The EMPF requires both tin lead (SnPb) and lead free wave soldering capabilities to support process development, prototype manufacturing functions, and training. When addressing this need for a dual process system, the EMPF decided to convert an existing Technical Devices Nu/Era Wave Soldering System (Figure 5-1) to accommodate both solder alloys rather than purchase a new, dedicated lead free wave soldering system.

When converting to a lead free system, the following four process elements provide the most effective opportunities to incorporate economical practices:

1. Solder recovery. The greatest cost savings can be gained through the use of a solder recovery system. As much as 75 percent of the solder bath (depending on pump design) can oxidize over time to become dross, the main component of which is pure solder. Manufacturers can process their own dross and reap the financial benefits in the form of reduced solder expenditures. With every dollar counting, this money-saving procedure
cannot be ignored.

2. Lead free process control. One of the major contributors to defects, such as bridging, icicling, and insufficient topside fillets, is poor heating of the printed circuit board (PCB) assembly during the preheat stage. Too much heat is just as bad as too little, a fact that is especially true in a lead free process. In fact, preheating must be more exact in lead free applications simply because of the higher temperatures involved. Some lead free solders will require melting temperatures near 700°F.

3. Preheater types. Manufacturers of wave soldering equipment use a variety of heating methods. Quartz lamps, infrared (IR) tubes, and Calrod elements all operate at high temperatures (1,330° to 2,000°F) to bring the PCB to its optimum topside board temperature of 190-240°F before entering the wave. Obviously, this high Delta T constitutes a very inefficient use of energy. Furthermore, the assembly does not absorb much of the heat emitted by such sources. Attempting to reach this relatively low temperature using such a high heat source greatly increases the possibility of burning the flux. (Tubular and quartz type preheaters randomly spray heat waves in all directions, providing as little as a 50 percent absorption rate.)

In contrast, the most energy-efficient method to heat a PCB is by using low-watt-density black-body IR heating strips. These IR units emit long IR wavelengths that are easily absorbed by the board. Hence, the Delta T between the heat source and the board is much lower than that produced by lamps, tubes, etc. Black-bodied IR radiant emitters direct heat at the PCB with absorption rates as high as 85 percent.

4. Preheater design. Most wave soldering machines offer a variety of preheater configurations. The optimum design for a preheating system should include more than one type of heater – e.g., a system that includes black-body IR heating strips on the bottom, coupled with forced-air convection from above.

Another contributing factor to heating the board evenly and gradually is the physical design of the preheater. For example, if there are gaps between the end of the preheater and the beginning of the wave, cooling will occur. Similarly, the distance between the board and the heat source as it travels on the conveyor can play a role in how the board is heated. Ideally, the board should get closer to the heat source as it approaches the wave. In addition, because lead free solder will be at a higher temperature together with the PCB upon entering the wave bath, the most efficient preheating system should minimize defects and the cost of repair and replacement, while yielding longer production runs to meet quantity goals.

A different alloy in every pot
If some production runs require lead free solder while others do not, the most economical solution is to have separate rollout solder pots and carts for each. This eliminates the downtime required to drain one pot and refill it with another alloy, as well as avoiding cross-contamination between alloys.

Besides the financial benefits, there are other reasons for not using lead free solder exclusively. It does not wet as well as tin lead and yields a duller surface finish. This may cause a higher percentage of good boards to be rated as defective, increasing rework time using current pass/fail criteria.

Controlling parameters
With different processes and solder alloys, one must be able to adjust the wave soldering machine to different parameters. To that end, temperature controls for the preheater and solder baths are easily adjustable, as is conveyor speed. The latter parameter affects the amount of time that the PCB is in the preheat zone as well as the duration the board is in the solder wave. Conveyor height is also crucial because the thickness of the board and the size of the components will vary. Other variables include conveyor width, nitrogen control, and all fluxing controls – all easily set and monitored on wave soldering machines.

For each process, an optimal set of parameters exists. These can be set and monitored best using a computer-controlled wave soldering machine with the capacity to remember an infinite number of recipes (parameter sets). Another advantage is the ability to view what is happening inside the machine via a monitor.

As new, lead free processes continue to evolve, wave soldering machines are adapting to the new parameter requirements. While economic resources are tight for most companies, wave soldering continues to be an efficient and cost-effective tool for production.

The EMPF has a Technical Devices Nu/Era Wave Soldering System with two pots on site. If you would like a demonstration of this system, please contact Jeff Stong at the EMPF at (610) 362-1200, extension 224 or jstong@aciusa.org.

For more information concering Lead Free processes and surrounding issues, please stop by ACI's new Lead Free Manufacturing Page to download articles contributed to ACI by some of the industry's most knowledgable individuals and organizations, as well as material generated by ACI, and documents on the legislation surrounding the Lead Free issue.