Automated Visual Inspection (AVI)

Automated Visual Inspection (AVI) is a relatively new process that visually inspects the final bare PCB before being assembled. The AVI process removes the human factor in the final inspection step. However, there are many experienced human inspectors who can determine an acceptable board or rejectable board faster than an AVI machine. The AVI machine when setup adequately can consistently give the same inspection board by board or lot by lot. All PCBs are accepted or rejected with the same algorithm.

AVI, automated visual inspection is an automated vision inspection of PCB is performed in one of the final steps before the PCB are shipped to the assembler or are assembled. After scanning by an AVI machine, the defects which do not meet the manufacturer’s requirement will be identified by the machine. Therefore, AVI will increase the quality/cosmetic quality of PCBs.

About the AVI Machines

The basic principles of the AVI machines are to have a motion system in combination with a camera and specialized software. AVI machines take advantage of colour cameras and led lights. Modern AVIs use a conveyor to transport the PCB into the machine and have cameras on the top and bottom side to inspect both sides of the PCB at the same time.

The Camera

On all AVI machines, colour is an important concern as it is used for cosmetic inspection. They must detect some contamination or discoloration on PCBs, so color cameras are more commonly used than monochrome camera for AOI machines.

In digital imaging, a pixel (or picture element) is a single point in a rasterized image. The pixel is the smallest addressable screen element; it is the smallest unit of picture that can be controlled. Each pixel has its own address. The address of a pixel corresponds to its coordinates. Pixels are normally arranged in a two-dimensional grid and are often represented using dots or squares. Each pixel is a sample of an original image; more samples typically provide more accurate representations of the original.

The intensity of each pixel is variable. In color image systems, a color is typically represented by three or four component intensities such as red, green, and blue, or cyan, magenta, yellow, and black.

The number of pixels is also an important parameter of cameras. Larger number of pixels does not mean it can detect smaller defects. To detect smaller defects, smaller pixel size is required. Pixel size is calculated by scan width / number of pixels. For example, the scan width is 2 inches and number of pixels of camera is 2000. Pixel size =2 inches/2000 = 1mil. Some machines use cameras with fewer pixels. But they can use more cameras to compensate for it. Normally the exposure time for more pixels CCD is longer. So, the scanning time of more pixels CCDs is longer too. Therefore, some machines use multiple CCDs with less pixels rather than using one camera with more pixels.

Detecting Defects

For AVI, the main two factors that determine whether the defects can be identified are the size and brightness/color of defects. The pixel size of the AVI machine determines the minimum size of defects it can detect. The pixel size can be seen as the basic unit of the scanned image. Therefore, the minimum defect size detectable must be larger or equal to pixel size. But even if the defect size is the same as the pixel size, the defect might be very difficult to detect. For example, in the following case, defect size is same as pixel size.

For case A, it is the best situation where the defect is inside the pixel, so the defect is easily detectable. For case B, it is the worst situation as each of the four pixels can only see the 25% of the defect, so it may not detect the defect. Therefore, if the defect size is very small and is close to the pixel size, then it may not be detected by the machine.

Case for defect size equal to pixel size

Another factor which makes the defect difficult to detect is brightness/color of defect. If the brightness/color of the defects is close to normal area, then it will be difficult for the machine to detect the defects. The common missing defects on AOI and AVI are scratches and dents.

Blue– Missing pad area

Red– Scratch on gold surface

Gold- Open under the mask

Case for defect size equal to pixel size

A verification station is used offline to the AOI to verify or repair defects.

Why Gardien’s Solution?

Your Gardien Local Service Centre may offer this service. The AOI tooling and production services are controlled by Gardien’s proprietary job flow system called Ontrack (link to Ontrack). This creates a seamless and error free flow of information from the customer supplied data to the Service Floors at Gardien.

Gardien’s team is trained and qualified on all internal process as documented Quality Management System. The internal process has specific inputs for incoming, certified, and not good boards as well as descriptive educational programs on various board types, and surface finishes.

Gardien certifies each order processed with a Certificate of Compliance with details about how the order was processed, what specifications were used to certify the product, equipment used with calibration expiry date, team member who processed the order, quantity, and failure analysis.

Gardien strongly recommends that the test sequence and parameters are clearly stated on the manufacturing drawing or at a minimum agreed upon during the quoting or contract realization phase. In addition, any PCB sent to us for processing should have a unique identifier on each peace for electronic traceability.