Can a Wave Solder Air Knife Cause Defects?

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting here today with Board Talk to answer your questions on soldering technology, surface mount technology, procedures, materials, equipment, and whatever bedazzles and amazes you.

We're in the soldering lab today because we have a wave solder question from Mr. JS in Kansas City, Kansas, he's saying, "We use a hot air knife in our SMT wave soldering process. We're seeing what appear to be deformed, unwetted solder joints as the board is moved past the air knife. Can anyone tell me what's causing this? We've adjusted the air pressure up and down but we can't seem to find a setting that keeps this from happening."

Oh, man, well this is an interesting one.

Jim
I think we have to start back and say what is an air knife because I don't think a lot of people have seen one and some people may not even know that they exist. An air knife is a tool that's an option that's added to the end of a wave soldering machine below the conveyer right after the final laminator wave that directs a focused stream of hot air or hot nitrogen on the board to remove bridges.

They can be very effective tools but they are somewhat controversial and they're not a panacea for removing the most difficult bridges, but they can be effective.

Phil
This is an interesting case because every once in a while we encounter situations where a tool or a methodology is masking a defect and in this case, it's actually working to reveal a defect, the root cause problem.

Jim
That's right. The basic principle of the air knife process is that it can blow off a bridge hanging between the joints that's only held by the cohesive forces within the molten solder.

It cannot under virtually any circumstances remove the solder that is wetted to the leads and the pads around the through holes. Those are the adhesive forces bonding the inner metallic formation and that's very strong and you can't strip those off.

So if you were seeing truly unwetted solder joints after you've gone through the air knife, there's not a problem with the air knife, the problem is that you do not have good wetting in your basic wave process.

Phil
Let's throw out a few ideas in terms of what may be affecting the wetting. Of course, there's some of the common problems, for example, you're not getting good hole fill and that could be a number of things from the wave solder settings, conveyer height, wave height, right down to actually the plating inside the holes and the finish inside the holes. We've even seen black pad effect on hole fill on some gold-plated boards.

Jim
It could be any of the basic processes for your wave. Your flux. Do you have enough flux? Your preheat. Is the flux activated or is it preheated too much and burned off.

I think you suggested a design of experiments, a DOE. Remember, wave soldering is a very sophisticated process. There's a lot going on. We're adding flux. We're adding heat in two or three different stages. We're adding solder and then we have to remove bridges. All the factors, times, temperatures, and materials have to play together so it's not a trivial process.

Don't forget the possibility of border lead contamination. Even with the best process, if you get some dirty materials going in, the solder isn't going to stick and the air knife is going to blow the solder away.

So step back. Put your hands up. Do not touch that air knife.

Go back and check out your process. Maybe turn the air knife off and run some sample boards, check the soldering on all the leads. When that's optimized, then go back and attempt to optimize the air knife pressure to remove the bridges and give you the correct shaped fillets.

Phil
That's about it for today for us, the Assembly Brothers, Phil Zarrow and Jim Hall, we bid you a good one and remember, don't solder like my brother.

Jim
Don't solder like my brother. For heaven sakes, keep those kids away from the solder pot.

A co-worker and I were just discussing solving some flux impregnation issues using an air knife after spray flux application. We are of course fighting hole-fill with OSP issues. He has seen this drive the flux further into small holes prior to pre-heat.

Just listened to discussion on the answer to "Can a Wave Solder Air Knife Cause Defects?" and I want to commend Jim and Phil on their answer to this question, they are right on, air knives do not cause defects but highlight the weakness of the wetting process.

Nice job from an old wave solder air knife process engineer.

Enjoy reading and using the techniques you cover.