PCB inspection, in its simplest form, is the process of identifying and correcting faults that might have occurred during the fabrication and assembly stages. This is accomplished via a number of mostly automated processes.
PCB inspection is necessary at many points during the assembly process. A printed circuit board is inspected after it has been created to look for any potential faults or flaws before shipping. After the PCB assembly components are joined to the board, there is a second inspection.
The IPC has established standards for what makes a good board. For assembly inspection, the IPC-A-610 inspection standard is utilised, but for bare board inspection, the IPC-A-600 is the standard.
There may be stricter regulations for testing that must be followed depending on the class of circuit board being made. The highest inspection frequency is used for Class 3 printed circuit boards since they have no room for error and are used in high temperature situations including commercial aircraft, space shuttles, and PCB environments.
THE EVOLUTION OF PCB INSPECTION METHODS
Compared to modern designs, the early printed circuit boards were substantially simpler. The main type of inspection was called MVI, or manual visual inspection, and it consisted of significantly fewer solder connections and parts. This procedure was as easy as inspecting a board to make sure all of the parts were connected correctly.
Manual visual inspection has fallen out of favour due to the exponential increase in printed circuit board complexity over time. This approach was viewed as producing subpar printed circuit board quality as printed circuit board complexity increased and was highly susceptible to failure due to human mistake. If flaws are not discovered in a timely manner, they result in faulty boards that get more expensive and wasteful as they move down the production line because they get harder to correct.
The process of visual inspection has mostly been automated to reduce human mistake. Inspections can go considerably more quickly and accurately when combined with surface mount technology. Smaller PCBs and multilayered designs made possible by surface mount technology necessitate the use of additional techniques, including X-rays, for early fault diagnosis.
TYPES OF PCB INSPECTION
Automated Optical Inspection (AOI)
Automated optical inspection employs cameras to take pictures of printed circuit boards, which are then compared against intricate schematics to make sure the board is error-free. A comprehensive profile of the board is created by taking many images of it at various angles and combining them.
It is especially helpful for spotting problems like scratches, nodules, stains, thin solder, open circuits, and short circuits early on in the printed circuit board production process. Additionally, AOI systems can find ineffective or missing parts.
In order to produce even more trustworthy results, 3D cameras have lately been used in AOI systems. For height-sensitive gadgets, this is crucial because 2D camera systems rely on coloured lighting and side-angle cameras to produce these effects.
The technology known as AOI is widely used in PCB production and is capable of spotting the most frequent PCB flaws. To find mistakes early, AOI devices are frequently installed at various locations across a facility. These systems, however, are constrained by what they can see because they are built around cameras. Although 3D cameras can identify height differences, they are unable to see concealed connections that are present on multilayered boards. They might not be able to inspect components that are obscured or shadowed on densely loaded boards.
Due to these shortcomings, AOI is not the only inspection technique used by the majority of printed circuit board producers. The requirement for faultless inspection, particularly in Class 3 PCB manufacture, has led to the creation of new inspection technology.
X-Ray Inspection (AXI)
Smaller components and the introduction of surface mount technology, or SMT, have allowed boards to be more densely populated and have over 20,000 solder connections. These boards’ solder connections are so tiny that AOI equipment cannot inspect them. Additionally, multilayered boards have parts that are hidden beneath layers that are not visible from the surface. A printed circuit board may be seen in a lot more detail with an X-ray inspection than by merely looking at its surface.
The fact that the components used in printed circuit boards are substantially lighter than the solder used to connect them is a benefit. This enables these details to be seen on X-ray inspections with a high level of clarity. Since X-rays don’t rely on visible light to see through objects, they can detect faults that could be caused by shadows or concealed parts.
Why don’t X-ray inspections serve as the primary method of PCB inspection? For lower-class printed circuit boards that lack the intricacy seen in modern chip packages, it is not necessary to employ this newer technology because it is expensive to buy and operate. The usage of these equipment necessitates experienced operators, and the procedure can be very time-consuming, adding to the cost to customers.
PCB DEFECTS FOUND THROUGH INSPECTION
The process of making PCBs is both an art and a science, and even though the stakes are higher than ever, mistakes and faults can still occur. The deposition of solder paste is responsible for the majority of mistakes. A printed circuit board becomes unusable when there is too much solder because it might lead to bridged circuits and shorts.
The best strategy for total cost reduction in PCB production is to find flaws as early as possible. Inspection is a crucial tool for fixing mistakes before they become disproportionately expensive, but it must be used in conjunction with PCB testing techniques to guarantee that the circuit boards are operational. For instance, while almost all soldering flaws may be detected by AXI systems, some component flaws can only be discovered by inspecting the board directly.