The Top Seven PCB Inspection Methods

As demand for PCBs increase, so does the need for faster, comprehensive testing

Published: Tuesday, December 20, 2022 - 13:02

The demand for printed circuit boards (PCBs) will only increase until a superior technology comes of age. The global market for PCBs could rise to $72.3 billion by 2026. PCBs have become essential everywhere, from infrastructure to consumer products.

The overwhelming demand for PCBs for the internet of things (IoT), automotive and aerospace products, healthcare devices, and general manufacturing makes PCB quality control supremely important. With demand being what it is, manufacturers must find every method at their disposal to make quality consistent and rework nonexistent.

Here’s a summary of the most common PCB inspection protocols and what they offer:

1. Visual inspection

The simplest and cheapest type of PCB inspection protocol is a straightforward visual inspection. It’s also quick to carry out because it requires little more than a comparison of each board against official schematics and design documents. Primarily accomplished using the naked eye and a magnifier, it involves the inspector seeking out board stains, components in the improper orientation, and other visually detectable defects.

Pros:
• Is the most inexpensive PCB testing technique available
• Requires no extensive fixture setup or apparatus calibration
• Identifies most soldering and similar defects

Cons:
• Relies on the skill level and attention of the inspector
• Subject to errors more rigorous tests likely wouldn’t miss
• Hidden components of the PCB can’t be evaluated this way

2. Bed-of-nails test

If visual inspections are the cheapest method, then in-circuit testing (ICT) is the most common. This is typically considered the most comprehensive PCB test, but it’s also the costliest. One type of ICT is called a “bed of nails” test because the circuit board rests on a bed of pins that make contact with relevant test points. This is best for mature product lines where you don’t expect to carry out many design iterations. It effectively roots out problems with capacitance, resistance, shorts and opens, and other indications of faulty or successful fabrication.

Pros:
• Coverage for PCB faults is about 98 percent
• Highly effective at scale, as in mass manufacturing

Cons:
• Carries some additional expenses, including testing jigs
• Not a good solution for prototyping needs or small-batch manufacturing
• Not sufficient for inspecting every PCB feature, including voids

3. Flying probe test

Another ICT method is the flying probe test, which also involves test pins coming into contact with testing points on the PCB. However, unlike the bed-of-nails fixture, which is stationary and unique to each PCB, a flying probe tester has a group of pins on the end of a robotic arm controlled by software to move around the PCB and touch different points to conduct different tests.

It offers an improvement over in-circuit tests because it eliminates some of the costs related to specialized testing equipment. In-circuit, bed-of-nails testing also mechanically stresses the PCB—a problem that flying probe tests alleviate. It may not be cost-effective at scale, but it enables testing of voltage performance, component orientation, passive components, and more.

Pros:
• More cost-effective than in-circuit testing
• Less impact on the PCB than other methods
• Manufacturers save space on the board by eliminating the need to add test points

Cons:
• The process isn’t fast enough for mass manufacturing
• The test is inadequate for appraising voids or insufficient solder

4. Automated optical inspection

An automated optical inspection (AOI) uses one or more cameras to take detailed photos and compare them to the expected results. It doesn’t power up the board for functional testing, but it can detect mechanical issues early in the process. Manufacturers choose this method when they need a noncontact protocol for inspecting the quality and completeness of their boards. They can also add it to various steps of the assembly process.

Pros:
• Is a cost-effective addition to assembly lines for catching defects early
• Offers more consistent performance than visual inspections carried out manually
• Is suitable for inspecting PCBs fabricated at scale

Cons:
• Offers coverage only for surface-level defects as a passive testing protocol
• Involves design changeovers with each new product when programming the automated inspection software

5. Burn-in testing

Burn-in testing has engineers powering up PCBs and keeping them powered—and their firmware active—for between 12 and 48 hours. Products carrying PCBs destined for the medical and military industries often require burn-in testing to detect “infant mortality,” the name given to PCBs that fail early under load. This test can destroy the item it’s carried out on or shorten its life, so it sometimes requires adjustments to the testing period.

