Voids in solder joint under high environmental pressure

Well, that makes two of us. Somehow I was reading 0.1 as .01. That makes all the numbers go up by two orders of magnitude, to some millions of cycles if you believe they scale that far.

Generally there is some threshold strain below which fatigue failure doesn't occur no matter how many cycles you give it.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Go to Harbor Freight and get one of their hand operated hydraulic pumps. If memory serves, these produce up to 10,000 psi. A bottle of fluid & a steel block with a suitable hole would produce a test chamber.

Hul

Rune wrote:

Some solder pastes have much lower tendency to produce voids than others, you might want to search for low voiding solder pastes to minimize the problem (although I have mostly seen these advertised for lead-free; tin lead has become a low production niche product with far fewer options).

You might also consider pressure testing to a higher pressure, hydrostatic test pumps are readily available to 10,000 psi and over at reasonable prices, for example:

Short term hydrotest pump rentals are also available.

If your device is small a test chamber could be quite simple, for example a piece of heavy wall steel tubing closed with plates held by segmented rings in a groove and sealed with o-rings.

Best regards, Glen

Or down if it's a percentage.

Hmm, at the elastic limit (EL)? I'm not sure what the EL is. At a guess when you start making dislocations in the lattice. I should be able to do a Purcell, back of the envelope, calculation (Binding energy of one atom to another.. over an atomic distance?) I'll have to think about it.

George H.

I think you would only really be sure to have no voids if, after potting/soldering under vacuum, you then let the air back in whilst the solder / potting compound / whatever is still molten (liquid).

When you make a vacuum around a liquid and it froths as lots of air comes out, some of the last few bubbles don't always pop. There is very little air/gas still inside the bubbles, (depending on surface tension of the bubble and how good your vacuum is), but the bubble is still there and it still doesn't have the potting compound / solder in it. Such bubbles full of vacuum could still cause you problems later, if you let the potting compound / solder solidify whilst still under vacuum. In the case of potted high voltage assemblies, bubbles full of rough vacuum probably break down at least as easily as air does, and in the case of solder, voids full of vacuum will still allow stresses to occur when the space around your assembly is highly pressurised.

Therefore, if the vacuum soldering is to help, the vacuum would have to be released just after the peak temperature is reached and the solder has all reflowed, but before the solder has solidified.

Chris

It's my understanding that most materials are inverse, so aluminum, solder, etc. get you 10x more cycles for 10x less strain and so on (it's not a direct proportion, but probably an inverse power law or something -- I forget), and materials with a "fatigue limit" (steel, titanium, others?) are inverse exponential (sort of), so for 10x less, you get 10^10 more cycles, or something like that. Point being, it's not hyperbolic (infinite fatigue below a threshold), but being exponential, it might as well be (below a certain point, even rapid ultrasonic stresses won't do jack over the age of the universe).

I don't know how well that applies to solder, because solder, or some of the metals in it at least, are prone to creep. I don't have the numbers on tin, or the actual alloys, but lead at least is notorious for creep. Alloys need not exhibit creep because of crystal pinning and stuff (probably characteristic of the hard, high-melt, brittle, lead-free solders). In essence, because the melting point is so low, the crystal sort of anneals itself at room temperature, allowing deformation at a certain rate. An apparent example: take some solder from your spool and make it stand erect at an angle on your bench. Observe. If the solder droops down to the bench over time, that's creep. If it stays there forever (more or less), it must be the sucky kind!

I don't know how well any of this is related to things like lattice energy and coordination number and differential electronegativity (for alloys) and so on. Materials science is messy. I know a bit of metallurgy but I couldn't tell you anything that in depth.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

Sorry my bad The PCB will be encapsulated in a potting material, and placed subsea at 350 bar water pressure. I was thinking some type of soft and flexible (low shore value), maybe PUR. But I am not sure how soft and flexible PUR will be at 350 bar, any ideas?

New entry in "Not to do list": Do not put electronics inside scuba tank!...

Yes, that is a nice one. We actually had a mechanical engineer here now retired that did exactly that we used water instead of oil in the pump, not a very god long term solution I know but it worked for the test at

350Bar.

Wow! Thanks, Nice numbers and references. I am far lesser concerned about those voids now.

Cheers Rune

Assuming that the length expansion relates to bulk modulus as

dl = l * -dp / E

dl is shrinkage in any direction dp is pressure difference in Mpa E is the bulk modulus Gpa. l is the original length in that direction. assuming two materials, potting (dlP), and PCB FR4 (dlF).

then the differences in length compression would be

dlP-dlF=l*dp(1/EF - 1/EP)

Assuming PCB length 5cm and pressure difference 35MPa EF = 9Gpa EP =

2.3GPa this gives a a length compression difrence of 50*35e9*(1/9e9-1/2.3e9) = -0.57mm. I am not entierly sure about that EF an EP value, just some suggestions found on the Internet.

So the potting material must be elastic or viscous enough to take up that expansion difference of 0.57mm without transferring any significant forces on the SMD components. Any suggestions of such a material? can polyurethane rubber be used? or maybe some kind of gel like material? How to calculate this?

An other concern would be the bulk modulus difference between copper on the board and the FR4 PCB. copper has bulk modulus of 140 Gpa according to wikipedia. So the compression difference would be approx 0.2mm over the pcb length of 5cm. this does not seems so much but I would need to consider the elasticity in the bond between PCB and the copper and also the elasitcity of copper it self.

Yes.

Yes I am considering hole components but I guess the potting material will still need to be some degree of elastic or viscous.

Yes but only with encapsulation i bulky 1-atmospheric containers. And I wish I had a large group of experienced subsea enginners on my back, even one would be fine. Unfortunately its only me and my boss here "Small company" the large upside is, I get to do everything ;).

Regards, Rune

The first paper I cited shows PbSn eutectic being just about exactly quadratic, i.e. 1/10 the strain gets you 100x the cycles to failure.

The threshold for fatigue failure exists in most materials, especially steel. (See e.g. the last figure in

All solids have a range of stresses over which their creep rate is zero--that's the definition of a solid. Even paint. Some paint is a liquid, but some is a very soft solid, which helps amazingly in reducing running and drips. (Folks of a certain age will remember the Lucite Wall Paint ads from the '60s--"Like having an army of painters on the job". Lucite was the first soft-solid paint formulation AFAIK.)

Cheers

Phil Hobbs

You should dig around for information on the structure of the black boxes used in aviation. I believe they have active pingers that can go to 20k feet or more.

Good latex paints are non-Newtonian fluids. Their viscosity is inversely proportional to the pressure applied, often only in one direction. Is that what you mean by a "soft-solid". I guess I'm not seeing it because solids don't tend to "wet" things.

and coordination number and differential electronegativity (for alloys)

Grin I agree with that! My materials science knowledge is next to zilch.

George H.

A solid is certainly not a Newtonian fluid. ;)

The criterion for being a solid is that the creep rate goes to zero at nonzero stress.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

It's certainly not a fluid at all.

So solids can wet other solids? Gotta process that.

Solder on clean copper. If it doesn't wet, it doesn't stick.

Joe Gwinn

Solder doesn't wet copper when it's in the solid phase. Latex paint will wet (whatever is being painted) just sitting there. It takes work to move it around though (by design).

Sure it does, if the contacting surfaces are clean enough. But it's really slow at low temperatures, and we run out of patience.

Joe Gwinn