AOI - Automatic optical inspection systems

Automatic or automated optical inspection, AOI, is a key technique used in the manufacture and test of electronics printed circuit boards, PCBs. Automatic optical inspection, AOI enables fast and accurate inspection of electronics assemblies and in particular, PCBs to ensure that the quality of product leaving the production line is high and the items are built correctly and without manufacturing faults.

It’s almost impossible for current electronic devices to work without a circuit board. A printed circuit board plays a critical role in electrical connection between components such as integrated circuits (ICs) that are assembled on bare boards applied in different electronic devices ranging from relatively simple equipment such as electronic watches, calculators, handheld computers, electronic communication equipment to advanced electronic devices like military, medical or aerospace products.

Need for AOI, automatic optical inspection

Despite the major improvements that have been made, modern circuits are far more complicated than boards were even a few years ago. The introduction of surface mount technology and the subsequent further reductions in size mean that boards are particularly compact. Even relatively average boards have thousands of soldered joints, and these are where the majority of problems are found.

This increase in the complexity of boards also means that manual inspection is not a viable option these days. Even when it was an acceptable approach, it was realized that it was not particularly effective as inspectors soon tired and poor and incorrect construction was easily missed. With the marketplace now requiring high volume, high-quality products to be brought to market very quickly very reliable and fast methods are needed to ensure that product quality remains high. AOI, automatic optical inspection is an essential tool in an integrated electronics test strategy that ensures costs are kept as low as possible by detecting faults early in the production line.

One of the solutions to this is to use automated or automatic optical inspection systems. Automated optical inspection systems can be placed into the production line just after the soldering process. In this way, they can be used to catch problems early in the production process. This has a number of advantages. With faults costing more to fix the further along the production process they are found, this is obviously the optimum place to find faults. Additionally, process problems in the solder and assembly area can be seen early in the production process and information used to feedback quickly to earlier stages. In this way, a rapid response can ensure that problems are recognized quickly and rectified before too many boards are built with the same problem.

Comparison Of The Major Defect Detection Capabilities Of AOI, AXI AND ICT

Defect Type AOI AXI ICT
Soldering defects
Open circuits Y Y Y
Solder bridges Y Y Y
Solder shorts Y Y Y
Insufficient solder Y (not heel of joint) Y N
Solder void N (not heel of joint) Y N
Excess solder Y Y N
Solder quality N Y N
Component defects
Lifted lead Y Y Y
Missing component Y Y Y
Misaligned or misplaced component Y Y Y
Incorrect component value N N Y
Faulty component N N Y
BGA and CSP defects
BGA shorts N Y Y
BGA open circuit connections N Y Y

Attributes and Capabilities of AOI Test

Depending on visual methods, AOI test can be applied to detect a lot of surface defects including scratches, nodules, stains, opens, shorts, insufficient or excessive solder, incorrect components, missing components, the incorrect polarity of components etc. As a matter of fact, these defects AOI is capable of going through belong to items of manual visual inspection that was mainly applied prior to the popularity of AOI test which is, however, capable of performing them quickly and accurately.

Multiple inspection objects – Automated Optical Inspection test works perfectly for both bare PCBs and PCBAs. For PCB inspection, defects are checked such as shorts, opens and insufficient solder. For PCBA inspection, issues are inspected including component soldering, polarity, and values.

Flexibility – Automated Optical Inspection can be arranged in any stage of the manufacturing line in accordance with customers’ needs and cost tolerance capacity. In order to increase inspection efficiency and reduce the corresponding cost, it’s optimal to place it after reflow soldering because most defects result from nonconformance occurring during the soldering process. The flexibility attribute of AOI test leads to a reduction of cost since it is an in-process test and as soon as issues are discovered, manufacturing or assembly parameters can be instantly modified so that later products will be correctly produced. As a result, more products won’t be rejected during a functional test or final inspection.

Low cost – Compared with Automated/Automatic X-ray Inspection (AXI), post-reflow AOI is less expensive when it comes to the solder joint defects inspection including bridges, broken joints, dry joints etc.

Working Principle of AOI Test

AOI test achieves its inspection process by scanning board surface. Depending on one or more high-definition cameras, this equipment can capture images of PCB surface with the help of numerous light sources including fluorescent lighting, LED lighting or infra-red or ultra-violet lighting. Then, a comparison will be made between the captured image and board parameters that have been input into computer in advance so that differences, abnormalities or even errors can be clearly indicated by its built-in processing software. The whole process can be monitored at any second.

The analysis of whether a board is perfect or features issues can be drawn by different algorithms applied by Automated Optical Inspection. When it comes to simple applications inspection, pixel-counting algorithm is used containing the following methods.

Template matching – Template-matching method, also called the area-based method or correlation-like method, has been popular for a couple of decades. Based on the pixel-by-pixel method, a small template is applied to a large inspection image through sliding template window. Then, normalized cross-correlation (NCC) will be calculated between the template and inspection image. The larger the value of NCC is, the higher the matching between them will be.

Object recognition – Object recognition conforms to the same principle as view method in that difference is observed through comparison between an ideal image and the captured image of an object.

Blob analysis – Blob analysis refers to a process during which the object is separated from the background first. Then with a pixel-based image applied, a blob is generated through a grouping of object pixels to inspect issues of objects.

Owing to the progress of technologies and higher demand from market for efficiency and effectiveness, some new methods start to play a major role in PCB or PCBA inspection, among which vectoral imaging technology stands out.

As a pattern location search technology, vector imaging is based on geometric feature extraction instead of complete grey scale pixel values. Under such circumstances, the image analysis is not influenced by color changes or non-linear changes. Furthermore, vectoral imaging performs better in the reduction of false failures due to the attribute of background elimination.

Automated Optical Inspection Test from East India Technologies

As a primary testing technique in PCB assembly, Automated Optical Inspection applies to fast and accurate inspection of errors or defects occurring in PCB assembly process so that high quality of PCB assemblies can be ensured with no defects after their leaving assembly line. AOI can be applied both to bare PCBs and PCB assembly. In East India Technology, however, it is mainly applied to inspect SMT (Surface Mount Technology) assembly line and for testing of bare circuit boards, a bed of nail and flying probe is used instead.

EITPL provides both online and offline AOI test to meet clients’ different demands. Online Automated Optical Inspection test features a higher degree of automation with no need of manual operation whereas offline AOI test features a lower degree of automation with the need for manual operation. Nevertheless, both of them share the same inspection principles, inspection objects and inspection methods. Click below button to get PCB Assembly Quote within AOI option.