Lean Philosophy and EMS Realities

Non-value activities are sometimes unavoidable.

One challenge for electronics manufacturing services (EMS) providers implementing Lean manufacturing practices can be customer buy-in. While an original equipment manufacturer (OEM) can design a process with minimal non-value-added activity, customer requirements may revive some necessary non-value-added activity and it is up to the EMS provider to find a compromise that satisfies the customer requirements while minimizing the cost of resources associated with the non-value activity.

A recent example of this occurred in SigmaTron International’s Chihuahua, Mexico, facility. A consumer product customer added conformal coating to a product experiencing field failures related to operating environment issues. An automated selective coating machine is used that controls conformal coating deposition areas and application thickness. The customer wanted an automated visual inspection step added to ensure any overspray or missed coating was caught.

From a Lean perspective, the inspection step is a non-value-added process, particularly given the level of spray control associated with the selective coating machine. It was a customer requirement, however. The EMS provider’s team evaluated available off-the-shelf automated optical inspection equipment and found that most had more inspection functions than the customer required and the process cost would exceed the customer’s target price.

The compromise was to develop a proprietary automated visual inspection system that incorporated only the inspection functions the customer required. The cost of this system was less than one-tenth of the cost of an off-the-shelf system. While off-the-shelf systems were faster, the customer only required inspection based on a sampling plan consistent with IPC-A-610 workmanship requirements, thus only a portion of the 3,000 units per day would undergo inspection, making slower throughput an acceptable option, particularly given the cost differences.

The systems were validated using a measurement system analysis (MSA) which included a Gage repeatability & reproducibility (R&R) study. The goal of an MSA is to quantify measurement uncertainty and determine whether a system’s accuracy, precision and stability is consistent with requirements. In this MSA, three appraisers performed three trials on a sample size of nine units. The characteristic evaluated was the percentage of conformal coating coverage on the studied samples.

To support the analysis, the team created a statistical process control (SPC) plan to determine whether the measurement data fell within control limits. The analysis looked at repeatability in terms of equipment variation to determine whether the machine consistently achieved the same results. It also looked at reproducibility in terms of appraiser variation to determine if results changed if machine operators changed. Gage R&R studies were conducted to determine the amount of uncertainty within the measurement system. The impact of part variation was also studied. The data showed that the system’s ability to discriminate among samples was acceptable across all samples and all appraisers.

The end-result of this example is a compromise that gives the customer exactly what they required at a cost aligned with acceptable pricing. While a non-value-added activity, it eliminates the variability found with manual inspection and requires minimal resources. It also helps ensure that any system variability found in the selective coating machine, such as issues driven by tooling wear or coating buildup, will be identified early. Consequently, the automated inspection step contributes to eliminating the waste of defects.

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