What Causes Blow Holes?

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.

Sorry to disagree with Phil and Jim the number one reason for outgassing and blowholing is the PCB, the plating thickness or openings in the copper wall. the amount of moisture in the board allows this to outgass during soldering by hand, wave selective or intrusive reflow see Defect of the Month video to see gassing happen live https://www.youtube.com/watch?v=xaauETZkBPs

A long time ago we tested lots of PCBs and captured the outgassing and analized the escaping gas. In addition there are other casues and cures if you take a look at this webinar on testing https://www.bobwillis.co.uk/event/pcb-outgassing-and-how-to-test-bare-or-assembled-boards/

Another thing that we have done to eliminate same problem is to change the solder mask so that it forms 3 or 4 stand offs, allowing the flux vapor to escape prior to the solder wave. Flux measuring may be required to avoid over fluxing. Some times PCBs can be cause of the problem and thicker copper plating always helps. We sometimes supply IPC class 2 for all, except class 3 for copper in plated holes which provides minimum 1 mil, 25 microns, in holes instead of minimum 20 microns. Good subject!

Two other causes of blow holes that I encountered were with boards which had plating voids in the holes and water vapour in the laminate escaped into the solder joint.

The second was poor hand soldering were a solder bridge was formed, the solder was taken away from the solder joint, and then brought back into the joint. Flux from the second application of solder was caught between two layers of solder.

Be sure your PCB is free of moisture. Bad drilling and plating from the PCB manufacturer, combined with moisture will always lead to blowholes.

We had the same blow holes problem and we deal with it by loosing the clinch. Due to this we solve the blow holes problem but some of the PCB arrive to wave soldering process with missing components. At the end we redesign the component.

How about high temp solder, such as SN5 (5% tin, 95% lead. If it does not melt in the solder wave, because it is "hi-temp" and not meant to melt, will it contaminate a lead-free pot when using it to attach a profile T/C? Or, is it just too risky? Is it "OK" if you only attach T/C on the top and none on the bottom using Hi-Temp lead lead solder?