Why is Solder Dross Sticking to Our PCBAs?

Phil
And welcome to Board Talk.  This is Phil Zarrow and Jim Hall, the Assembly Brothers pick and place and place.

Jim
Today's question comes from K.U. We are wave soldering using 63/37 tin lead solder. After wave we are finding solder dross particles sticking to the printed circuit board surface. We cleaned the dross from the solder bath and yet have the same problem. Do we likely have a contamination problem with the solder bath, what else could cause dross particles to stick to our PWBs?  

Phil
One thing that immediately comes to mind is how often are you cleaning the dross from your solder pot? Are you doing it like once a week, once a month, once a year, because at the very least you should be doing it once per shift.

Jim
What my brother is saying is if you don't clean the dross regularly, dross particles can build up within the bath and cause your problems even though you now start to skim to clean it. You really need to keep on top of it.

But now, in addition your second part is much more cogent.  

Phil
Question? When was the last time you had the contents of your solder pot evaluated, because as my brother will tell you, contamination will definitely have an effect on dross.  

Jim
If you get bad enough contamination in the pot, often are metallics dissolve from the different metals, and if you build up high enough concentrations of other metallics, they actually become particles in the wave and some of them don't float. 

Some of them are not lighter than the tin lead and so they will not be skimmed off with the dross but will circulate continuously in the pot. What you are describing as dross might be other metallic contaminants other than tin and lead oxides that are not floating and are re-circulating in the pot but again, you have the core of it.  You should have your pot analyzed. 

Check that the chemicals that they analyze your pot for are within the specifications because yes, they can definitely cause other particles that can stick to your board and look like dross.  

Phil
Hopefully, that stirs you in the right direction.

Whether you are wave soldering, with or without dross. 

Jim
Don't solder like my brother.  

Phil
Please don't solder like my brother.

I have seen this before and it was mostly from the lack of flux on the surface of the PCB. Flux acts as a "Lubricant" to wash away any impurities on the board.

I had the same problem and I was scarred of dedrossing the solder pot regular as the cost to replace new solder can be high. I have used an EVS solder recovery system for many years and I clean my pot everyday. I am getting back 70% solder from my dross. Once I did this this problem went away.

All comments are very good. I particularly agree with Mario regarding the negative fall on the wave flow, as we have experienced this first hand. Laminar waves with negative fall will cause exactly what you are seeing.

This can happen when you attempt to sweat dross in house. Up to 20% dross inclusion from this material has caused problems like this for several customers.

Check to see that your fluxer is working properly and that your preheats are bringing the board to proper temperature.

Might also be undercured solder mask. Fine webs of dross will stick to the porous surface. To check mask cure, get a cotton swab and some MEK. Swab the mask and if color comes off on the swab it's not properly cured.

Make sure you use High Purity Tin Phos treated solder, this will stop the dross sticking to the nozzles and coming off and attaching to your PCB once the wave comes on. Poor quality solder will always do this. It is also worth periodically cleaning out the wave former to remove this powdery dross. If you cannot get to the bottom former then increase the pump speed to dislodge loose dross in the channels from the pumps. Tin/Phos again helps this dramatically. If the dross is sticking to waxy greasy residues on the PCB and you have a doughy type excess dross in your pot then check the solder resist cure.

The problem can be related to maintenance of the pump/solder pot. The pump assembly should be removed from the solder pot at least once or twice a year depending on how many hours of run time you accumulate for the pump. The dross will build up under the pump and eventually it will start going up with the solder that is pumped to create the wave. That dross will be on the surface of the wave, so when the PCB passes over the wave it will pick up dross and solder.

The Guys are so right all none inerted waves used regularly should be de-drossed once per shift. And have a solder analysis once per quarter, this should be a Free Service from your solder supplier.

With out this information you are flying BLIND not knowing what is in your pot as the contaminates will build up with use.

I have seen this issue quite regularly on the wave solder, this is what happens: right after you dedross your pot, immediately after that you will see a layer of dross forming and that's ok, you will never be able to remove all the dross due to the exposure to oxygen, remember that is not recommended to dedross the pot so often you will gain nothing with these, it's always good to have a thin layer of dross since it will prevent more dross from forming.

Ok so back to the magic, once you set up the wave height to solder, you have to be very careful to set the height just at the point where there is no negative fall, that means there is no solder falling on the back of the former, so when the wave pallet or board passes through the wave it will touch the solder surface and push that thin layer of dross to the back. If you set it right you will avoid dross sticking to the board and will reduce the amount of solder bridges and insufficiences considerably. This is just the concept so if need more detail let me know, there are a whole bunch of things you need to be watching when setting up the wave solder process.