Evaluating Equipment Tradeoffs

Do multiple single-spray-nozzle conformal coaters outperform a three-nozzle machine for volume builds?

Electronics manufacturing services (EMS) providers often have to evaluate equipment selection tradeoffs on more stringent standards than original equipment manufacturers due to their needs for flexible capacity across multiple clients.

In one recent example, the engineering team at SigmaTron International’s Chihuahua, Mexico, facility needed to evaluate whether multiple single-spray-nozzle automated conformal coating machines would be a better choice for a high-volume project than a single machine customized with three spray nozzles.

The argument in favor of multiple single-nozzle machines was the flexibility they could provide in addressing the needs of other projects, plus the redundancy they provided should one machine have unanticipated downtime. Customizing a single machine with three nozzles cost less than three machines, took up less space on the factory floor and reduced setup and cleanup time. The potential negative to the single machine option, in addition to the lack of redundancy for unanticipated downtime, was that most other projects would require only a single nozzle, utilizing only a third of the custom machine’s capacity.

The choice was made to customize one machine with three nozzles. Six Sigma core tools were used to evaluate the best choice for nozzle type, assumptions related to takt time with three nozzles, accuracy of spray, optimum spray pattern and repeatability of the process.

Two coating types, four coverage options and two nozzle types were tested over 10 board sample sets, evaluating the area covered, coating time, curing time, material use, capacity required, pallets required and unit cost.

Once the conformal coating choice was approved, a CPK study verified conformal coating thickness. The sample size was 20 printed circuit board assemblies (PCBAs), and a PosiTector 6000 was used for measurements. The results were in line with expectations.

A measurement systems analysis (MSA) study was performed on the selected coating to verify that the required coating thicknesses could be achieved using the planned equipment configuration. Ten PCBAs were coated and measured three times by three different appraisers using a PosiTector 6000. The study included a gage repeatability & reproducibility study (R&R) that tested repeatability against equipment variation, reproducibility against appraiser variation, R&R, part variation and total variation. The results showed detectable variation was within statistical process control limits.

An additional MSA used the optical inspection system to determine the percentage of coverage. The sample size was nine PCBAs with three appraisers and three trials. Results showed detectable variation within statistical process control limits.

The results of these detailed studies were shared with the OEM to determine the preferred coating and nozzle type. By utilizing Six Sigma core tools to evaluate the results of variations and validate assumptions about key characteristics, the OEM could see the impact of various choices.

A production part approval process (PPAP) and validation run of 100 pieces were performed, following OEM confirmation of machine configuration and coating choices.

The results validated that the single machine with three nozzles would meet cost, capacity and quality requirements. From a Lean perspective, the single machine choice reduces the non-value-added setup and cleanup time. It also uses less energy. The operation’s footprint is minimized on the factory floor. The studies related to coating options, pattern choice and nozzle type helped reduce the overall takt time of the operation. The process as currently designed requires 18 seconds per unit application time plus load and transition time into the machine. The coating is applied to three units at a time. Two operators are required: one to load PCBAs on pallets and one to inspect for correct application at the end of the line. The engineering team is currently evaluating a conveyor option that would turn and return the coated units to the first operator to eliminate the need for the second.

While this option was best for a long-term, high-volume program, it might not have been ideal for an EMS provider engaged in higher-mix production, particularly if PCBA sizes have significant variance among different projects. Utilizing Lean philosophy to consider the most efficient process and Six Sigma core tools to analyze best equipment configuration assumptions helps teams make the best choice for their production environment. This approach also helps drive continuous improvement over time as opportunities are identified.

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