|Ask the Experts|
June 19, 2020
Wave Solder Pot Contaminated with Lead
Our wave solder pot contains lead free solder. Analysis indicates it contains 0.12% lead. The maximum allowed is 0.10% lead.
Is there anything we can do to promptly bring the lead concentration into compliance?
|Expert Panel Responses|
If you add fresh Pb free alloy of the correct amount to reduce the % of Pb to below the 0.1% this should be enough. How ever this would involve probably taking out some of the alloy already in the pot and adding fresh material.
In theory this could become a large volume, for instance if you wanted to reduce the Pb contamination to below 0.1% to give yourself a fair degree of confidence to say 0.06% you might want to remove 50% of the contents of the pot and add fresh alloy.
I suppose the big question would be where did the contamination come from in the first place. If you know and can be sure it will never happen then it is a one of cost and you can live with it.
If you are not sure then this could be a process you may have to repeat a number of times and this could become a costly procedure. Sources of contamination now tend to be counterfeit components or a genuine mistake in a factory where you are running both process.
If you are running both process try to put a line down your stores and keep PB materials on one side and Pb free on the other (can just be tapes on shelves or on the floor) anything to try to stop it from happening again.
Global Product Champion
The only way to lower the amount of lead contamination is to dump at least a portion of the pot and replace it with fresh, uncontaminated lead-free solder. Since you are only slightly over the 0.10% limit, you should be able to get by with replacing about 1/3 to 1/2 of the pot with fresh solder.
The exact amount that should be replaced is dependent on the purity level (or lead contaminant level) of the fresh solder that will be used for replacement. If your fresh bar solder source has, for example, a lead amount of 0.04%, a 1/2 pot dump would only lower your contamination level from 0.12% to 0.08%.
Check with the Certificate of Analysis from your bar solder supplier and then work through the math to see just how much of the existing contaminated solder pot will need to be scrapped.
This will help you avoid the cost of a complete pot replacement.
General Manager - Electronic Assembly Americas
Yes. You can dilute your pot to get to the right level. For instance if your pot size is 1,000 Lbs. with a reading of .12% and you add back new material at a value of .02% then for you to get to .1% you will need to decant off based on the following formula:
(New Lead Content %/100)(X) + (.12/100)(Pot volume - X) = .10% of Lead content
.02X/100 + .12(1,000)/100 - .12X/100 = 1,000(.1)/100
-10X/100 + 1.2 = 1 or -.10X/100 = -.2 or .10X = 20 or X = 200 Lbs.
So in this case you would need to decant off 200 Lbs and add back fresh solder. The equation is simple of course if there is no lead in the material you are purchasing but typically most mfg's will have a small amount of lead.
My first question: How did the lead get into the solder pot?
I'm not sure there is an easy way to correct this condition. Changing the physical solder pot is the first choice, I don't mean changing the solder, I mean physically changing the solder pot and charging it with new virgin grade lead free solder.
My second choice would be to find out how much solder is in the pot, by weight. Then figuring out how much of the existing pot has to be drained out so when it is replenished with new material it has knocked down the percentage of lead to within the limits of the regulations. Your solder manufacturer should be able to supply you with this calculation.
The third method is to drain the pot completely, flush it out with a high temperature oil, to clean out all the solder which would be attached to the inside of the pot and recharge with new lead-free material.
Cross contamination of the materials is an expensive adventure and needs to be monitored very closely to prevent it from happening.
Vice President, Technical Director
I do not know of any simple lead scavengers that you could put in the pot to lock up the lead in-situ which leaves the only options of dilution which is relatively inefficient (depending on the level of lead in your dilutant alloy you may need to replace half the pot) or replacement.
The key to the future is to determine where the lead is coming from and stop it at he source other wise any measure will be only temporary.
Senior Applications Chemist
The most efficient method to bring the solder pot back into tolerance would be to decant about 50% of the pot and replace it with lead free solder. This should work for a period of time until the lead level grows again.
Director of Marketing
Unfortunately the only thing you can do is bail out an amount of solder to bring you back under 0.1 and then refill with new.
Technical Sales Manager
BLT Circuit Services Ltd
Easy, just remove about 1/3rd of your solder and replace it with pure lead free solder.
President and Founder
The easiest would be to add new pure solder to the bath. This will reduce the percentage of lead contamination.
