Using unique solder pots for each alloy is a low-cost, highly efficient solution for mid-volume environments.

The manufacturer converting wave soldering operations from SnPb to Pb-free has three options:

• Acquire a new system to support Pb-free alloys.

• Convert the existing wave soldering system to accommodate Pb-free alloys.

• Implement Flexible Manufacturing Systems (FMS) concepts to switch alloys as required.

As a case study, the American Competitiveness Institute and Technical Devices developed a flexible manufacturing workcell for wave soldering applications. With the solder pots cold, it is possible to change from SnPb to a Pb-free alloy within 15 minutes, minimizing equipment downtime due to solder alloy conversion. This article identifies the requirements to implement this process, and the challenges in maintaining an FMS workcell in a production environment.

FMS workcells used in manufacturing processes offer unique opportunities to the production environment:

• Production of several different families of assemblies and subassemblies.

• Random launching of assemblies and subassemblies onto the systems.

• Reduced in-manufacturing lead time.

• Reduced in-process inventory.

• Increased machine utilization.

FMS workcells have a successful track record in SMT and wave solder applications. Due to environmental legislation, FMS concepts will have to be reconsidered for specific manufacturing processes.

As a host of literature has detailed, the RoHS Directive minimizes use of specific materials in electronics. For SMT applications, changing the solder alloy from SnPb to Pb-free is a straightforward process. Assuming that the reflow soldering equipment can reach the Pb-free alloy's higher temperatures, adjusting the thermal profile to accommodate Pb-free alloys is the primary issue.

For wave solder applications, changing from one solder alloy to another offers unique challenges. Changing a solder pot from a SnPb to a Pb-free alloy can be time consuming and labor intensive (up to one week of production time could be lost during the conversion). There is the risk of lead contamination during the changeover. Purchasing new wave solder equipment or retrofitting to support Pb-free soldering applications may be cost prohibitive. Finally, one has to ensure that once the solder pot is converted to a Pb-free alloy, the amount of lead within the solder pot is less than the 0.l% upper limit defined by RoHS.

Acquiring a new system. If the manufacturer installs a new wave soldering process while maintaining its existing SnPb capability, no downtime will be caused by the solder alloy changeover. As Figure 1 illustrates, a process firewall can be implemented, separating both processes, minimizing the probability of lead contamination within the Pb-free processes.

The prime disadvantages to this option are price and floor space. A separate wave for Pb-free soldering could cost as much as $250,000. The dual wave soldering process capability requires extra floor space and utilities. Finally, the manufacturer must determine whether its customers need the volume required to support separate SnPb and Pb-free waves.

Retrofitting the existing system. A second option is to retrofit the SnPb wave soldering process to support Pb-free alloys. With an estimated price of $80,000, it is less expensive than adding a new wave soldering system.

There are several disadvantages to this option. The first is the process capability of the current wave soldering system. Most Pb-free solder alloys melt at a higher temperature than eutectic SnPb solder (Table 1). Further, Pb-free alloys do not wet as well as SnPb solder.1 Therefore, preheat temperatures and active solder fluxes are required to support Pb-free alloys.

The second disadvantage is the probability of lead contamination. SnPb residues remaining on the solder pot and wave soldering fixtures can raise lead contamination above the 0.1% level.

A third disadvantage is the solder pot material. Most solder pots are made from stainless steel or cast iron. Solder alloys with high tin concentrations can erode the solder pot. Tin erosion can damage an unfinished solder pot and its fixtures.² To prevent tin erosion, wave solder manufacturers are finishing solder pots and wave soldering fixtures with ceramic, Teflon and titanium nitride.

Another disadvantage is that the manufacturer will not be able to process SnPb and Pb-free waves simultaneously. The manufacturer will lack the flexibility to support both and will have to choose between SnPb or Pb-free alloys.

The final disadvantage is the conversion process. Emptying the SnPb solder pot, purging any SnPb residues and introducing a Pb-free alloy is time consuming and can be an environmental health and safety hazard. Under the best conditions, the process would take three days to complete, reducing machine utilization (Figure 2). This situation is compounded if the manufacturer has to convert the process from a Pb-free solder back to eutectic SnPb.

A "flexible" alternative. To implement a flexible wave soldering process, a solder pot for each solder alloy is required. A heavy duty cart is required to transport the solder pots. Assuming that the solder pots are cold, changing from one solder alloy to another can be accomplished in 15 minutes, not three days (Figure 3). Depending upon the wave soldering equipment manufacturer, this process can be implemented with less than $40,000.

