Question: Is “wet sponge” solder tip cleaning mandatory for leaded and RoHs soldering? Are there any downfalls to only using the dry “copper sponges”?
Answer: Cleaning solder iron tips with wet or moist sponges has been the technique used by industry for many years. It has worked wonderfully with minor problems. The problems encountered were contaminated sponges, dirty sponges, etc. With the introduction of lead free products in 1986, the electronic industry was tasked with finding a lead-free alternative to the traditional 63/37 or 60/40 Tin/Lead alloys. The new alloys were not drop in replacement for the existing materials and were also composed of alloy compositions which had higher melting points.
Now the dilemma was how to introduce the materials into the workplace and still keep them separate to prevent the intermixing of the alloys.
The higher melting temperatures also created new issues, as questions arose as to whether or not the temperature of the soldering irons should be raised to accommodate those new parameters. It was found through experimentation and experience that the existing soldering iron temperatures were fine, but the soldering technique had to be modified somewhat to accommodate a longer dwell time on the solder joint to get the good wetting needed to make an acceptable solder joint.
It was also found, that the high tin alloy solders were very corrosive to the metals with which it came into contact. Hence the industry had to review the manual soldering operation and separate the tools used for lead-free soldering and leaded soldering. It also became apparent that the lead-free alloys when clean with a wet or moist sponge would accelerate the cracking of the plating on the tip of the soldering iron. This cracking allowed the tin to penetrate the plating on the tip, which prevented the tip from being tinned, thereby increasing the oxidation on the tip, rendering the tip useless and needing to be replaced. By using the brass tip cleaner the fluxes and excess solder were easily removed and the tip did not experience a thermal shock which ended up providing a longer tip life. This is why many facilities today use the brass metal cleaner to clean the tips.
One still has to be cautious as contamination can still occur with this process and the method suggested to prevent this from happening is to have dedicated tooling for both processes.