As electronic products become increasingly complex and smaller in size, multilayer boards have become the preferred choice for PCBAs (Printed Circuit Board Assemblies) for many high-performance electronic devices due to their higher wiring density and better electrical performance. From smartphones and servers to medical devices and automotive electronics, multilayer boards are ubiquitous. However, the introduction of multilayer structures, while enhancing design capabilities, has created significant challenges for PCBA testing. Unlike single- or double-layer boards, the complexity within multilayer boards makes defects more difficult to find and locate. This article will delve into the major challenges facing multilayer PCBA testing and analyse the key strategies within the industry to address these challenges, while highlighting the fundamental role of high quality PCBA processing within it.
The unique construction of multilayer PCBAs poses several major testing challenges:
1、Internal defects are difficult to detect: the wires and connection points of multilayer boards are distributed over multiple layers, including inner layers, buried holes and blind holes. These internal structural defects (such as short circuits in the inner layer, circuit breakers, buried holes or blind holes in the connection problems) in the PCBA processing is completely ‘hidden’ up, the naked eye or ordinary optical inspection can not reach.
2、Test point access is limited: In order to achieve high-density wiring, design engineers tend to arrange the signal line in the inner layer, or use buried blind hole technology to reduce the outer pad. This leads to many circuit nodes can not be set in the outer layer of the physical test points, making it difficult to achieve comprehensive coverage of the traditional probe-based test (such as ICT).
3、Processing defects hidden high: multilayer board manufacturing involves lamination, drilling, plating and other complex processes. pcba processing may occur during the interlayer alignment is poor, uneven plating, internal pad contamination and other issues, the defects caused by its hidden inside the board, increasing the difficulty of testing.
4、Signal integrity testing is complex: high-speed digital signals or high-frequency analogue signals transmitted within the multilayer board, susceptible to impedance mismatch, crosstalk, interlayer coupling and other issues, these electrical characteristics of the problem is difficult to verify through a simple pass test, the need for more complex dynamic or functional testing.
5、The difficulty of repair and high risk: If found in the test is a multilayer board internal (such as the inner layer of the line or buried blind holes) problems, usually difficult to carry out effective repair. Even if it is a component solder joint problem, due to the board layer, heat dissipation, improper rework is easy to damage the internal structure.
To cope with the test challenges of multilayer PCBA, it is necessary to comprehensively apply a variety of test methods and strategies, and work in concert:
1、Strengthen the manufacturing process testing: start from the source of the multilayer board.
Bare board stage test: before the PCBA assembly, the multilayer PCB bare board for rigorous electrical testing (flying probe test or special fixture test), to detect whether there is an internal short-circuit, circuit breakage or poor connection, which is the most effective means of discovering structural defects in the multilayer board itself.
High-precision X-ray inspection: After PCBA processing is completed, X-ray inspection is essential to check the quality of hidden solder joints (e.g., voids, offsets, consecutive tin) underneath packages such as BGAs and QFNs, as well as some of the internal over-hole connections.
Advanced AOI/SPI: AOI and SPI combined with 3D technology and algorithms can provide more accurate information about the outer layer of solder paste and component placement.
2、Design-based electrical testing: Using design means to compensate for the lack of physical test points.
In-circuit testing (ICT) and test point optimisation: Despite its limitations, ICT is an important means of detecting manufacturing defects. ICT coverage can be maximised by strategically placing critical test points in the outer layers during the design phase (DFT – Design for Testability) in collaboration with the test team.
Boundary Scan: For multilayer PCBAs containing boundary-scan compatible (IEEE 1149.x standard) chips (e.g. processors, FPGAs), boundary-scan technology can test the interconnections between chip pins non-contactly through the test logic inside the chip, which greatly reduces the dependence on the physical test points, and is a powerful tool for testing complex digital multilayer boards. It is a powerful tool for testing complex digital multilayer boards.
Flying probe test: In the case of restricted test points or small batches, flying probe test is still a flexible electrical test option.
3、Comprehensive functional test (FCT): to verify the overall performance of the PCBA in the simulation of the actual working environment.
FCT can test the actual transmission quality of high-speed signals, the complex interaction of control logic, as well as the power consumption and stability of the entire board, to make up for the performance of the electrical test can not fully cover the blind spot.
4、Environmental and Reliability Testing: Expose potential hidden defects.
High and low temperature cycling, heat and humidity, vibration and other environmental stress test, especially HASS/HALT highly accelerated stress test, can simulate long-term use or extreme environmental conditions, accelerate the exposure of multilayer board internal connections (especially over the hole) or components due to the accumulation of stress and the potential defects.
The quality of PCBA processing, especially the multilayer board manufacturing process itself, has a decisive impact on the validity of subsequent tests. If in the multilayer board manufacturing process of poor interlayer alignment, the inner layer line defects or buried / blind hole problems, these defects in the PCBA processing assembly is almost impossible to repair, and difficult to fully detected by conventional electrical testing. Even the most advanced X-ray or boundary scan can only detect specific types of defects.
Therefore, a high standard of PCBA processing, from the selection of raw materials, inner layer production, lamination, drilling, plating to the outer layer of graphic production, soldermask printing and other aspects of strict control, is to reduce the potential defects of multilayer boards, improve test efficiency and ensure the reliability of the final product of the cornerstone. If you fail to strictly control the quality at the PCBA processing stage, even if you invest more testing resources, it is still difficult to completely screen out all potential quality problems, which ultimately affects the performance and reliability of the product.
Multilayer PCBA testing faces unique challenges due to its complex structure and hidden defects. Addressing these challenges requires a combination of bare board testing, advanced design-assisted testing techniques such as X-ray inspection and boundary-scan, and comprehensive functional and environmental testing. At the same time, it is important to recognise that high-quality PCBA processing is a key prerequisite for ensuring successful multilayer board testing. Only by controlling quality at the source and adopting a targeted approach in the testing process can the performance and reliability of multilayer PCBAs be effectively tested and guaranteed to support the development of modern complex electronic systems.