Simple Test for Flux Penetration

Phil
And welcome to Board Talk. You are here with Phil and Jim, the assembly brothers pick and place. And we are here to talk surface mount processes, equipment, materials, procedures, and whatever may ail you or make your day.

Jim, what's today's question.

Jim
Well Phil, it actually isn't surface mount, it is the old technology, wave soldering. It is a pretty straight-forward question and comes from J.V.

What is a good test for flux penetration through the plated hole barrels?

Phil
Good question, and I'm glad he is checking it out. So many people neglect their wave solder machine and the proper setup. It is kind of refreshing to hear. 

There are some procedures that are used. You can buy some fancy tools. But we like to do the old board sandwich.

One way of doing that is essentially you take your board, your through hole board that you are wanting to test penetration on. And you put a piece of thermal style fax paper and you sandwich that in. Then you put another like board on top of it. Then you clamp them together.

If you don't have any fax paper you can use ...

Jim
Well, technically any paper, but to really make the dots show up, if you have alcohol based flux, thermal fax paper is the best. If you are using a water-based flux litmus paper will give you a better reaction.

Any paper will work but it won't turn color, and it will be more problematic to see the dots.

Phil
You make that sandwich and you run it through your fluxer as you would normally operate it.

When it comes out you look at the paper and look at the dots. You will be able to determine if your flux has penetrated with the current settings through the holes on the board. Pretty straight-forward.

Jim
Do not run the board through full pre-heat or the wave with paper in it. You take severe risks to the well-being of your factory.

Phil
Regardless of whether you are trying to run litmus paper through your machine or whatever, when you go to solder whatever you do, don't solder like my brother.

Jim
And don't solder like my brother.

AIM has a helpful video found at: https://youtu.be/MRPRmOYORT8

I still use this method quite often when optimizing wave solder processes. It is also a great way to see if flux is penetrating into designated keep out areas when using a selective solder pallet. Anything that is getting under the pallet is at risk of remaining active as that area is shielded from the full thermal excursion required to render most no-clean type fluxes near benign.