Several disadvantages to this concept exist. As with retrofitting, the manufacturer will not be able to run SnPb and Pb-free wave soldering processes simultaneously. This concept assumes that the wave has the process infrastructure to support Pb-free wave soldering process requirements. The final disadvantage is process discipline. Given the possibility of lead contamination, equipment maintenance will be required.

This concept may not be applicable for all production requirements. In high-volume commercial applications converting a system from SnPb to Pb-free may consume too much time. However, for low- and medium-volume production, with a large variety of assemblies and subassemblies, this concept may be applicable (Figure 4). The manufacturer should consider its production volume, assemblies and subassemblies, and market requirements prior to implementation.

Case History

ACI required SnPb and Pb-free wave soldering capability to support its Demonstration Factory and Electronics Manufacturing Learning Center customers. Adding a second wave soldering process was too expensive to justify. Therefore, ACI, with assistance from Technical Devices, implemented the FMS concept.

Electrical and mechanical quick disconnect fixtures (Figure 5) provided utilities services to the solder pots. Separate wave solder pumps and fixtures were used to reduce the possibility of lead contamination. Prior to implementation, ACI developed Pb-free thermal profiles on the wave, ensuring that the preheat zones had the thermal capability to support Pb-free wave soldering processes. The Pb-free solder pot and fixtures were coated with titanium nitride to reduce the tin erosion effect. With the solder pots cold and using two heavy duty carts, changeover from one alloy to another was accomplished within 15 min.¹

Equipment preventative maintenance and discipline is required to support the process. When changing alloys, the wave soldering system is inspected for solder splashes and residues (Figure 6). The preheat panels, conveyor and conveyor fingers are inspected and cleaned. Solder splashes and residues in the system's interior are removed.

Each solder pot is clearly identified, indicating which alloy the solder pot contains. On the wave soldering system, a placard displays which solder alloy is being used. Finally, prior to using the wave, all solder bars are removed from the workcenter. This prevents a SnPb bar from accidentally being placed into the Pb-free solder pot. For a solder pot with an 800 lb. capacity, two eutectic SnPb solder bars can sufficiently contaminate the solder pot above the maximum contamination level.

Solder pot maintenance becomes more critical with Pb-free soldering. The electronics manufacturer will have to assure the solder pot's lead level remains below the upper control limit to be compliant to the RoHS Directive and the IPC J-STD-001 (Table 2). Monthly samples will have to be analyzed to ensure the solder pots contamination levels are within these specifications

Conclusions

It is feasible to implement FMS concepts to the wave soldering process. Table 3 details the challenges associated with each conversion process alternative. ACI demonstrated the capability to convert its wave soldering system from SnPb to Pb-free solder alloys. It is less expensive than retrofitting or acquiring a new system. Assuming both solder pots are cold, converting from a SnPb line to a Pb-free line will take approximately 15 min., utilizing quick disconnect fixtures and heavy duty carts. Once installed in the wave solder machine, the solder pot will take approximately two to three hours to melt the solder, making it available for use. Taking into account up to three hours for the solder pot to reach temperature, this changeover is less than the three to five days to replace the solder within a solder pot, as in the case of retrofitting.

There are several caveats to using a FMS wave soldering system. Preventative maintenance procedures and operation discipline are required to reduce the risk of lead contamination. Furthermore, the bath must have the infrastructure to support Pb-free solder alloys.

Finally, wave soldering equipment manufacturers and academics should consider developing automated material transfer capabilities. Developing the ability to safely remove and replace a solder pot while hot will reduce changeover from 15 min. to zero. It will also minimize the time required for the solder pot to reach operating temperature.

Acknowledgments

The author would like to acknowledge the American Competitiveness Institute, Jeff Stong of Essemtec and Debbie Alavezos of Technical Devices for their contributions to this article.

References

1. Dr. Ning-Cheng Lee, "The Relationship of Components, Alloys and Fluxes," Circuits Assembly, October 2005.

2. L. Whiteman, L. Stong and Debbie Alavezos, "Converting Wave Soldering Equipment to Pb-Free," Circuits Assembly, August 2005.

Bibliography

• M. P. Groover, Automation, Production Systems, and Computer Aided Manufacturing, Prentice Hall, 1980.

• W. J. Hopp and M. L. Spearman, Factory Physics, Irwin McGraw-Hill, 2001.

 

Lee Whiteman, Jr., PMP, is a senior electronics manufacturing engineer with expertise in electronics packaging, electronics manufacturing and project management of process development and engineering programs. He has a master's in mechanical engineering from Rensselaer Polytechnic Institute; lwhitemn@optonline.net.

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