Dross Contamination After Selective Soldering

Dross Contamination After Selective Soldering
We have discovered a fine low density dross on PCBs after selective soldering. The dross is difficult to see. Is there an effective way to clean this? Jim Hall and Phil Zarrow, The Assembly Brothers, answer this question while sharing their own experiences.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Authored By:

Contribution from
Bob Klenke
Business Development Manager
ACE Component Services


And welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers. Today we are coming to you from the through-hole technology lab at ITM's headquarters, high atop Mount Rialto.

We are here to talk about electronic assembly, materials, equipment, components, practices and procedures, among other things. Jim, today our question is ...

It is about selective soldering, Phil. Perhaps we should search out some superior knowledge and wisdom.

Oh my, look who just walked in. It's Bob Klenke, world-renowned guru of selective and wave soldering and all things like that. Hey Bob, how are you doing?

Well, I'm doing fine. How are you guys?

Great timing Bob. So good to see you.

Today's question is indeed about selective soldering. It is from M.K.

We have discovered an extremely fine low density dross on PCBs after selective soldering. The dross is difficult to see without careful examination. Is there an effective way to clean or remove this? Out batch washing machine is unable to remove it.

We are using SN 100 C alloy and a no-clean flux applied with drop jet. The dross dust appears in the vicinity of solder joints, but not immediately adjacent to them. It seems that the dross may be carried to the board via the nitrogen blanket surrounding the nozzle.

We de-dross the pot once at the beginning of each eight hour shift and de-dross the pump assembly once per week. What do you guys think? What do think on this one Bob?

Well, from what they are describing if it can't be cleaned off, I am tending to think that the residue they are talking about could perhaps be organic, rather than metallic.

Dross is made up of tin-oxide, which can typically be removed, but excessive flux especially if it has been over-heated is organic in nature and that is perhaps why it can't be cleaned off in their washing machine.

It is not uncommon in the field we see people tending to put an excessive amount of flux in selective. You don't need much flux at all.

People tend to over-flux. It is sort of one of the ten deadly sins of selective. The other thing is the writer was saying that they are using a no-clean, which I like to call a less-clean because these is always some residue.

The other thing is it could be excessive flux in combination with bottom-side pre-heating and not bottom and top heating. You really want to heat your flux so that you can drive the carrier vehicle, activate the solids.

You want to measure top-side board to make sure to activate your flux. You should also monitor the bottom-side board temperature for flux survivability.

Bob, do you mean you have to profile your board for selective soldering?

Well, funny you should ask Jim. When I teach select workshops I always ask a show of hands of how many people profile their reflow oven.

Every hand in the room goes up. I ask how many people profile their wave machine. Maybe about half the hands go up.

When I ask how many people profile their selective I'm lucky if I get one or two. I don't understand because soldering is about time and temperature.

What this gentlemen is describing sounds like it could be an excessive amount of no-clean flux that is staying on the board. It could be over-heated on the bottom-side and therefore it is leaving organic material.

I have seen it where people are doing crazy things in their selective solder pot that they shouldn't be, like doing manual tinning as well as soldering.

They will see a black dust that they think is dross but it is burnt flux. That's what I tend to think it is.

Yeah, when you say I can't clean it I always think of over-heating flux. That is one of the things that will inhibit cleaning.

Right, because of excessive amount I can't emphasize enough that whenever I am out in the field addressing a selective soldering process issue invariably people are putting more flux on than they really need to. They are not heating it properly or monitoring the heat properly.

Whether you have latent heating or infrared heating or convection or convection IR, you need to profile the board. You don't have to do it every single time but a first article in process development is certainly in order. That is what I am thinking here.

You're the expert Bob, so I put a lot of credence in it. I hope our listeners will also.

I do too. The excessive flux thing, we see it in through-hole soldering only because you can't really do an excessive flux thing in reflow soldering.

The old adage if you add enough flux you can solder anything, more flux will make it right. Doesn't surprise me, missing the point. Hey Bob, thanks for tripping by.

You're welcome.


You have been listening to Board Talk with Phil and Jim and your other brother, Bob. Remember, if you can't laugh at yourself, we certainly will. And whatever you do, don't solder like my brothers.

Don't solder like my brothers.

Same thing.


The other dimension to consider is the amount of non-volitile resin/rosin in the flux. This has a major effect on the measured amount of flux residue. The balance is the amount of rosin/resin in the formula to prevent low SIR masurements and/or electromigration versus the amount of activitation in the selective soldering flux.Witha no-clean, you need enough rosin to prevent electromigration.

The good news is that selective soldering uses inrt gas (N2) at the point of contact with the solder and the PTH leads/through holes that have been coated with flux. Balancing the rosin/resin content with the activity of the flux is something that Alpha has extensively studied and published on.
Mitch Holtzer, Alpha Assembly Solutions
We've run into this exact same issue on our selective soldering machine too. What M.K. describes here is exactly the same thing we do too. SN100C alloy, no-clean flux, dedross in the morning and clean the whole pump once a week.

What we found was that something, dross, flux, not sure exactly, was getting clogged in our nitrogen distribution system. This is a screen like material that distributes the nitrogen evenly around the wetted nozzle. If we take this screen out and clean it really well, the dross on the boards goes away for a while until it builds up again.

I don't think it's necessarily burned on flux. I think what Bob mentions is all absolutely true, but not necessarily related to the problem M.K. is having.

Still, I would love to get to the bottom of this. It's driving us nuts, but we're able to handle it be cleaning that nitrogen system.
Chris Denney, Worthington Assembly

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