It is common practice to change the characteristics of the laminar solder nozzle since a longer contact time may be required due to the lower wetting properties of lead-free alloys. Often the distance between the chip wave and laminar wave may be reduced to minimize any temperature drop between contact points. Increasing the length of the chip wave improves wetting while increasing the preheat output produces a similar effect. Reducing the fall height of the wave to decrease the distance of overflowing solder reduces the amount of dross with lead-free alloys.
 Compared to tin-lead solder, most lead-free alloys oxidize more rapidly when the solder is liquidous due to their increased tin content. Tin oxide, consisting of tin-oxygen (SnO) and (SnO2), forms at a higher rate because of the higher processing temperature and results in more oxidation and dross. Nitrogen inerting of the solder pot is recommended and minimizes exposure of the liquidous solder to oxygen and decreases the amount of dross. Needless to say, reducing the rate of oxidation and the resulting dross build-up significantly improves the performance of the wave soldering process.

Solder Pot Maintenance
 With the introduction of lead-free assemblies, many sources of lead bearing material have to be eliminated from the wave soldering process. OSP coating over bare copper are quickly becoming a replacement for traditional hot air solder leveled (HASL) board finishes resulting in a trade off between a potential source of lead contamination and a potential source of copper contamination. As lead-free wave soldering becomes more widespread, questions have been raised over increased solder pot maintenance brought about by the high copper dissolution rates of lead-free alloys.
 During long periods of operation, some lead-free alloys begin to show sluggish behavior in the solder pot. This is caused by a buildup of a copper-tin intermetallic (CuSn) that forms at the bottom of the solder pot. The problem didn?t exist with standard tin-lead wave soldering because the copper-tin intermetallic floats and can be easily removed.

 In a standard tin-lead wave pot, as impurities such as copper build up they form intermetallics with the tin. Reducing the temperature of the solder pot, allowing the pot to sit idle for a few hours, and skimming the top surface can easily remove the intermetallics. The method works quite well since the density of the copper-tin intermetallic (CuSn) is 8.28, and tin-lead (SnPb) is 8.80, allowing the copper-tin intermetallic to float. Periodic maintenance of a tin-lead solder pot can easily maintain copper levels between an acceptable range of 0.15-3.0%. With the increased use of tin-copper or tinsilver- copper lead-free solder alloys, the situation changes since the density of both alloys is less than that of tin-lead (Table).

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  As a result, instead of floating on the surface where it can be easily removed, the copper-tin intermetallics sink in lead-free alloys and are dispersed throughout the solder pot. In addition, some lead-free alloys dissolve copper at a faster rate than tin-lead. The effect can be a higher rate of copper build-up and contamination of the solder pot.
 The result can be the need to dump the solder pot more often, or a complete changeover of the pot. Studies suggest that solder pots containing lead-free alloys may have to be dumped when the copper contamination levels reach 1.55%. Above this point most lead-free alloys become sluggish and above 1.9 to 2.0% damage can result to the impeller, baffles and solder pot.
 Practice has shown many lead-free alloys cause corrosion to the base metals used for solder pots, impellers and baffles because of aggressive nature of tin at high temperatures. The surface of many base metals such as stainless steel or cast iron generally show signs of pitting and start to dissolve after prolonged contact with leadfree alloys. This leaching process releases iron (Fe) particles resulting in increased iron content and contamination of the solder alloy.

Experience so far indicates that solder pot materials such as stainless steel can become damaged after only a few months of operation when using lead-free alloys. The use of high-grade stainless steel can reduce this effect somewhat as does applying a proprietary corrosive resistant surface coating. However, surface coatings are very susceptible to scratching because of the higher maintenance levels needed to remove the copper intermetallics that sink to the bottom of the solder pot. Over time repeated scratching breaks down the surface coating and results in corrosion of the base metal and contamination of the solder pot. Without protection, parts made from these base metals degrade to the point where they may require replacement after only one to two years of operation. In one long-term production application using a tin-silver alloy, it has been reported that the solder became contaminated and was changed once a month, the impeller needed replacing once every six months, and the solder pot required replacement once a year.
  Base metals do exist that are resistant to the aggressive nature of tin scavenging but until now the cost of these materials was considered excessive. So what is the solution? Titanium is one material that is ideally suited for lead-free solder pots, impellers and baffles since it is impervious to the effects of tin scavenging.
  The frequency with which solder pumps and baffles made of traditional base metals wear out and have to be replaced in leadfree wave soldering applications has caused a rethinking of the use of titanium. Retrofit kits are now available that replace the impeller, baffles and solder nozzles with parts made of titanium that withstand prolonged service in lead-free applications eliminating periodic replacement of surface coated parts and prevent contamination of the solder alloy (Figure 1).