Does Flux Volume Impact Solderability?

Phil
Welcome to Board Talk. This is Jim Hall, Phil Zarrow and our guest, Joe Belmonte, all of ITM Consulting. I believe today we have a wave soldering question.

Jim
We do. It comes from Harry Jay. "I'd like to hear your thoughts on the impact or balance of usage of flux to improve solderability and its impact on post-assembly processes, like in-circuit tests where excess flux can cause contact problems."

Phil
This is like the guys at the wave solder machine pouring flux all over the whole board saying, "Wow, we get enough flux, I can solder anything."

Jim
The initial topic is you've got a solderability problem. So you'd like to have more flux. I don't think that's a good idea or a good practice. The manufacturers recommend the amount of flux that you should use and then if you put too much, you have to adjust your preheat or you may not get it all activated. You may have other problems.

Phil
Yeah, like a fire.

Jim
Certainly, in a no-clean process, you do not have that option. You can't put excess flux on or you'll have a residue that's unacceptable. Likewise if you're using a water-soluble cleaner you can choose a stronger, higher solids flux to counter solderability but, that's a slippery slope to go down.

If you have poor solderability, you should be dealing with that up front, not trying to solve it in the wave solder machine. With a water-soluble flux, you can use a stronger flux and maybe use a little more, but make sure you preheat it properly. But you should be cleaning before you go to in-circuit test.

So if I read this correctly, if you have flux reside, that means you're using a no-clean and you should use what the manufacturer recommends. Joe, do you want to talk about the best way to make sure that you're using the right amount of flux while wave soldering?

Joe
Flux is a very sophisticated product. We know it's not beautiful. We know it's not glamorous and we know it doesn't smell very good, but please respect the sophistication of fluxes because they're designed to work within certain parameters. Now every specification for flux has what Jim just mentioned; the precise amount of flux that should be applied to the product.

So the way that volume usage is checked is usually by weighing the circuit board assembly before and after. The second thing is to find out if the flux has been applied in the right place. The key here, is hole fill because the real problem with waste soldering is top-side hole fill, especially with thick, multi-layer boards that are lead-free. So what do we do? There's many sophisticated tools on the market which you can research and they're wonderful tools.

One way to get started is just to take two boards with no components and put a piece of paper in between them, like a paper sandwich. You run this board sandwich over the wave system then then take the boards apart. You examine the paper to see if you got penetration of flux up through the holes onto the paper. This will ensure that you've got enough flux in those holes to get proper topside fillets.

There are many ways to apply flux. We have spray, we still have people using a static method, such as foam fluxing and wave flux, so all those things have to be put into consideration, but the key is to get the right amount of flux and get it where you need it.

Jim
Thank you, I think that sums it up very well. So I hope we've answered your question.

Phil
So on that note, we'll get the flux outta here. This has been Phil Zarrow, Jim Hall and Joe Belmonte.

Jim
Special thanks to Joe and whatever you do, don't solder like my brothers.

Phil
Hey, don't solder like my brothers.

Excellent question and good answers by Dr. Joe Belmonte!

I coined a phrase to cover this application many years ago. I said, "Once you're wet, you can't get wetter." An even coating of a minimum amount of flux will get the job done. Any more will only drip off the assembly with a resultant potential fire, solder balls, voids etc. My 2 cents.

Your conclusion of the key is to get the right amount of flux and get it where you need it is exactly why we came up with a new technology to apply flux for wave soldering. Using a high speed jet technology you can do exactly what was mentioned in your conclusion with the additional advantages of clean machines, no smell, etc.