What Is a PCB Inspection System?
A PCB is a device that inspects for problems such as misalignment, disconnection, shorts, cracks, floating parts, and soldering of components mounted on the board. This inspection is called board visual inspection.
In addition to functional testing of electronic circuit boards to ensure correct operation, board visual inspection (also called board inspection or mounting inspection) inspects whether each electronic component is mounted in the correct position without defects.
Electronic board inspection is called AOI (Automated Optical Inspection).
If there are no problems in the board visual inspection, a functional inspection is performed to ensure that the product actually works as designed.
Function testers are used for this functional inspection.
Uses of PCB Inspection Systems
Until now, board visual inspections have been performed visually by human eyes.
However, with visual inspections, the criteria for acceptance or rejection may differ depending on the subjectivity of the inspector.
In addition, the more inspection items are required, the more personnel are needed, resulting in higher labor costs.
In addition, electronic circuit boards are generally manufactured in a factory on a production line. Visual inspections have a limited processing capacity, and this limits the speed of the production line.
PCB visual inspection systems are introduced to increase efficiency and reduce costs by automating inspections that were previously performed by human labor.
Principle of PCB Inspection Systems
Since PCBs use a machine to perform what was originally done visually, it requires “eyes” to see the appearance and “brains” to judge whether it is good or bad, just like a human being.
Therefore, the PCBs consist of a camera as the “eye” and a computer equipped with image processing software as the “brain”.
The most common defect in board visual inspection is soldering defects.
The PCBs determine whether soldering is good or bad by using a straight line connecting the boundary of the solder adhesion surface and the electronic component adhesion surface as a threshold value and whether or not it exceeds this value.
Since the threshold value varies depending on the shape of the component and other factors, such as the electronic board, it is necessary to input all kinds of threshold data into the image processing software.
In recent years, to alleviate this complication, three-dimensional imaging with multiple cameras and the use of X-ray cameras have made it possible to detect defects that cannot be detected with ordinary cameras.
False Alarm Rate
The false alarm rate is one of the most important indicators for inspection equipment.
Although it is as old as 2009, here is an article describing the false alarm rate data from a survey on the current status of product inspections conducted by the Japan Institute of Electronics Packaging to Japanese manufacturers of printed circuit boards and electronic devices.
Inspection of Board Mounting by PCB Inspection Systems
Electronic boards manufactured by manufacturers can have various types of defects. For this reason, various types of PCB inspection systems are used to identify boards with defects. There are various types of board defects, the most common of which are listed below.
1. Component Defects
- Component Not Mounted: Component is not mounted in the correct position
- Component misalignment: A component is removed from the pad and mounted out of the correct position.
- Component floating: Only one side of the component is soldered and the other side stands up.
2. Solder Defects
- Disconnection: No solder is applied
- Short: Excessive amount of solder adhering to adjacent pads
- Void: A defect caused by air bubbles during soldering
- Poor wetting: Solder is clean but electrical contact is incomplete
- Solder ball: A ball-shaped lump of solder
- Bridging: Solder is connected between adjacent IC pins
- Cracks: Cracks on the surface of the solder
- Imo-solder: Solder does not bond well and the solder is bumpy like a potato
What Is AOI?
AOI stands for Automated Optical Inspection. It is an automated visual inspection in which an electronic board is photographed by a camera or other means to inspect for serious defects and quality defects. Because optical inspection is non-contact and non-destructive, it has been introduced in many board manufacturing processes. Methods used in automated optical inspection include camera-based imaging, laser reflection, and X-ray transmission.
Visual inspections used to be performed by human operators, but they are now widely used for a number of reasons, including the fact that it is no longer easy to distinguish between small, integrated boards due to the increasing integration of circuits, cost reductions and productivity increases due to labor and manpower savings, and quality value enhancement by reducing human error.