Concerns with Flux During Vapor Phase Reflow

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, also members of the ITM Consulting team.

We have an interesting question today about reflow and vapor phase from B.K. B.K. says, "In a vapor phase system, is there an issue with condensing fluids, removing flux from the items to be soldered."

He's saying that vapor degreasers are very similar. Who still has vapor degreaser? You don't see those very often. Anyway, they work really well at removing anything that was soluble in the fluid. I can't think of a better expert anywhere on vapor phase than our one and only, W. James Hall, known affectionately as Jim Hall.

Jim
Actually, this question is answered almost by itself. And it says, "Removing anything that is soluble in the fluid. One of the foundation premises of vapor phase is that the fluids boil and condense on the board. This heats the board very uniformly, and rapidly, and all that good stuff, that these fluids are totally chemically inert.

That means they will not dissolve anything. Remember dissolution is basically a chemical reaction of a sort, and so, the answer is "no." These fluids will condense on the boards. They will flow over the solder paste and they will not remove any of the flux.

So there is no need to use a special flux. There have never been, to the best of my knowledge, any special fluxes that were formulated specifically for vapor phase. Perhaps, for a special temperature profiling issue, but even then, I've never known of a specific paste formulated for vapor phase.

One of the inherent advantages that you get as part of the process is this chemically inert fluid that doesn't damage or interact chemically with anything on the board.

Phil
Very good, that sums up. I notice you always get excited when we talk about vapor phase. It's like a blast from the past.

Jim
I probably should say, with respect for people who are considering vapor phase, that as the flux residue comes to the surface, some of it will be physically washed off into the vapor phase sump.

So in any vapor phase system, you need to have some sort of filtration. Any good vapor phase system should have a filtration system to remove this flux that is physically washed into the liquid sump. This flux removal is happening during the warm-up process, and it will not in any way, negatively impact the wetting.

Phil
Well, thanks, Jim, for enlightening us.

Jim
This is 30-year old technology that's come back. Everything that goes around, comes around.

Phil
We are the Assembly Brothers, and whatever you do –

Jim
Don't solder like my brother.

Phil
And whatever methodology you use, don't solder like my brother.

Flux density is lighter than the Galden, so any melted flux, in theory, would float to the surface. It's also difficult for flux volatiles to evaporate in the Dense Galden vapour. The vapour phase does have drawbacks.

We tend to see little or no loss of flux in vapor phase reflow. This may depend on who's system you are using and how preheat is accomplished.