Rune wrote:

After checking my reference fro the Bulk modulus of FR4 i found that it is probably around 18GPa, not 9 GPa as previously suggested.
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