Irrespective of the goal of having a printed circuit board or PCB in a device, it is imperative that the PCB functions properly. As the performance of a product depends primarily on its PCB, malfunctions can be serious.
To ensure the PCB will function as intended, manufacturers inspect it at different stages throughout the design, fabrication, and assembly processes. Inspection is a crucial activity for ensuring the product not only operates as expected, but also meets quality standards. The increasing complexity of PCBs today leads to new capabilities, and at the same time, gives rise to higher chances of failure. Therefore, inspection techniques and technologies have also evolved to keep pace and to ensure the quality.
Need for Inspection of PCBs
Inspection is a critical step during any PCB fabrication and assembly process. Inspection helps to locate defects, and correcting them leads to an overall improvement of the operation. It also helps in revealing existing design flaws. Inspecting PCBs after each phase of the process enables locating flaws before moving on to the next stage. Correcting the process for eliminating flaws requires much less effort and time than when repairing a defective product in the field. The inspection process helps in ensuring consistent quality of the products, builds confidence in customers, and reinforces the manufacturer’s reputation.
Without inspection, there is a high probability of defective boards reaching customers. Customers receiving defective products may ask for returns or warranty payments, and the manufacturer may lose money that way. A still bigger issue is the loss of trust customers had on the manufacturer, and the resulting loss of reputation. The situation may even lead to customers taking their business elsewhere, and the manufacturer missing business opportunities.
A defective PCB may cause injury when it is part of an automation component or even death if it belongs to a medical device. Although this is a worst-case scenario, such disasters can lead to severe damage to reputation and even expensive litigation for the manufacturer.
Inspection during the manufacturing and assembly process of PCBs can help in improving the overall production process. AOI assists in uncovering frequent defects, allowing staff to take immediate action to correct the flaw at the process level. Regular PCB inspection therefore, contributes to continuous improvement of businesses.
Automated Optical Inspection (AOI)
Although manual optical methods of inspection are still common during fabrication of boards, the manual process of optically inspecting a PCB is rather slow considering the high volumes of production. It leads to operator fatigue and human errors. As a result, the automated method is becoming more popular in the PCB industry.
Fig 1: An AOI System
During PCB manufacturing, an AOI system typically uses a rotating camera to scan the board for capturing images of copper pads and traces on the PCB. It then uses computers and algorithms to compare the captured images to eligible parameters in a database. The system processes the images to detect and highlight defects, displaying them on a monitor to allow the staff to realign work processes.
Inspection Goals for AOI
Manufacturers use AOI mainly for achieving the following inspection goals:
Final Quality: this establishes the status of the PCB before they leave the manufacturing line. This inspection goal does not consider manufacturing issues, but is an important process when the PCB is complex, and speed and throughput is a major concern.
Implementation requires placing the AOI equipment at the end of the fabrication line, where the system can generate a large amount of information related to process control.
Process Quality: this monitors the fabrication processes for generating detailed information on defects and their classification. Manufacturers typically place AOI equipment at several places along the PCB fabrication line to monitor specific manufacturing situations, thereby providing necessary adjustments to manufacturing processes and techniques.
Strategic Locations for AOI
Manufacturers prefer to place AOI systems at strategic locations where they can recognize most defects and let the staff make necessary adjustments. Two strategic locations are most popular:
1. After Creation of Each Layer
2. After Bonding of Final Layer Pair
After Creation of Each Layer
For a multi-layered board, the fabricator must drill and etch the two copper layers on the central core first, and then drill and etch each layer independently, before bonding them in pairs on to the central core. Manufacturers use AOI systems for detecting defects on the core and each layer separately, such as for:
- Pin Holes
- Open Circuits
- Missing Conductors
- Missing Pads
- Missing Holes
- Wrong-Size Holes
- Drill-Hole Breakages (Hole Outside Pad)
- Conductor Spacing too Close
- Spurious Copper
- Excess Copper
Presence of most of the above defects in a layer may be catastrophic or may affect the quality of the final board. Either way, it is important to detect them before bonding the layers to alter the process parameters effectively, thereby preventing their occurrence—AOI systems do this very effectively.
Fig 2: Defects during PCB Manufacturing
Some defects may affect the functioning of the final board. For instance, under-etching or overetching may affect the conductor width resulting in violations of system integrity. Improper conductor width and spacing can change the impedance of the section in a board handling highfrequencies, high data transmission rates, and high-power loads.
The AOI system verifies the copper conductor trace image on the layer for deviations from the Gerber data, and highlights the errors.
After Bonding of Final Layer Pair
The final layer pair undergoes a somewhat different process than that for the inner layers—pattern exposure, copper plating, tin plating, drilling, etching, and tin stripping. Therefore, the creation of the final layer pair requires a separate AOI process, although the type of defects the AOI system detects remains the same as for the other layers.
AOI Strategies in PCB Manufacturing
The fabricator programs the corresponding Gerber data the designer has used for creating the circuits into the AOI system. The system uses this data as a baseline reference for comparing the circuits on each layer of the board. The camera of the system scans the panel for any obvious defects and consistency with the data in its memory.
When it detects a flaw, the AOI system brings up the image of the area on the display screen. The fabricator inspects the area and decides on the action necessary for correcting the problem. Usually, they mark the area and complete the scanning of the entire board before assigning any steps necessary for maintaining the quality of the board.
AOI systems function mainly through processing the images that the camera captures. Effective image capturing and processing is highly dependent on proper illumination of the board. The main lighting parameters affecting the systems capabilities are:
- Spectral Profile
The camera is never an ideal optical system, and requires calibration and compensation for different types of distortion to obtain an acceptable image.
AOI systems use various algorithms for edge detection, smoothing, image subtraction, segmentation, thresholding, and transformations for generating a proper image of the board for comparison and matching.
Most AOI systems use template and feature matching to compare the captured and processed image to the Gerber data. The matching process involves complex mathematical calculations for generating the final data.
AOI in PCB manufacturing is a highly significant process that keeps a check on the performance standard and quality of the entire manufacturing line, while ensuring on-time delivery. With improvement in technology, AOI systems have evolved to a stage where they can accurately predict defects, leading to continuous process improvements. As such, AOI systems form a necessary part of sophisticated manufacturing environments.