Can We Skip Cleaning After Rework?

Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, who by day work at ITM Consulting but at night are here on Board Talk to answer your electronic assembly questions, problems, dilemmas and maybe give you some solutions.

Jim
The question today is pretty fundamental. It's from D.B. and it goes: what short-term and long-term concerns should we have with product reliability if we skip cleaning after rework? And D.B. further says, our rework process will leave organic flux residues on our circuit boards. What do you mean by "organic"?

Phil
Well first of all are we talking organic in the term of organically activate or organic acid flux? Or are we talking about organic in organic rosin fluxes? Or are we talking about free range fluxes? But we won't get into that today.

Jim
The more common way to talk about flux is, is it a no-clean flux or is it an OA water soluble flux? Is it designed for the residues to be left on the board, or is it designed to be washed off? If it is in fact an organic acid flux it needs to be removed from the board.

If you leave it, the materials are acids, they are active full-time even at room temperature, they will perhaps in the long-term and perhaps even in short-term, cause corrosion, promote electro-migration and dendritic growth and shorting, and damage. They're not designed to be left on the boards.

No-clean, on the other hand, which might include low concentrations of organic rosin are designed to be left on the board. But in rework, and this is something we deal with our customers a lot, a no clean flux is active when you put it on the board, and the only way that that residue becomes safe to leave on the board is if you properly heat it through a reflow, a wave, or in this case a rework cycle. And that means all of the flux, all of the flux volume.

So when you are reworking you have to be really careful with no clean flux to make sure that you heat all of it so that you deactivate the active chemicals and that's such that the residue is safe to leave on the board. If you don't you can get into some of the same problems that you do with an OA flux is you have active chemicals around that that corrode things, attack things, cause electro-migration and so forth.

The real issue in no-clean flux, if you're talking about organic rosin no-clean fluxes, is don't put too much of them on the area, because if you use a lot of flux during rework, it's almost guaranteed you're never going to be able to heat all of that flux sufficiently to deactivate all of it. So therefore you deactivate part of it, but you still have some of that flux hanging around that's got active chemicals that can damage the reliability of your board.

That leads us to cleaning no-cleans. And that's a whole other topic that we will leave for another day.

Phil
So you've just squandered another five minutes listening to us, but obviously had great wisdom imparted on you.

So remember: no matter what you're soldering and what kind of flux you're using --

Jim
Don't solder like my brother.

Phil
And don't solder like my brother.

Jim
And keeps the kids away from the flux pot whether it's organic or not.