Adafruit’s products that are made on-site do not contain lead, this is called RoHS compliant (Restriction of Hazardous Substances). So, when we were looking into purchasing a selective solder machine we knew we were going to have to use a lead-free alloy in our process. What we discovered was that not every lead-free solder is the same!
Designing a manufacturing process involves weighing many options. The reliability over time of a given parameter as well as cost, are two of the most critical factors involved in making a final decision. Your overall throughput can condition both of these factors as well.
Leaded vs. lead-free selective soldering
There are pro’s and con’s to every process. Your final needs should act as a guiding principle in determining which process or aspect of a process best suits these needs. Though leaded solder creates great solder joints it’s harmful to the environment, especially when disposed of. Lead solder is also restricted in Europe, so being RoHS compliant also opened markets to us which we wouldn’t have otherwise been able to reach.
Selective soldering (as well as wave soldering) is known as somewhat of a dark art. Process integrity is reliant upon how well the machine is maintained; there is an inherently alchemical feel to the proceedings. By all accounts lead-free selective soldering is an even hairier beast! Lead-free solder alloys generally display decreased wetting characteristics and slower wetting times compared with tin-lead solder alloys. Lead-free solder alloys also have melting points that are considerably higher (30-40℃).
What results is a much hotter pot of solder (315-320℃). Because wetting times are slower, users end up soaking their PCB’s and thru-hole (TH) components with far greater heat than they otherwise would have with a leaded process, for a greater duration of time. The longer a board soaks under high heat the greater the chance of there being thermal shock. This could potentially damage the board. Longer wetting times also mean an inherent increase in cycle times.
Lead-free alloys (due to higher contents of tin) oxidize far more rapidly than leaded solder when in a liquidus state. This simply means that when you’re running a job, the integrity of your solder wave degrades much faster-and may require more frequent fluxing to achieve good 360 degree coverage. More frequent fluxing may result in greater dross and particulate buildup within the pot but also the nozzle itself.
Excellent lead-free solder joints rely on a well-maintained machine and pot, the correct amount of nitrogen shielding, the surface finish of the PCB, and highly accurate flux deposition.
But which alloy?
We still had a decision to make about which lead-free alloy we would use. The candidates were SAC305, Sn100C, and Sn97Ag03…
SAC305 is a lead-free solder alloy containing tin, silver and copper. SAC305 has a similar melting point to Sn100C however SAC305 has been shown to corrode some of the steels used in older solder machines. Persistent corrosion over time can result in catastrophic failure-such as a solder pot leak. Sn100C has been shown to be non-corrosive. SAC305 is also about 30% more expensive than Sn100C due to it’s silver content.
Sn97Ag03 is a lead=free solder alloy containing tin and silver. Similar to SAC305, this alloy is made more expensive than Sn100C due its silver content.
Sn100C is a lead-free solder alloy containing tin, copper, nickel and germanium. Sn100C is also cheaper than SAC305 or Sn97Ag03. Though the germanium may degrade over time, it helps to prevent dross, unlike the other two lead-free alloys. Dross prevention is a key to stable throughput. Though germanium content may decrease over time, an additive like AO1000 will help maintain stable levels within the alloy.
We finally decided to go with Sn100C bar and wire-spooled solder. We wanted an affordable eutectic solder alloy which would help us reduce dross (so critical in the lead-free selective soldering process) without damaging our machine.
Whatever solder alloy you decide to use for your selective soldering process consider the machine you’re using, the activity level of your flux, throughput, and the various costs involved. Happy soldering! ?