Table of Contents

• What are the core benefits of Design for Testability?
• Why is it important to consider testability in the early stages of a product’s ultimate design?
• How do DFT engineers influence product design?
  • In-circuit Testing or Bed-of-Nails Testing
  • Flying Probe
  • Automated Optical Testing
  • Burn-In Testing
  • Functional Tests
• How to guarantee a successful DFT experience with your EMS partner
• What other design methodologies work alongside Design for Testability?
• Conclusion

When it comes to circuit design, catching errors early in design and production is key to your product’s success. However, in order to test a printed circuit board (PCB), you have to design it in such a way that is conducive to testing procedures. Waiting until the final product is fully embedded in silicon limits—or completely precludes—the ability to test a chip’s performance or integrity.

Instead of a “wait and see” approach, leading electronics manufacturing services (EMS) providers adopt a methodology known as Design for Testability (DFT), a practice that makes an integrated circuit simple and cost effective to test. DFT experts have an intimate understanding of the old engineering adage “anything you cannot test will fail,” so they focus on designing chips with a debug-friendly layout from the outset. DFT-minded engineers architect the structural design of a PCB to provide access for the leads, cables and probes that will evaluate its performance throughout the manufacturing process. They consider where to place test nodes and whether to modify designs with spare gates. While these augmentations do not add operable functionalities for your clients or their end users, they bolster product reliability for years to come.

WHAT ARE THE CORE BENEFITS OF DESIGN FOR TESTABILITY?

C-suite executives may scoff at the added manufacturing costs but designing for testability can save your organization significant costs—and protect your brand reputation—over the long run. Consider the costs of a product recall: coordinating the return of thousands or millions of defective circuits, ascertaining the root cause of the failure and retooling your entire production line to incorporate the update. This process can take weeks, months and even years. By then, costs would have skyrocketed, and customers might have turned to a more reliable competitor with a similar offering. It’s a pitfall that can make or break the longevity of your entire company, leading to multi-billion dollars in losses and a tarnished brand reputation. By incorporating DFT into your product design mentality, you’ll shore up your product reliability rating and safeguard your organization’s livelihood. What are other advantages of this design philosophy?

• Shorten manufacturing timeline
• Reduce risk of defects
• Increase product reliability and brand goodwill
• Implement diagnostic features so the circuit can be tested in the future

The right electronics manufacturing service provider can help determine what tests are best for your integrated circuit design. For example, some tests take longer to conduct while others require custom tooling. By evaluating costs, customization needs, production volume and development time, a team of DFT experts and engineers can identify the right testing procedures needed to ensure a long-lasting product life cycle.

WHY IS IT IMPORTANT TO CONSIDER TESTABILITY IN THE EARLY STAGES OF A PRODUCT’S ULTIMATE DESIGN?

DFT engineers need to have a holistic awareness of a circuit’s intended use. Original Equipment Manufacturers (OEMs) are inserting PCBs into increasingly complex and sophisticated product designs, so testing measures need to simulate a chip’s intended environment. By enlisting DFT specialists early in the process, you have a greater chance of preventing future failures.

Adding the right testability features does not just benefit your organization during initial product launch, but has significant advantages over the life of a PCB. Many tests accelerate product infancy to simulate long-term wear and tear. Test engineers examine currents, wire stability and potential gate degradation to assess longevity.

Testing during new product development (NPD) diagnoses large-scale defects and poor manufacturing tolerances. However, it’s important to provide access for testing during latter stages of the life cycle. With the right measures in place, you can continue to perform tests to investigate random errors while a product is already on the market. You can also predict performance decline of long-life circuits as the product matures. By working closely with an EMS partner that fosters a collaborative environment between designers, supply chain experts and test engineers, you can create a circuit that you can test perfectly for its ultimate use.

HOW DO DFT ENGINEERS INFLUENCE PRODUCT DESIGN?

There are several tests that testing engineers are mindful of when they recommend architectural changes to hardware. Armed with diagnostic foresight, DFT specialists outfit products to undergo critical performance and structural evaluations.

In-circuit Testing or Bed-of-Nails Testing

Engineers can specially design a PCB layout to match up with the probe alignment of this test fixture. Like children’s building bricks, the circuit is designed to mirror the exact positioning of ICT test pins so when the two meet, the circuit fully powers up to enable testing of its various outputs. There are also many software simulations engineers can use to generate comparable results.

Identifies: Short circuits, open circuits, poor or weak solders; incorrect, missing or misoriented components
Strengths: Short testing cycles, provides full power to chip, highly repeatable
Disadvantages: Longer development time, higher tooling costs

Flying Probe

Instead of making architectural changes to mirror an ICT’s layout of test probes, a flying probe device uses a set of robotic needles. A testing engineer can program these precision needles to move and match up with certain points on a chip. While a flying probe test does not power up a circuit, it can provide highly accurate analysis of its performance. Many engineers complement flying probe tests with other software modelers to run through accessibility nodes and testing coverage percentage.

