Introduction: Design for Testability (DFT)

When electronics engineers and manufacturers consider testing too late in the design process, testing can become more expensive, less effective, and ultimately reduce the product’s reliability.

To that end, many leading electronics manufacturing services (EMS) providers have adopted the Design for Testability (DFT) methodology. The idea behind DFT is to design printed circuit boards (PCB) that are – from the start – simple, debug-friendly and cost-effective to test.

Using DFT principles, the PCB’s architecture will allow access for leads, cables and probes to evaluate performance throughout the manufacturing process. Benefits of incorporating a DFT methodology include:

• Shortened manufacturing timeline


• Reduced risk of defects


• Increased product reliability and brand goodwill


• Implementation of diagnostic features for easy future testing

When DFT specialists examine a product, there are a handful of tests they consider. The right DFT-minded EMS provider will help determine which testing procedures will best suit your circuit design & product application.

This blog will review five tests that are particularly critical in DFT evaluations. We’ll look at what the test identifies as well as its advantages and disadvantages.

1. Automated Optical Inspection (AOI)

• In automated optical inspection, optical images of the printed circuit board assembly (PCBA) are captured and analyzed by the processing software within the AOI system. These images are used to detect surface feature defects such as nodules and scratches as well as the more familiar dimensional defects such as open circuits, shorts and thinning of the solder. They can also detect incorrect components, missing components and incorrectly placed components. As technology improves, AOI systems are able to predict defects with a high level of accuracy and have very few instances where no defects are found. As such, AOI systems form a very useful element in a sophisticated manufacturing environment.


• Advantages: Quick way to find structural errors


• Disadvantages: Does not provide power to circuit

2. Flying Probe Testing

The flying probe test identifies inductance, short circuits, open circuits, capacitance, resistance and diode issues.

• Advantages: No custom tooling, low upfront costs, high accuracy, quick turn-around


• Disadvantages: Longest testing cycles, does not provide full power to chip

Unlike the “bed of nails” setup, a flying probe device utilizes a set of robotic needles. A testing engineer can program these needles to move and match up with points on the chip, rather than needing to make architectural changes to the board itself.

Another difference is that the flying probe test does not power up a circuit, but it can still provide highly accurate performance analysis. It is suitable for initial prototype stage where the test requirement is not fixed and the actual production tester is not available.

For a more complete picture, flying probe tests are often complemented with other software modelers to run through accessibility nodes and testing coverage percentage.

3. In-Circuit Testing or Bed-of-Nails Testing

ICT tests identify short circuits, open circuits, poor or weak solders and incorrect, missing or misoriented components. The term “bed-of-nails testing” refers to the test fixture, which includes multiple test pins with which the PCB will make contact.

• Advantages: Short testing cycles, provides full power to chip, highly repeatable


• Disadvantages: Longer development time, higher tooling costs

Engineers can design the PCB layout to match perfectly with the probe alignment of the test fixture. This way, when the circuit meets the ICT test pins, it fully powers up to enable testing of all its various outputs.

In addition, there are a number of software simulations engineers can use to generate comparable results.

4. Functional Tests

Like burn-in testing ensures a circuit will perform well over time, functional tests ensure it will perform well in its intended use-case. This is extremely important for OEMs. Often, a chip functions well independently in the lab, but falters in its ultimate application.

Functional testing simulates the actual electrical environment of the circuit’s intended use, identifying any issues with voltage, timing, current and frequency.

The PCB’s performance is tested against a series of signals and power levels customized to the unique specifications of the product’s actual application.

• Advantages: Evaluates product functionality and in-use performance


• Disadvantages: Doesn’t assess structural defects

5. Burn-In Testing

A circuit may perform perfectly when it’s brand new, but how can designers be sure it will maintain integrity over time? Burn-in testing identifies the circuit’s long-term reliability.

• Strengths: Estimates a chip’s lifetime, prevents launch failures


• Disadvantages: High cost, risks damaging the device

Burn-in testing works by simulating the circuit’s aging, increasing ambient temperature to see how the chip will perform months or years into its life cycle. It can be performed on both powered-down or fully powered-up chips.

Conclusion

As digital circuits grow rapidly more complex, designers cannot rely on a single one-size-fits-all testing procedure.

The five tests outlined here are among the most common, but a reliable EMS partner with a solid basis in DFT will be able to think outside the standard menu to identify the most suitable testing procedures for your needs and fully tailor them to your product.

If you’re on the lookout for an EMS provider, be sure to check that any prospect you evaluate has adequate prior experience in Design for Testing, and that they incorporate these principles as early as possible in product development to reap the greatest benefits.