Process yield has a direct impact on the bottom line. But many companies fail to properly address and analyze it.
Understanding how to troubleshoot yield problems in lead-free manufacturing can help reduce product cost and improve process yield in design and production of tin-lead and lead-free surface mount technology and mixed-use technology. “Yield impacts deliverables and profitability,” says Bob Willis, a U.K.-based consultant with a global client portfolio. “However, few people actually monitor process yields with real accuracy.”
“People don’t take the time to monitor yield problems and root causes,” Willis says. “They just ‘fix’ it. They believe it’s quicker to rework the product than monitor yield and analyze the causes of a problem.”
Identifying root causes of SMT and through-hole defects is critical to addressing the often complex challenges of effective yield management, says S. Manian Ramkumar, Ph.D., director and professor at the Center for Electronics Manufacturing & Assembly at the Rochester Institute of Technology in Rochester, N.Y.
Root causes can include mistakes in the design or layout of the printed circuit board, the choice of materials in everything from finishes to solder pastes, solder masking and printing of solder pastes.
A major technical concern is the time-and-temperature profile of the reflow process because exposure to the higher temperatures of lead-free can damage a board or its components, says Ramkumar, who instructed “Troubleshooting Yield Problems in a Lead-Free World” at the IPC Midwest Conference & Exhibition in September.
Compliant and Compatible
To Willis, the fundamental cause of defects that eventually hurt yields is design. Too often, OEMs do not understand the design-for-manufacture (DFM) process, he says.
Many companies also don’t properly evaluate components for manufacturing or assembly. Specifically, some companies confuse RoHS compliance with process compatibility. “Many marketers refer to their components, their laminates, their materials, as ‘RoHS-compatible,’” Willis says. “Then people assume that means it can go through a lead-free process. But what the marketers fail to tell you is that ‘RoHS-compatible’ actually means that the products are compliant with RoHS, which means they contain no lead or cadmium, and so on. But it doesn’t necessarily mean they will stand up to the soldering process.”
Additional Defects
Jasbir Bath, a Fremont, Calif.-based consultant who is former chair of the Flextronics Lead-Free Working Group and a member of the 2006 IPC Blue Ribbon Committee for RoHS Process Capability Audit, reflects a different perspective on root causes. “You can trace 60 percent of SMT defects back to the printing process,” Bath says. “If the printing process is not under control, you’re going to create more and more defects down the line.”
The other 40 percent of the problem originates with defects in components. Therefore, testing each component before it reaches the assembly floor is a key to optimum yield, Bath says.
In recent years, testing has become especially important since counterfeit parts have become more prevalent. Jim Hall, principal consultant at Durham, N.H.-based ITM Consulting, says counterfeit components will become a direct and serious threat to future yields.
Wetting is another root cause of process yield. “With lead-free, the issue is wetting during the soldering process, be it reflow or wave, and [related impact on] hole-fill,” says Hall. “The lead-free alloys do not wet as readily and completely as tin-lead. That is well-known. But you have to deal with it.”
Addressing the Process
A comprehensive approach to maximizing yield includes analysis of three factors: design, assembly and incoming materials.
The key to minimizing defects is to design for RoHS compatibility, not just compliance. “Principally, that means the materials can withstand the temperature cycles for lead-free soldering,” Hall says.
Ramkumar says assembly is another critical element of the process. “You can talk about design all you want,” he says. “But you will still not know whether you’re going to have a yield problem until that board goes through the process and you learn from that.”
The third leg of the stool is incoming materials. Always ensure the materials you use in manufacturing are what you ordered and specified, and are lead-free compliant, Hall says.
Most important, thoroughly analyze and fully integrate the DFM process. Effective troubleshooting for yield shortcomings must be a core priority based on broad consensus.
The best way to troubleshoot: “Have proper quality checks at every step of the process because each step is going to build upon the previous step, whether good or bad,” Ramkumar says. “So if you can check to make sure everything is fine at every stage, that automatically helps reduce yield problems at the end.”
