Which process offers fewer steps – and less contamination?

In a perfect world, the electronics industry would have migrated to 100% SMT by now. Unfortunately, through-hole remains a required technology for some products. In particular, through-hole connectors are often preferred over their SMT counterparts due to the robust solder joints they provide.

From a Lean perspective, a requirement for mixed technology can open the door to several of the seven wastes, as it can drive the need for processes not required for a 100% SMT printed circuit board assembly (PCBA). In particular, the wastes of transport and processing can occur when separate solder processes are required for the same PCBA. The need to do multiple thermal cycles when processing via reflow and wave solder also potentially adds to the waste of defects, as it can plant the seeds for premature component failure and handling damage.

SigmaTron International’s facility in Chihuahua, Mexico, utilizes pin-in-paste (PiP) as an alternative to using two separate solder processes. In the PiP process, solder paste is deposited inside through-hole vias and pads prior to component placement. SMT and through-hole components then go through the reflow soldering process. In one recent project, the team selected PiP as an alternative to wave soldering because the customer had concerns about wave soldering’s added ionic contamination.

In this specific PiP process, two- and 16-pin through-hole connectors are reflowed using a low-temperature tin-bismuth solder paste that reflows at 190°C. The connectors are tolerant of oven temperatures up to 260°-270°C. The critical-to-quality (CTQ) dimension is the lead-to-hole ratio. Good stencil design is imperative, as the amount of paste in the hole must be carefully controlled.

In developing the process, the team designed the stencil and validated its design through trials. X-ray inspection was used to control filling of the holes. The team utilized 3-D solder paste inspection (SPI) for process confirmation and process monitoring. Automated optical inspection (AOI) was used to confirm good solder fillets.

The advantages to this process include:

• Better solderability
• Ease in controlling the process
• One thermal cycle – which reduces the potential for component failure post-reflow or in the field
• Elimination of the additional handling – and potential handling damage – from a wave solder process
• Elimination of pallets and shielding required for wave soldering
• No added ionic contamination, eliminating the need for aqueous cleaning
• Improved cycle time
• Lower cost than using both reflow and wave solder.

Overall, this simpler approach is inherently Lean because it eliminates the additional processes and handling from using two separate solder processes.

As with any process, PiP is not universally the best choice for mixed-technology PCBAs. Wave solder and selective solder continue to be viable options in some applications. The number of through-hole components on the PCBA, PCB layout, through-hole component heat sensitivity, end-product application, product volumes and specific customer concerns relative to cleanliness or cost can impact preferred choice. Facilities continuously running wave solder equipment would see less of a savings factor than facilities that only intermittently use wave soldering. And while the PiP process is easier to control than traditional wave solder, it does require automated inspection to monitor solder paste deposition and solder fillet quality.

This example illustrates benefits of simple solutions when legacy technology would otherwise drive additional processing. It also underscores the value of inline inspection technology in maintaining levels of process control in solder paste deposition that enable a broader range of soldering options.

Alvaro Grado is manufacturing engineering & quality manager at SigmaTron International (sigmatronintl.com) in Chihuahua, Mexico; alvaro.grado@sigmatronintl.com.

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