Conducting product testing on printed circuit boards has the potential to become costly. Because of this, it’s easy for businesses to view PCB assembly testing as an expense without much value.
So, why test?
The answer is two-fold. First, manufacturers cannot guarantee product quality without testing. Second, when done correctly, testing saves companies time and money in the long term. Inadequate testing can result in inferior quality products getting shipped to end customers leading to warranty repairs, field failures which require replacement, an inferior product image, and—depending on the industry—even incite potential lawsuits.
On the other hand, adequate testing does not mean that every product requires 100 percent test coverage. The costs of the tests necessary to achieve the last few points toward 100 percent are often not only unnecessary but costly to a point where both production costs and the retail cost of the product can increase exponentially.
It is essential to assess the optimum amount of testing for a given product and industry to maintain a high-quality product while also limiting the overall cost of test. For example, military and aerospace products require a significantly higher investment in test when compared to other industries due to the nature of their mission-critical operation. On the other end of this spectrum, lower cost consumer electronic products, where some amount of product defects may be acceptable, typically require a smaller test investment.
Finding optimum testing coverage
Three main elements of a PCB assembly are evaluated during testing: (1) structural devices, (2) structural connections, and (3) functional connections. These elements are assessed using a combination of popular visual, electrical, and functional testing methods, including automated optical inspection (AOI), automated X-ray inspection (AXI), in-circuit testing (ICT), JTAG boundary-scan, and customized test software.
Many PCB assembly manufacturers use an approach known as PCOLA/SOQ/FAM when evaluating the effectiveness of test systems for their products. This approach defines a defect-spectrum of fault categories for assembled PCBs that can be used by test engineers to calculate a score for defect-coverage. The “PCOLA” segment of the test evaluates the structural devices of a populated PCB using visual testing methods. This portion of the test looks at (1) presence, (2) correctness, (3) orientation, (4) live, and (5) alignment.
The “SOQ” segment uses electrical testing methods to evaluate (1) shorts, (2) opens, and (3) quality. These are connectivity tests to ensure an accurate electrical connection between components as well as identify the quality of those connections.
Functional testing, which is the last type of test process in the PCOLA/SOQ/FAM defect-spectrum evaluates (1) features, (2) at-speed, and (3) measurement. These tests ensure components and connections in the system are functioning as expected, and they measure performance against known benchmarks.
While there are several testing methods available, manufacturers tend to select the methods that provide comprehensive coverage of the PCOLA/SOQ/FAM spectrum.
A breakdown of the PCOLA/SOQ/FAM approach and the applicable tests is identified in the table below:
When making decisions about testing and achieving the optimum amount of coverage, it is ideal to consider which tests can evaluate a significant amount of the PCOLA/SOQ/FAM defect-spectrum. JTAG boundary-scan, a method for testing printed circuit boards when physical access is limited, is ideal for this reason.
Unlike other methods, boundary-scan can perform visual, structural and functional testing. Because of this, JTAG boundary-scan testing can evaluate a PCB for much of the PCOLA/SOQ/FAM defect-spectrum.
Other benefits of JTAG boundary-scan testing
JTAG boundary-scan testing provides a high-level of coverage for PCB testing, but it also offers cost savings in other ways. Equipment acquisition, test procedure development, training and labor expenses are among the inherent costs of performing boundary-scan testing. These costs may sound expensive, but in reality, performing boundary-scan testing is actually less costly than using popular PCB testing methods like ICT. In fact, in addition to the cost of equipment acquisitions, procedure development, and training and labor, ICT also requires test fixture development and equipment maintenance, which are costs that are not incurred in boundary-scan testing.
Additional benefits to consider are:
- Significant product yield increase
- Reusable test patterns
- Reduced test time
- Reduced time to market