Reliability is paramount to how electronic products and brands are perceived. For most people, electronics troubleshooting goes as far as swapping batteries or disconnecting/reconnecting power. I’m reminded of when I fixed an LED end lamp for my mom by noticing a resistor was no longer making contact with the circuit. After a quick run to a friend’s house to borrow a soldering iron, I reestablished continuity. I was lucky that this was about as minor of a fix as possible, but tasks like these are not always something a person has the knowledge, tools, or time to handle.
Manufacturers are vested in testing PCBs to verify and refine production processes. The earlier they catch design or production errors, the less time and money will be lost. To meet the challenging standards associated with making PCBs, manufacturers employ many quality assurance tests as feedback.
Looking At (and Through) Boards for Manufacturing Process Validation
Visual inspection forms an invaluable lead-in to reliability testing. Advancements in optical technology have increased the throughput of these systems while increasing total accuracy. What’s more, the implementation of machine learning has enabled deep learning models in optical machines that analyze and improve their processes with large enough sample sizes:
- Automated Optical Inspection (AOI): A system of cameras that uses photos or videos to detect manufacturing defects. When compared against a board deemed a gold standard (typically provided by operators, although recent developments have allowed for the aforementioned deep learning models), the system can catch many assembly issues at the solder bonds or relating to part placement. Another update to the technology is 3-D visibility, allowing for confirmation of component height for verifying fit within the enclosure or interfacing connectors as aligned in the z-axis. AOI is only capable of a direct-line view, so it cannot assess bonds occurring beneath devices such as in BGAs. Additionally, the method can be impacted by camera positioning, light angles, and component placement density. All of these factors can result in features being partially obscured or shadowed, increasing the verification’s difficulty.
- Automated X-ray Inspection (AXI): AXI goes a step beyond AOI by using a high-frequency light source that pierces through the various items of the board instead of reflecting from the surface. Notably, this allows the viewer to see through any packages with solder joints hidden from a top-down perspective. Even for components with adequate visibility in AOI, AXI can capture the detail of the solder heel (the portion of the solder joint that extends inward towards the component side) for a more thorough evaluation. Finally, AXI can see within the solder profile, pinpointing any material vacancies that could contribute to early failure.
- Scanning Electron Microscope (SEM): SEM is an extremely powerful resolution tool, able to view at magnifications magnitudes greater than contemporary visual technology. Unlike AOI and AXI, SEM typically involves destructive testing where the workpiece is sacrificed for a cross-sectional view of the item. Importantly, SEM can identify granular failures like cracks, voids, and inclusions even when the processes used to produce these boards can pass less invasive tests. Discerning failures by this route provides the most immediate and diagnosable recognition of failure routes. Advantageously, SEM can also use coupons built on the edge of the panel that is intended for destructive testing for fabrication process verification.
Testing PCBs for Ruggedness, Reliability, and Response
Once the inspection has concluded, the board is ready to test properly. Though the specific feature or functionality under testing may differ, the general purpose is to assure the board will operate as intended in conditions similar to what it will experience as a final product:
- Burn-in testing: Burn-in testing is a stressor to determine the components’ longevity and the overall assembly. Boards are operated beyond the manufacturer’s rated guidelines for voltage and temperature as a secondary check for infant mortality after the components leave the manufacturer. Running boards at values beyond the manufacturer’s recommendations for a period (depending on requirements, up to two days) effectively simulates a board operating under lengthy yet standard end-use.
- In-circuit testing (ICT): A board may have thousands of connections, and shops require a method to evaluate these simultaneously. Boards are placed in a test fixture known as a bed-of-nails, where spring-loaded pins make contact with test points (placed by the designer) that check for basic circuit characteristics such as resistance, capacitance, and the presence of opens/shorts. There is more run-up time and expense involved with a bed-of-nails fixture than a flying probe, but over large production runs, the per-board speed of each bed-of-nails test proves more valuable than the alternative. However, the flexibility of the flying probes can prove useful in small lots, prototyping (where a revision would necessitate a new bed-of-nails fixture), and densely populated boards.
- Functional testing: The final operation of most PCBs is a synthesis of hardware and software, and unlike the tests mentioned above, functional testing engages the latter. Scrutinizing functionality is much more ambiguous than evaluating a binary pass/fail condition, and the cost and testing conditions reflect a more specialized approach. It is far more difficult to provide a blanket statement of what constitutes a successful functional test. Therefore, engineers and technicians must be intimately familiar with the board, its features, and the most valid expression of its design intent.
Your Contract Manufacturer Is Up To The Mark of PCB Testing
Testing PCBs is as involved and technical as their construction, and for a good reason: shops want the confidence of knowing their methods are sound before engaging in mass production or further processing. End-users demand highly reliable devices that meet their expected service life with minimal (ideally, zero) interruptions. There is no singular analysis that can accurately diagnose a board at any stage of design. Still, a composite of several tests can paint a full picture with varying levels of destructibility. If your boards are experiencing issues, whether in production or out in the field, VSE can help. At VSE, we’re a team of experienced engineers who build electronics for our customers. Coupled with our trusted manufacturing partners, we aim to deliver nothing less than excellence in all our PCBs.
