How do you ready a solder pot for lead-free solder?
It is important to first insure the solder is lead-free solder compatible. High tin alloys tend to leach iron causing dissolution of iron and solder contamination. The dissolution can advance to the point of causing micro-cracks and thinning of the walls, eventually resulting in a solder spill.
Materials that are lead-free solder compatible are:
• Titanium
• Cast iron
• Ceramic coatings
• Melonite coatings
• Specialty coatings, unique to the equipment maker
If a pot is lead-free compatible but contains leaded solder it can be cleaned before lead free solder is added. Cleaning is very important to avoid the introduction of lead. The limit required as per RoHS Directive is only 0.1%. Most lead-free solders will contain a small amount of lead in the range of 0.05%; there is little room for unintentional contamination.
A standard procedure for wave machine cleaning is detailed below:
1.Empty the pot completely
2. Drain any remain solder
3. After cooling use a scraper to gently remove any visible leaded solder
Avoid damage to the metal finish during scrap down
4.Clean ducts, baffles and impeller mechanism thoroughly
5. Clean conveyor chain and fingers with a steel brush, removing all lead-bearing solder particulates.
6. Fill machine with pure tin to clean any remaining leaded solder
7. Run system at 500º F for 2 hours, circulating the solder
8. Empty completely, removing all visible excesses
9. Refill with lead-free solder
10. Do an analysis for lead and iron after running pot for one hour
For dip pots and selective soldering pots, the cleaning is simpler but complete removal
of leaded solder is required and a tin fill is a less demanding process since the volume is
substantially less. A pot analysis for lead and iron is still required.
Note: If a tin wash is not performed it becomes imperative to remove all traces of leaded
solder in all parts of the solder pot area. The risk is highly increased in reference
to lead contamination. Tin washing is therefore preferred
About the author:
Source: Peter Biocca, Senior Market Development Engineer with Kester Inc.