Using Lean Six Sigma to balance the increasing cost of solder.

While the death of through-hole technology has been predicted for decades, the reality is some applications have components that require a level of solder joint robustness that only through-hole technology can deliver. In low- and medium-volume operations, the cost-effectiveness of soldering those mixed-technology printed circuit board assemblies using a selective solder machine is an easy calculation because it may eliminate the cost of operating a wave solder machine. However, operations doing high-volume assembly of predominantly through-hole PCBAs may find determining the cost-effectiveness of selective solder is more challenging since their wave solder machines operate continuously. In those cases, the question becomes: What is the point at which use of wave soldering becomes inefficient when the percentage of through-hole components on printed circuit board assemblies drops?

The cost of solder, along with other material and production costs, is increasing globally. While these cost increases are unavoidable, implementing efficiency improvements can help balance these costs by reducing the amount of solder needed and eliminating solder dross.

SigmaTron’s Suzhou, China, team recently used Lean Six Sigma core tools to analyze whether an automated soldering machine would be a better choice than a wave solder machine on assemblies with only a few through-hole components.

The team used a define, measure, analyze, improve, control (DMAIC) process for the project. In the define phase, it selected metrics from September and set a goal to determine if automated soldering equipment could reduce solder bar use and solder dross generation by 40% to 45%. Two assemblies were selected for process comparison.

In the measure phase, the team determined that while less solder was used on the sample PCBAs due to the small number of through-hole components, solder dross generation remained the same since it was a byproduct of machine operation regardless of through-hole component count.

Wave solder dross studies from SigmaTron’s Lean Six Sigma team in Tijuana, Mexico, provided baseline data on typical dross metrics to better validate team assumptions related to dross generation. The ability of teams in different facilities to compare past projects helps shorten learning curves in similar projects.
In the analyze phase, the team used fishbone diagrams to evaluate man, machine, environment, method and material in a brainstorming activity. The conclusion was the number of through-hole components did not justify the cost elements of wave solder.

In the improve phase, the team developed component-count-based criteria to determine which process was more cost-effective when tooling, energy use, solder use and solder-dross generation were considered. It also found bill of materials solder usage assumptions were incorrect when actual solder use was measured.

In the control phase, the team defined the new process and determined its use reduced cost more than 72%. Cost-reduction assumptions did not include the savings likely to be found in indirect labor associated with wave soldering machine maintenance, wave fixture cleaning and solder dross removal.

Final conclusions found PCBAs with fewer than 20 through-hole pins were good candidates for automated soldering. The machine itself had a cost of less than $10,000 and generated almost zero solder dross, resulting in no solder loss. Plus, the automated soldering machine is much easier to clean and maintain; there is no solder dross to be recycled or handled and no excess flux residue left on the PCBA. The automated soldering machine option typically delivers a more consistent result than wave soldering when the solder joint is close to the edge of the wave solder fixture. There are temperature-sensitive components with a narrow process window, or specific solder volume is required in a through-hole.

That said, the team found ensuring consistent results across a range of assembly types required fixturing modifications. They performed a design of experiments to test their assumptions. The basic fixture design is concave with multiple levels of rectangular, square or half-circle shapes to accommodate typical PCBA sizes and shapes. PCBAs that easily fit within these cutouts need no further fixturing. A clamshell fixture attached by dowel pins to the soldering machine platform is used to stabilize PCBAs whose shape or size does not fit within the concave portions of the standard fixture. All fixturing is designed for fast load and unload. The machines process two PCBAs or arrays simultaneously, depending on product configuration.

Using analytical continuous improvement processes such as Lean Six Sigma addresses the challenges of rising costs by evaluating evolving product designs against more cost-effective processing options.

Hom-Ming Chang is SigmaTron International’s vice president, China operations (sigmatronintl.com); homming.chang@sigmatronintl.com.

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