Identifies: Inductance, short circuits, open circuits, capacitance, resistance, diode issues
Strengths: No custom tooling, low upfront costs, high accuracy
Disadvantages: Longest testing cycles, does not provide full power to chip

Automated Optical Testing

Optical testing harnesses the power of artificial intelligence to compare a highly detailed photograph of a circuit against a diagram of its intended design. If the chip does not meet standards, it’s passed on to a technician formanual examination.

Identifies: Misaligned joints, shapes and other out-of-place elements
Strengths: Quick way to find structural errors
Disadvantages: Does not provide power to circuit

Burn-In Testing

While a circuit may perform perfectly fresh off the production line, burn-in testing accelerates circuit aging to try to uncover potential usage errors. The test increases ambient temperature to simulate how a chip might perform months or years into its life cycle. Technicians can perform a burn-in test on a powered-down or fully powered-up chip.

Identifies: Long-term reliability
Strengths: Estimate a chip’s lifetime, prevent launch failures
Disadvantages: High cost, risks device damage

Functional Tests

Functional tests simulate the intended electrical environment of a circuit’s intended use. This is critical for OEMs—a chip might function well independently but it’s important to test it within its ultimate context. Technicians subject a PCB to a series of signals and power levels to test its performance. It’s a heavily customized test where engineers design unique specifications to suit the needs of each product.

Identifies: Voltage, timing, current, frequency
Strengths: Evaluates product functionality and in-use performance
Disadvantages: Doesn’t assess structural defects

Digital circuits are skyrocketing in complexity and there is not one single magic bullet to evaluate PCB performance. While these are the most common testing solutions, it’s important to select an EMS team that is also capable of thinking outside of the box—not one that is limited by a standard menu of testing procedures. Creative testing engineers are able to design unique and fully customized tests that suit each product during its life cycle.

HOW TO GUARANTEE A SUCCESSFUL DFT EXPERIENCE WITH YOUR EMS PARTNER

A successful DFT experience will require the expertise of DFT experts and correct timing. Whether it’s a DFT expert or a production development specialist who has experience in DFT, it’s important to ensure that there is close collaboration between the team/person responsible for DFT and the product engineer. On top of this, check with your EMS partner what prior experience they have in DFT. When your EMS partner takes initiatives to think about what needs to be tested and how prior to the adding of testability features, it shows that your EMS partner is well-equipped with DFT know-how.

DFT should also be incorporated right at the beginning of product development. Starting it later in the product development process can lead to very expensive solutions and even result in product failure. Good DFT ensures all parts of the circuit board and the final product get tested. If testability wasn’t planned prior development, it might be too late to address issues that are detected later in the development stage. A good EMS partner with the right DFT experience will ensure that circuit boards have enough test points in the early stages of product development as adding them in the subsequent stages is a lot of work which may include coming up with a new design that’s testable.

WHAT OTHER DESIGN METHODOLOGIES WORK ALONGSIDE DESIGN FOR TESTABILITY?

Design for Testability, like any design mindset, cannot live in a bubble. Design philosophies are meant to collaborate together—an ideology that forms the foundation of the respected Design for X (DFX) strategy, where X represents a particular design objective. This provides a framework for a design and engineering team to use when assessing the commercialization of a given project. In this case, X stands for ‘testability’ but there are other methodologies that can also influence your PCB project:

• Design for Procurement (DFP): Does a design call for an exotic material? Is a necessary component nearing its end of life? DFP experts evaluate the availability of parts and raw materials and suggest design changes based on their findings.
• Design for Manufacturing and Assembly (DFMA): Through a DFMA lens, specialists evaluate how easy and cost effective a product is to manufacture and assemble. Are parts easily manipulated by assemblers? Does a given design suit the nuances of a particular manufacturing process? DFMA dives into the nuances of how a product actually gets made and makes design changes to ensure its success.

Design for testability is just one member of this family of rich design frameworks. The right ECM partner will use abest-in-class hybrid approach, merging an optimal mix of DFX mindsets best for your project.

CONCLUSION

When it comes to the manufacturing of highly reliable printed circuit boards, DFT reigns as one of the most important design methodologies. After all, “anything you cannot test will fail.” Without augmenting your design with the right diagnostic features, it’s impossible to foretell a fault-free, seamless product launch. Getting this stage right has a lasting impact on your product’s success across its entire life cycle.

Working with an EMS partner that ignores testing considerations until the latter stages of your development process is a strategic misfire. Instead, the right EMS partner will collaborate with you using a diagnostic mindset from the beginning; always looking for ways to design your PCB so it can sustain various tests throughout different phases of its life cycle to ensure the success of your product—and your business.