Pros:
• Is an excellent way to test load capacity for PCB-based products
• Assesses product performance using a lens of real-world environmental pressures—an essential priority in military, medical, aerospace, and automotive manufacturing
• Is an integral part of establishing excellent product durability and reliability

Cons:
• Is destructive and reduces manufacturing yield
• Requires more labor and time to carry out than other types of PCB testing

6. X-ray inspection

Manufacturers sometimes need a test protocol capable of identifying hidden defects. Also called automated X-ray inspection (AXI), this method penetrates hard-to-reach components and the core of the PCB to find flaws that other test methods can’t. Like AOI, this test is an excellent addition to manufacturing production lines for early error detection. It can’t catch defects in every layer of the PCB, but it’s effective at finding problems other methods—like camera-based inspections—may miss.

Pros:
• Offers more consistent performance than visual-only inspection
• Is capable of identifying almost any solder defect
• Is an effective testing protocol for testing mature products at high volume

Cons:
• Requires both X-ray equipment and trained personnel to operate it and know how to spot defects
• Like AOI, AXI requires some software know-how and time to input the preferred templates for the machine logic to compare against

7. Functional testing

Numerous functional tests may apply to your project. These include solderability tests, contamination testing, peel tests, time-domain reflectometer tests, and more. The design’s complexity will determine the course to take; a power on/off test may suffice, or a more comprehensive battery of tests may be required. The advantage of functional tests is they simulate typical operational environments and behaviors, and can be tailored to the unique requirements of your assembly process and board design. Functional testing will find current and voltage problems, power integrity issues, signal distortion, and multiple other signs of failure.

Pros:
• Can adapt to any type of PCB design
• Typically doesn’t require the specialized equipment other tests do
• Is a good choice for smaller-batch manufacturers that use other tests in conjunction

Cons:
• Is only as effective as it is comprehensive. Designers and engineers can be as cursory or as rigorous as they want, which may be an advantage for you.
• Takes trained technicians, not just machine tenders, to carry out functional tests

Other PCB testing and quality control recommendations

Although these testing methods are comprehensive, there are additional considerations to smooth out the process. Here are a few PCB testing and quality-control recommendations to use throughout the production process:

Log all data

Testers often only log some available data from quality control steps, such as ICT. Pass/fail is a helpful metric, but it doesn’t say anything about what may need improvement. Data are a manufacturer’s best friends in PCB quality control.

Dave Huntley, director of business development at PDF Solutions, points out, “Having assembly and consumable data that are stored and can be correlated to test data” is essential for “finding and isolating problems early in the process.”

Factory managers must ensure that they gather usable data from every step of the PCB fabrication and quality-control processes. With this flow of data and the touchpoints they provide, manufacturing environments will enable inspectors to identify trends, solve bottlenecks, and make changes that improve quality and consistency over time.

Optimize the environment

Fine-tuning the facility’s design is an often-overlooked part of bolstering PCB quality control. For example, highly angled light sources can skew the results of some machine-vision inspection protocols. Machine vision is an automated and highly accurate QC technology, but it requires low-angle lighting and other considerations during implementation.

Train and retrain appropriately

Knowing about the nuances of implementation and deployment is a matter of employee training. Workers should feel empowered and confident using the available tools, and learn how to log and organize relevant process data. Maintaining quality in PCB manufacturing requires attention to the organization’s expectations as well as changes to those expectations over time as the nature of the work shifts.

Invest in manufacturing tech

Many of the PCBs manufactured today are destined for IoT products and similar connected devices. PCB manufacturing environments benefit in turn from adopting the capabilities of the IoT. Manufacturers should retrofit existing equipment for the IoT or bring new equipment aboard that makes process data easy to gather, organize, and evaluate.

PCB quality control: The tools are in your hands

The world is in the grips of a chip shortage, distribution and sourcing challenges, and new pressures to do more with less of the Earth’s natural resources. PCB manufacturing is no different, and it requires the same level of attention to minimizing defects and waste.

About The Author

Emily Newton

Emily Newton is the editor-in-chief of Revolutionized, an online magazine exploring the innovations disrupting the scientific and industrial sectors.