With the move toward lead-free electronic products, an increasing number of manufacturers are preparing to implement lead-free soldering processes in compliance with pending regulations. A great deal of effort has been placed on converting the reflow soldering process to lead-free due to the dominance of SMT components. Wave soldering must also be changed to leadfree to avoid mixing of lead bearing and non-lead bearing alloys on the same assembly.
 Implementing lead-free wave soldering involves more than replacing one solder alloy with another because no ?drop-in? replacement strategy exists. A common notion is that one can simply switch from tin-lead (SnPb) to lead-free by dropping lead-free solder into an existing wave machine. Another general misunderstanding is that you need to buy a new wave machine for lead-free processing. Neither of these tactics is correct because there are alternative ways to minimize the cost of implementing lead-free wave soldering.
 For lead-free wave soldering to be successful in a production environment, necessary changes to the entire process must be considered. The majority of lead-free solder alloys possess good solderability but exhibit decreased wetting characteristics compared to tinlead solders. Since wetting is a critical factor affected by solder temperature, contact time, flux, use of nitrogen and wave configuration, changes are required that will affect the majority of machine parameters.

 Introducing lead-free wave soldering generally involves two major process changes because lead-free alloys have a significantly higher tin content than tinlead solder and require higher processing temperatures. Many products will be converted to lead-free over a gradual phase-in, however numerous manufacturers are forging ahead by making their wave soldering machines lead-free compatible now.

Process Parameters
Because the wetting characteristics of leadfree alloys tend to be less than tin-lead solder, using good flux chemistry is critical. Additionally, the higher temperatures needed for lead-free soldering require a flux chemistry that can withstand preheat temperatures up to 130�C and liquidous solder temperatures as high as 280�C for up to 3 seconds. A VOC-free, water-based flux is generally recommended since they meet these higher temperature requirements.
 Often an existing wave solder machine may need upgrading with a newer spray fluxer to be suitable for processing VOC, water-based fluxes. An ultrasonic or nozzle type spray fluxer works best since the flux droplet size can be controlled and a continuous and uniform spray pattern can be applied across the entire PCB. This is essential since it is important to achieve the smallest possible droplet size with VOC-free fluxes to obtain good throughhole penetration.

More preheating is generally required because of the higher melting point of lead-free alloys. A longer preheating section is often needed to reach these higher temperatures and avoid thermal shocking of the PCB when entering the chip wave. Achieving proper preheat temperatures on the top of the PCB has the greatest single effect in reducing solder defects such as bridging and insufficient topside fillets. Optimum preheating of a PCB can best be achieved with a combination of infrared heating from the bottom and convection heating from the top.
  Preheating for lead-free wave soldering can require a heating length of up to 1.8 meters for conveyor speeds as high as 120 mm/minute and as long as 2.4 meters for conveyor speeds greater than 180 mm/minute. An effective upgrade strategy is to replace an existing spray fluxer with an external spray fluxer. This not only improves the quality of flux application but frees up space inside the wave solder machine that can be used to extend the preheat capacity.
  The operating temperature of the solder pot will generally increase depending on the lead-free solder alloy. For tin-silvercopper alloys (SnAgCu) with a melting point of 217�C, the solder pot temperature may be between 260-270�C. For high melting point alloys such as tin-copper (SnCu), the solder temperature may be as high as 270-280�C.