Regional Sales Manager
OK International Inc.
You have all of the answers you need as to howto dilute the pot with fresh lead-free solder (and isn't it amazing how many engineers don't have any math skills?), but before you jump in and do that, run a solder analysis on your lead-free bar supply already in stock. It could very well be that your bar solder (as purchased) does not meet the specification limit for lead, or is so close to the limit that any lead leached from the component leads during the soldering process is enough to bring you over the limit. Consider the following possible sources:
Do not assume your SMT component terminations are not the source, as many of the topside SMT components may be soldered to vias or are connected by short traces to through-holes, and during wave solder the topside SMT joints will go into at least a partial reflow also. Of course, the bottom side SMT component terminations must be considered whether or not they are masked by the wave solder fixture for the same reasons.
First, I completely agree that you need to address the root causes of the Pb contamination to avoid a recurrence. As far as getting your pot back in spec, if you need to do this immediately then replacing at least a part of the pot volume is the way to go. If you have capacity and can take the machine offline for a period of time, there may be another path.
It's not widely known, but Pb will stratify at the bottom of a pot that is held near the melting point and left undisturbed. What you would do is to turn down the temperature to the minimum set-point that does not produce solidification, then let the machine sit for 48 hours or more. Now, without stirring the pot at all, carefully decant perhaps 2.5% to 5% of the pot volume from the lowest point. Typically, this can be done through the drain provided by the manufacturer.
It should be done as slowly as possible. Now re-fill the decanted volume, and re-analyze the pot. The decanted volume will be much higher in Pb content, and can be sent for reclaim.
I agree that find the root causes of the Pb contamination is the first action. I do not know what your lead-free alloy solution is, but maybe you should check also if concentration of copper andiron are above of 0.30% and 0.020%, respectively. In past I was working as consultant and I was presented to a contamination case.Glayson Figueiredo, Philips Medical Systems, Brazil
In the first approach the customer told me that the issue was only about 0.50% of lead concentration.When I got access to the machine and the chemical analysis reports and I could see that copper and iron limits were also above the limits. For the iron contamination a quick examination show some machine parts were eroded...
Some parts were changed, others not, remained in the state. They don't tell me but the machine was retrofitted (a bad one) from SnPb to lead-free. For the copper, I could see that machine was dedicated to run OSP mother boards (thousands and thousands boards per week) and they never had controlled copper concentration in the bath. Also they didn't want fully replace solder in the pot. So, I did the calculations and I recommended they should remove 80% of soldering in the pot (about 560kg) putting melted solder into minor molds with capacity of 20kg each.
After solder cooling, each mold produced a contaminated solder bar with 20kg each. So, I asked to complete the machine solder pot with new solder (SN100 alloy), since I could not use SAC305 due to copper be above the limits.When the machine was completed with melted solder (about 700kg) they sent samples to external chemical analysis in the solder supplier lab. After the analytical results, all the concentrations were within the limits again (as expected) and the machine was released to restart production.
No major changes in the yields or line quality indicators were observed in during the week I was there. The contaminated solder bar were used following the rule: for each 100kg of new SAC305 solder bar disposed into the pot a 20kg contaminated solder bar could be used to feed the solder pot. As well, the chemical analysis frequency changed from once per month to once per week (the solder bar supplier did this without cost!).
Also we created a SPC for the chemical elements inside the machine to make easier take decision to abandon use of contaminated solder bar in the pot (if necessary). For my happiness they used all the contaminated solder (split in 20kg minor bars) without cause a new contamination of solder pot. In terms of root cause, I gave them some directions, but I did not take part of the investigation team.
And the best answer goes to Richard Stadem from General Dynamics. He mentioned the Pb will stratify at the bottom of a still pot held at a minimal temperature, the material with the lead concentrate will be more liquid and heavier, so use the bottom port to decant with still solder.Bruce Webster, Iridium Communications, USA
This would best be done after a minimal soak temp over a 12 hour period to obtain the concentration of lead at the lowest possible layer of stratification in the pot.
Decant about 40-50% of the pot and replace it with the same lead free solder bars. The solder decanted out from the pot can be saved and gradually add it back to the pot with the fresh solder in 1:1 ratio.
Director New Product Development
Metallic Resources, Inc
|Submit A Comment|
Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.
Insert Your Email Address