The challenge created by system variation is sometimes best solved by moving test operations offline.

A paced assembly line with inline functional test balanced through careful application of Lean manufacturing principles is a model of efficiency. Achieving that level of efficiency requires careful coordination among engineering and production personnel.

Paced lines that integrate functional testers deal with several challenges, including:

• Length of test time vs. standard time per assembly station.
• Accommodation of system variation.
• Determining the best mix of automation and human interaction.
• Fixture reliability in a high-volume environment.
• Fail-safing the process from operator error.

The test time equation. The most important part of determining whether an inline functional test option is possible is understanding the length of test time vs. the standard time per assembly station. For example, if the standard time per station is 15 sec. and the total test time is 1 min., it would be logical to assume four testers could handle the line output efficiently. If each station is operated by an individual operator, however, the cost becomes inefficient because the operator likely has a minute of idle time with every board tested. In SigmaTron’s facility in Suzhou, that issue is addressed by assigning multiple testers to a single operator.

System variation. In this example, the biggest cause of system variation is the variance in time likely caused by test operator variations in load/unload speed. Humans are not robots. Even the best test operator will have variations in speed throughout a production shift. Consequently, the assumptions related to number of testers needed to support a line typically need to factor in system variation. In the example above, four testers might be manageable, but the operator would need to work at precise speed with every load and unload to keep pace with the line. If the test time were 1.5 min., the better solution might be one operator per three testers, since managing six testers when a paced line is operating at full speed is a skill few test operators can handle for an extended period of time.

The challenge created by system variation is sometimes best solved by moving the test operation offline. The rationale goes back to the earlier example of the inefficiency of one operator per tester. If one operator can efficiently operate three testers, but the line output requires four testers, splitting the test stations into two testers each is inherently inefficient. While the best Lean solution is to test inline since it eliminates excess processing, transport and handling, the most efficient solution may be to test offline. While the operator will have a standard time to match, the stress of keeping up an assembly line constantly depositing new products at set intervals is eliminated, as is the possibility of damage from product pileups. In that scenario, small amounts of work-in-process (WIP) may accumulate due to system variation, but that WIP is safely handled and not slowing down the line. Additionally, in that scenario, presence of WIP ensures that if multiple operators are required, each operator is fully loaded in terms of capacity. In that scenario, the Lean solution is cross-trained operators who are available for spikes in demand, but working in other operations when there is minimal WIP present.  

Test operator input is a key part of the determination on whether to perform test inline, divide test among multiple operators, or perform test offline. In the Suzhou facility, if the complexity of an inline functional test operation appears to be causing operators significant stress, or if there is wide variation in the ability of operators to keep pace with the line, the process is redesigned.

Automating the process. The facility’s test engineering staff utilizes Lean principles in test system design. Where possible, variation is minimized through utilization of a standard test platform. Fixtures are designed to be operated with one hand, typically using a clamshell press-down fixture with an automated opening process at the end of the test.

Similarly, operator interaction during test is minimized by designing testers that do not require the operator to push buttons or turn knobs. Step motors handle this process instead. Parallel programming is used to shorten test time by testing multiple elements simultaneously.
At SigmaTron, data collection at each test station is integrated with the company’s proprietary Tango shop floor control system. Each product carries a barcode with a serial number to facilitate this tracking. The operator scans the product during load and unload. As a result, the operator can focus on the speed of load/unload, scanning and appropriately sorting the product based on test results.

Fixture reliability. In a high-volume test environment, clamshell press-down fixtures or bed-of-nails fixtures are preferred over connector-based test, since plugging and unplugging can weaken the connector and exceed target test time.

Fail-safing the process. In a paced production environment, the question isn’t “will operator error occur?”

Instead, it is “when will operator error occur?” In SigmaTron’s process, automated data collection acts as the fail-safe that identifies operator error. Each internally designed clamshell press-down automated fixture accesses test points and ensures necessary data are stored in the database as units are tested. Should an operator inadvertently pass a failed product to the next station, the receiving operator will be locked out from scanning that product until it passes test.
Keeping functional test balanced on a paced line can be a complex process. Taking a team approach that considers both test engineering and test operator input, combined with cost-effective automation, is the best way to design an efficient inline functional test.

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

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