X-ray inspection

[IconicPCB]

Interested to hear of Your experiences with X ray inspection.

Presently have an epoxy potted assembly which has failed.
Would be keen to know whether for some reason a solder joint had failed due to some movement in epoxy compound.
Would an X ray inspection be able to reveal a failed ( cracked, sheared ) solder  joint?

[WattsThat]

Certainly. But, you’ll need several views to confirm. These were potted or molded resistors, looking for failed welds. Two connections, four or two welds, depending on product.

Back in the day, we used to use Polaroid black and white film and you’d have a positive print in 60 seconds. Repeat until you had enough image views to confirm/support your hypothesis/suspicions. Just like dental x-rays, which was the last holdout for chemistry based film, it’s no doubt all digital today.

[SerieZ]

Some Assembly Factories actually have Big X-Ray Machines in them where they can Inspect PCBs for solder joint (and other issues) from different angles in real time.
I have actually used such a machine myself to inspect BGA Solder Joint quality.

[SMTech]

Its pretty much essential if you work with BGA a lot and want to be able to verify solder joints etc, maybe not on every board but at least a sample. On automotive or other critical systems you might also have requirements for minimum % voiding under large contact areas, standard process often have much lower actual inter metallic contact than you might think and even careful layout, paste choice and stencil design struggles to meet some of these standards, at that point reflow with vacuum starts to become necessary and you need X-Ray to inspect/verify it.

So lots of places now have X-ray in-house, its not as expensive as it used to be and there are also places that will do it for you.

Some folks on here I believe have scored low powered X-ray units of fleabay, while not designed for this purpose they can see through at least some electronics assemblies but whether they can get through a potted thing is another question...

I guess it depends how many units have failed and what they are worth. We pot a few things, we expect some failures after potting, typically through failed seals, maybe the manufacturing defect / test miss.

[IconicPCB]

I contacted a local assembly house who do have X-ray gear with the view to having their report on suspect area of the assembly.

Their response has been positive. I am hopeful the inspection will illuminate the failure and we can do something to minimise repeats in the future.
We did have a failure on one earlier variant of the product. The product was dissected ( literally ) using a drop saw, epoxy removed to expose a dry joint ( a very slight tombstone effect ). The resistor electrode was just touching the solder pad, sufficiently so to pass functional test but then failed open post potting and after a while on the test bench.

The product is a UV  LED water steriliser operating in conjunction with a salt pool chlorinator in-line with the output of a recirculating filter pump.
Harsh environment. salt water under pressure,  calling for good epoxy seals to prevent water ingress into the assembly.

[james_s]

I can confirm that it's possible to image potted assemblies with a low power dental head. The limiting factor for me was digitizing the image on the film, good film scanners are expensive.

When dealing with inanimate objects where movement and absorbed dose is not an issue you can take a long exposure or even multiple exposures to get the desired mAs with a much lower tube current than would typically be used for medical diagnostic or industrial xrays. Dental tubes are normally used with a very short distance between the anode and film to image a very small area but you can use a greater distance to get a correspondingly larger beam, with the inverse square law applying to the intensity of course.

[IconicPCB]

Well the imaging is done.

It is such an incredible piece of kit.

The image showed presence of a solder ball between two pins on an IC. The ball was small enough not to be a problem initially, however after a period of operation at maximum voltage ( and therefore heat dissipation ) the image shows traces of tin whiskers growing within the epoxy potting volume between the solder ball and the two pins it is adjacent to.

A  result.

The two pins are power supply to a constant current driver and the constant current pin driving an LED.

The short created by the whisker/solder ball combination shorts out the LED. Expectation is that since this is a constant current driver, the overall assembly current draw should be the same as for a functioning assembly. This was tested and confirmed to be so ( good versus bad assembly).
Then a good assembly was modified by shorting out LED pins and no change in assembly current was observed as expected.
 Whisker/solderball combo confirmed.

[james_s]

If it's just a whisker maybe you can blow it out with a higher current? Just don't exceed the voltage that the IO pins can tolerate.