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Vergent Products - Blog

Test - Just Another Four Letter Word by Tracy Ireland

Wednesday, July 03, 2013

When it comes to manufacturing a product perhaps nothing is more frustrating and controversial than manufacturing test. The need to test in production is often a result of manufacturing defects, component margins, tolerance stack-ups, design or system integration issues. Most clients view manufacturing test as a necessary evil and some ignore it until the last possible moment at their own peril.

At Vergent Products we prefer a more proactive approach. While we strive to manufacture with zero defects and utilize Automated Optical Inspection with great success we still cannot control the processes of all manufactured components and unknown design margin or system integration issues.

Thinking about test strategy early in the design is very important because if you design a product that is hard to test or requires excessive test the manufacturing cost of this necessary evil escalates quickly and the test coverage can be significantly diminished.

The circuit designer must plan for how the product is tested and how much self-test, diagnostic capability, component isolation, boundary scan, and node access will be designed in. As the design proceeds to layout items such as tooling holes, probe access, probe density, component accessibility, and clearances should be considered.

At some point a decision must be made concerning manufacturing test strategy and platform. There are three popular test options: In-Circuit Test (ICT), Flying Probe, and Functional.

ICT is often a general purpose test platform like HP3070 series or Genrad/Teradyne with a bed of nails fixture. The advantages of ICT include low per-piece cost, exceptional coverage (if designed for test), power-up test for both analog and digital functions, integrated boundary scan, and device programming. The downside is initial cost of test development and dedicated fixtures. ICT can be very cost effective overall for high volume long life products compared to the development costs of a dedicated functional test and the higher per-piece test costs of other options.

Flying probe provides an excellent option for low volume short life products. Programming is often derived from CAD data and there are no fixture costs. Thus, the development cost is low but per-piece cost is often much higher than ICT due to higher test times. Flying probe is very effective at identification of missing, wrong, reversed and defective for two and three pole components. The approach is most effective for manually loaded through-hole assemblies. Some Flying Probe testers also provide for JTAG device programming and boundary scan for IC validation. Flying probe is frequently used to accelerate prototype validation – when turning on a new design it is helpful to have high confidence the assembly was correct and Flying Probe is a low cost means to that end.

Functional Test can cover a very broad range of options from a simple power-on self-test to installation of the assembly into a product and execution of product level tests. This approach often provides the very best coverage and validation but at a high price. Unless the dedicated equipment is automated, the per-piece time tends to be high and the test development time can be significantly higher than ICT or flying probe.

In some cases, like high energy power supplies or products with safety issues, a mixed approach may be in order. For example, you may wish to validate components on Flying Probe before you power-up an assembly in a product based functional tester to prevent potential damage to equipment or expose personnel to an unsafe condition.

It is important to make a test strategy decision early in the design process and Vergent products can help with cost/benefit analysis of the various options while also providing design for test and test development services.

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