Hi, Anyone have any clues on judging voids in solder joints for cyclic high pressure applications(0 -350 bar, 0-5000 PSI)? More specific how would a 0.1mm3 gas pocket under a D2pack on FR4 laminate with 135u copper layer affect that solder joint if environmental pressure is cycled between 0 and 350 bar?
Wow! 5,000 psi! I don't have a clue. Are you seeing failures of somethin g. I guess I'd also worry about the bulk modulus differences of the variou s materials. So calculate how much strain you get from just bulk modulus d ifferences and compare that to the strain from a little gas bubble. Which is bigger?
Assuming the void is low pressure, everything should be compression, at least. It might be okay, but WAG it's not with a lot of margin- a
0.5mm x 0.5mm void would result in the surrounding material carring ~
Since this is such a speciaized problem, if I couldn't find anyone who'd done tests, I think I'd slap together an FEA analysis in Solidworks or NASTRAN and see how it looks. The material characteristics should be available.
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No idea of the physical effect, but with such pressures and combustible materials I would also be concerned if the gas had any oxidant or reducing properties. I would expect oils and plastics to spontaneously burn under oxygen at these pressures!
To prevent voids in our compounds wrapped around electronics, we used to epoxy them in low atmospheric pressure, 10-3 torr [approx Bell Jar] where we would watch with surprise at how they foamed like crazy before settling down.
Perhaps, the soldering should be done in rarified atmosphere also, then your question would become a moot point.
On a slightly related note, I once watched a spate of failures in high power NPN transistors, TO-3 cans, used to drive high power ultrasonic cleaners. The final resolve was that the IC Mfgr had allowed an increase in voids under the die when mounting to the package - result: less thermal conductivity and massive high temp failures. Thankfully it was their quality issue and no need to change the design.
sorry to answer my own, but forgot! Is this solder ROSH????
Again, slightly related, back in 70's plastic IC's shipped from Bay Area to LA had a huge failure rate. The packages went by AIR, went through high altitude where the packages tended to equalize, then on landing the packages tended to 'suck' in the air, the smog from LA infused back in, ate up the IC's and thus large failure rate.
Lesson here? Depending on rate of compression and decompression you may have problems. Envision the void becoming higher pressure, then when decompressed outside the void now has energy to try and blow the chip off. From memory solder is pretty good under compression, but lousy under tension [breaks] and ROSH compliant solder is worst!
Hi, Thanks, god point, I must admit I have not considered the strain from the bulk modulus differences in the various involved materials. I suspect that will be a larger concern than the small voids in the solder joints. For the same reason encapsulating this in a hard Polyurethane potting material, as initially intended, and the surface mount D2pack probably is a no go to? I need some more calculations on this. Maybe a softer potting material would work better?
To save complexity, I would prefer not to go the oil and pressure compensation route on this design.. This is not a realized application yet but the circuit is intended to be used in a subsea installation rated for 350 bar.
The gas pockets or voids was discovered by an x-ray inspection and after some googling I find that this is a common issue especially for larger solder pads the gas forms from the flux used in the solder paste.
We use to vacumise the potting fluid before pouring it into the mold
Yes it does actually exist some reflow ovens that solders under vacum, mostly used for outer space applications, unfortunately can't afford one.
yes I wish those component manufacturers would provide some more available data of what kind of voids one can expect inside their potting material. It's kind of a gamble until you get your hand on a X-ray machine at least.
Nice. Thats an interesting way to measure pollution.
Good point, maybe I will have to go for a TO247 package or something instead.
I can't see it being a real issue. These pressures are routine (if at the high end) for normal industrial hydraulics, using flammable mineral oil and tanks open to atmosphere. It happens all the time that fluid gets aerated and then compressed etc.
That was sorta my 'gut feeling' but I really don't know! (Dang, I sometime s hate it when people think my advise is useful... I guess it's mostly when I don't really know what I'm talking about.) I assume there are others he re who have perhaps done some underwater circuits and will advise.
Is the potting to keep the water out?
Well this reminds me of differential thermal expansion. (I wonder how the numbers compare?) I've got this project that's been "on h old" for a while now. But I'd like to use little surface mount components at low temperatures. I've been told that they aren't reliable.. but of cou rse I want to try some testing of my own. (I haven't done it yet so I can' t say much more.) In your case, if it's possible, I might try through hole components. The l eads can flex a little and take up any strain.
So have you done underwater circuit before? Maybe there's an engineer in your group, that has made all these mistakes a lready? (So you don't have to :^)
Potting, unless gone properly will create more problems,guaranteed. And you will have to evaluate strength of materials, resilience, etc...
Oh. I mistakenly presumed you were talking of gas pockets under a SMD package (where the idea i proposed would be useful). Gas pockets from solder paste would be a big issue.
All surfaces that need soldering should then be de-oxidized by something that can be totally removed,leaving a perfectly clean surface. Then solder / paste WITHOUT organic (or other "activators") should be used. All of this in an inert atmosphere (helium in extreme) until have finished product. Makes no difference in those needs using tin/lead or RoHS tin/silver. BTW, do NOT use pissy SAC which can cause problems. For RoHS, PCBs are produced with immersion SILVER so the surface to be soldered is SILVER and NOT copper; having copper alloyed in a solder is counter-productive. Some idiot took the GOOD idea of copper alloyed tin/lead solder to create Savbit which prevented alloying copper from the soldering irons (bits) and transformed it into the BAD idea of SAC. Do not need copper: PCB surface is SILVER, solder is placed where needed in paste form,and then re-flowed. Where is the bit / beef?
In small-scale hand production, (nickel coated?) iron bits are used; NO copper.
The Artronix CAT scanner had ceramic CMOS multiplexer chips in a tank of Xenon (or was it krypton?) at several thousand PSI as the readout part of the X-ray detector. Sometimes the unit had to be opened up to replace a chip. I was amazed the metal lids didn't crumple inward when the gas was pumped in, or blow off when it was depressurized for service.
Real numbers! Phil you "da Man". Where did you get the 1% from? Is it from figure 2? I'm not sure how to read figure 2.
1000 fatigue life at delta_epsilon = 0.1... isn't that 10%, which seems like an outrageous strain... so maybe it's 0.1%? that seems like a more reasonable number.
As a point of reference (at least for moi) the thermal contraction from 300 to 77K is a few parts in 10^4 for many materials (nylon and teflon are worse.) (below 77K things don't contract much anymore.)
The Youngs modulus of FR4 is ~3E6 psi (if you can believe wiki)
Which seems to imply that 5000 psi is about the same as thermal cycling to 77K (But I'd have to go and relearn Bulk modulus, Youngs modulus, poissins ratio and all that.)