|Ask the Experts|
January 20, 2020
Dross Particles Sticking to PCBs
We are wave soldering using 63/37 tin-lead solder. After wave, we are finding solder dross particles sticking the the PCB surface. We clean 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 PCBs?
|Expert Panel Responses|
Dross sticking to wave soldered PCB surface may be an indication that the time-temperature profile is at fault or the flux volume dispensed is wrong or both. Wave solder flux should first dry up substantially during the pre-heat process, cleaning and protecting solderable surfaces. Next the flux should "burn off" when exposed to the solder wave.
If it doesn't it will become sticky and entrap not just dross but also solder fines. Take a look at the flux manufacturer's soldering profile recommendation and use that as the starting point for optimizing the soldering process. Flux manufacturer's generally include a recommendation for dispensed flux volume per unit area.
This should be a dry measure. Spraying on to microscope slides, gently drying and using a microbalance is a good methodology. Don't use the Fax paper or pH paper to assess flux volume.
Although these are used widely throughout our industry, these indicator papers should be used only qualitatively for assessing dispense sprayed flux pattern showing whether coverage is even and consistent.
Please note that I am assuming a spray flux process is in place as this is the most common flux dispense methodology today. Remember when a component lead is inserted in a through-hole, the flux spray will never make it to the top side of the board.
After all, the space between the pin and hole is miniscule and flux migration will be by capillary action rather than aerosol transport. Bottom line: 1) ensure wave profile matches with flux manufacturer's recommendation and 2) ensure that flux volume per unit area is in accordance with flux manufacturer's guideline.
Typically if you are getting dross or webbing you are using to little flux. The flux acts a lubricant between the board and solder that minimizes the dross to stick.
In cases where the flux is sufficient then typically the soldermask is undercured and highly porous and holding more of the dross surface that forms at the top of the wave. I would suggest you investigate the use of MS2 as a solderpot additive (PK Metal) this would minimize the dross in the pot.
President/Senior Technical Consultant
If the pot surface has been cleaned, then the dross particles are coming from immersed parts. Dross often gets entrained in the solder flow that is returning to the pump, and can become trapped in the system.
This dross can later be released and will be expelled through the nozzle. If it comes in contact with the board, it will often stick, giving you the symptoms that you are experiencing.
The answer to this situation is to have a good periodic maintenance program which involves pulling the pump and nozzles and cleaning the immersed parts.
Dross appearing on the board can be due to several process issues. The impingement of the board has to push away any small amount of dross that forms as solder is oxidized at the surface of the wave. Checking the back plate at the wave to insure this happens is important.
The other is to insure the board is immersed sufficiently into the wave solder to allow this skimming off of the dross to happen.Also in some cases dross forms around the impeller and is slowly released therefore checking the impeller and other areas of dross inclusion is important. Other causes can be hot spots in the solder pot creating areas of dross potential.
Another potential cause is solder contamination check to see if the solder is free of zinc,cadmium and high copper.
Senior Market Development Engineer
I agree with some of the comments, but from my perspective, the solder level in the wave is too low. The height of the solder in the wave should be checked as the wave is being pumped not in its static state.
A dry wave will create dross and this dross will be pulled down to the pump through cavitation of the pump shaft. Keep the solder level high and watch for cavitation at the pump shaft and the dross should be eliminated.
Vice President, Technical Director
It sounds like you have Undercured Solder Resist causing the surface of the resist to become tacky and not allowing the solder to drain off properly. This also puts Plasticizers back into your solder and causes even more dross so its a viscous circle.
The solder will have a brown Tar form on the metal work where the solder returns back into the pot this will be liquid when in the pot but once removed go crumbly and the solder will be very spongy and silver. Worth doing a Solvent wipe test on the resist to ascertain its level of cure.
Technical Sales Manager
BLT Circuit Services Ltd
If the problem is not caused by flux, wave height, flux and dross entrapped in the sump, pump, or impellers, and all of the other things the experts have mentioned, then check your alloy content.
Solder should be sampled as per J-STD-001, and if you find your tin level has fallen out of tolerance, below 62% or less, it will cause the solder to form strings on the bottom side of the PWB.
The suggestions for draining the pot and cleaning the solder delivery system once or twice per year, etc, are all a good, but also watch your alloy samples to make sure they are within the required ranges.
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