What Causes Blow Holes?

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting. Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes. Jim Hall A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil
Welcome to Board Talk. We're the Assembly Brothers, Phil Zarrow and Jim Hall, of ITM Consulting.

Today, we've got a question from M.J. out there in wave solder land. M.J. asks, "When wave soldering radial lead LEDs, we are seeing blow holes. We have found that loosening the clinch applied by the placement machine resolved the blow hole issue. Any idea why the tight clinch might be closing blow holes?" Well, this is interesting.

Jim
Let's get back to basics. Blow holes are caused by trapping solvents in the flux inside the hole. When they hit the wave, the solvents boil rapidly and expand. They form a bubble and blow solder out of the hole.

The traditional causes are inadequate preheat. You're not boiling off all the solvent deep in those holes or you've got too much flux. Contamination can also be an issue.

The fact that you have been able to resolve it by loosening the clinch tells me that in the initial setup, the clinch was obstructing the hole and preventing the solvent and flux from getting out of the hole during the preheat session and trapping some amount of liquid flux and solvent.

So that when you hit the wave, it exploded and gave you a blow hole. That could be the only reason if you've resolved it completely by loosening the clinch.

You obviously now have more clearance around that hole at the bottom where the principal amount of the evaporation is taking place. It's allowing that extra flux and solvent to be boiled out during preheat.

It surprises me that tight clinching would cause this problem. But I guess if the whole clearance was pretty tight when you clinched the lead over close to the board surface, it could restrict the flow of liquid and gas during the preheat process.

Phil
Let's say hypothetically it isn't totally resolved, that maybe it starts reoccurring again, particularly in those radial parts.

Jim 
One possibility would be that the radial parts are heavier than the other parts that you're inserting, and the preheat was marginal so that you were not completely heating all the other through the joints because the extra mass in these radial parts.

What's unique about the radial parts? Well, they're heavier. The leads are heavier. So maybe your preheat was on the marginal side.

Phil
You can certainly determine that by running a thermal couple on that particular part.

Jim
That's right. Put it on the top side and make sure it was at least 100 degrees C for alcohol flux or 120 degrees C for VOC free water base fluxes.

Phil
We also don't know if we're running a no clean or a water soluble. Of course, it brings up a question of the amount of flux going on, when the penetration is correct.

Jim
One of the classic things of blow holes is too much flux. Wave soldering never goes away. Wouldn't it be great if we could just retire it to its noble place and put it up in the museum?

Phil
So much for mature technology. Well, that's it for this session. And remember, whatever you do and whatever kind of flux you're using ...

Jim
Don't solder like my brother.

Phil
... and don't solder like my brother. And speaking of flux, keep the kids away from that flux pot.

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