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Designing a Quality System May 26, 2012

Posted by Tim Rodgers in Product design, Quality.
Tags: , , , , ,

The quality of a hardware product absolutely starts with a well-conceived design, but you can’t just do a DFM or any other DFX analysis and then hope for the best. You also need a quality system to monitor the physical processes that convert the design into a tangible product and ensure that they can do so repeatedly, within the required specifications, and with a minimum of variation. Unfortunately there are still a lot of people who think that end-of-line testing before final packaging and shipment is an adequate quality system, checking all finished products or an audit sample to see if performance requirements are met. I suppose that helps ensure that non-conforming products don’t get shipped, but non-conforming products then have to be repaired or scrapped after a lot of value has been built-in, and that’s hardly an efficient or lean use of parts, labor and other resources.

Similarly, reliability can’t be assured solely by taking a sample of a few finished products and repeatedly exercising them to simulate expected life, or subjecting them to other non-destructive stresses. It’s useful to test-to-failure to verify the most likely failure modes for prototypes, but it’s not a good idea to ship a product that’s been stressed or damaged, even if it passed the tests and still meets functional specs. Once again, routinely testing finished products is an expensive (and usually unacceptable) way to verify reliability.

A cost-effective quality system is based on the idea that it’s good to audit products before they’re shipped, but it’s better to prevent non-conforming and unreliable products from being built in the first place. That means working backwards from the performance and reliability requirements of the finished product, and conducting engineering analyses of the design (including analysis of test failures and customer returns) to understand those critical part characteristics and manufacturing process characteristics that must be monitored in order to reduce yield loss in the factory and reliability failures in the field.

Incoming inspection will help prevent non-conforming parts from reaching the factory floor, but it’s better to insist that part suppliers have quality systems of their own to meet the functionally critical dimensions and other measurable specifications to a Cpk of 1.33 or better. All assembly tools, jigs, and fixtures should be qualified using gauge R&R ANOVA studies. A design FMEA analysis can identify possible failure modes for the completed product, but a process FMEA analysis identifies the earlier, intermediate processes that require monitoring, either by measuring the output of the process, or the process’s operating conditions and parameters. Critical processes should be designed to be as robust as possible to minimize variability.

Prototype builds before the start of the production ramp are required to evaluate the quality system, looking for correlations between incoming part quality, intermediate process control, and end-of-line factory yield. Reliability testing of a sample of prototype units can help confirm that the manufacturing process can deliver products within the expected life. The quality system itself must also be qualified before full production can be turned on, and then continuously managed to look for improvements.

None of this happens without a committed engineering team that understands that it’s not enough to create a design that can be used to build a single product within specs.



1. Jim Shattuck - May 28, 2012

Tim don’t forget a focused approach to gauge selection with standards and a defined traceability path to NIST. It will minimize/limit your variation due to source and make your data clearer. Many organization think its just stickers but there is use issues and the way gauge activties are classified within a QMS. Great article!

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