I am the Process Engineer at a small contract electronics firm and I could use some help.
I have been asked to find out what kinds of problems might arise from washing lead free boards in our wash line that is used every day to wash products containing tin lead solder? Is this recommended or approved as a safe practice?
Our wash line has solder visible in the bottom of the pre-wash tank at all times it is setup in the cascade mode this allows about 3 gallons per min to go to drain after being filtered.
What do you think the chances of depositing lead on our lead free product might be? I have heard differing opinions on the subject and I am concerned about cross contamination.
I think in practice it is extremely unlikely that there could be any lead contamination that would violate RoHS in using a cleaning process for both leaded and lead-free product. Solder is so heavy it sinks as you observe. Any wash residues will be low enough to pass SIR testing, so there could not really be any significant lead salt residues. The lack of lead could easily be demonstrated by using an X-ray hand scanner.
Having said this, many reasonable people would say that in principle you should never mix leaded and lead-free processes, no matter what argument is used to defend it. In some cases identical products, one leaded and one lead-free might be cleaned successfully on the same machine (i.e. no lead residue on the lead free PCBs) and then mistakenly switched before being shipped!
So, it would not surprise me if someone auditing a facility for lead-free production would want to see completely separate lines.
Note: The image at the above right is courtesy of Speedline, it is one of their Aquastorm cleaners.
All the best,