Automated optical inspection (AOI) For PCB Quality Control

Automated Optical Inspection (AOI)

PCBWay

Automated optical inspection  or Automatic Optical Inspection (AOI) is a technique used for the quality control of  Printed Circuit Board (PCB) fabrication as well as PCB Assembly (PCBA) .

So basically this AOI  technique finds defects in both bare PC boards and assemblies to ensure that the quality of product leaving the production line are built correctly and without manufacturing faults. Hence PCB manufacturing companies like PCBWay.com use AOI Testing to ensure fault-free PCB manufacturing.

The AOI process finds uneven soldering, tombstones, missing components, and misaligned parts in the PCB assembly process.

The AOI machine takes a series of high resolution photos and stitches them together using software. There all almost no shadows and little distortion in the image of the board.

Need of Automatic Optical Inspection

Printed circuit boards (PCBs) play an important role in all integrated circuits. Its realy impossible to imagine an IC without a PCB, whether it is a relatively simple equipment such as electronic watches, calculators, handheld computers, electronic communication equipment or an advanced electronic devices such as  military, medical or aerospace products.

Despite of the rapid developments in technology, modern circuits are much more complicated than what they were just a few years ago.

The introduction of surface mount technology, and the subsequent further reductions in size make the boards even more compact. You can find an average board with thousands of soldered joints,  where the majority of problems  are actually found.

In the early periods the inspection approach for PCBs and PCBAs include manual inspection which was later realised not so effective as the inspectors soon got 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.

Current market now demands high volume, high quality products to be brought to market very quickly and hence it is required that the  inspection methods should be very reliable and fast to ensure high product quality .

This is why AOI, automatic/automated optical inspection is an important tool in an integrated electronics quality inspection strategy which detects the faults early in the production line and hence ensure the low costs  of the device.

Automated optical inspection systems can be placed into the production line just after the solder process. In this way they can be used to catch problems early in the production process.

Working of Automated Optical Inspection (AOI)

AOI systems use visual methods to monitor printed circuit boards for any defects.

They are able to detect a variety of surface feature defects such as scratches, nodules, stains, open circuits, shorts, insufficient or excessive solder, incorrect components, missing components, incorrect polarity of components etc.

The AOI system can perform all the visual checks far more swiftly and accurately as compared to the previously used manual operators.

This is achieved by visually scanning the surface of the board.

This system uses on one or more high-definition cameras, to capture images of PCB surface with the help of numerous light sources such as fluorescent lighting, LED lighting or infra-red or ultra-violet lighting.

In this way the AOI machine is able to build up a picture of the board.

Then the AOI system make a comparison between the processed captured image and the board parameters that have been already fed into the computer in advance.

Now using this comparison the built-in processing software of the AOI system is able to detect and highlight any differences, defects or even errors . This whole process can be monitored at any second.

The AOI system uses the following methods or techniques to detect whether a board is satisfactory or has any defects :

1. Template-matching Method :

Template-matching method is also known as area-based method or correlation-like method.

In this method, a small template is applied to a large inspection image through sliding template window. Then, normalized cross correlation (NCC) is calculated between the template and inspection image.

The larger the value of NCC is, the higher is the matching between them.

2. Pattern matching :

In this method the AOI system stores information of both good and bad PCB assemblies and then match the obtained image to these obtained information.

3. Statistical pattern matching :

This approach also use the same principle as pattern matching method, except that it uses a statistically based method of addressing problems.

By storing the results of several boards and several types of failure, it is able to accommodate minor acceptable deviations without flagging errors.

In order to build up the database of what the board should be, both known status boards and PCB design information is used.

As technology has improved it has been able for AOI systems to very accurately predict defects and have a small number of no defect found scenarios.

AOI image capture and analysis

The important feature of the AOI system is the image capture system. This system first captures an image of the printed circuit board, PCB assembly and then it is analysed by the processing software within the AOI system.

Depending on the application, complexity and cost of the AOI system, there are different types of image capture system available.

This image capture system may contain a single camera or there may be more than one camera to provide better image quality .

The cameras should also be able to move under software control. This will enable them to move to the optimum position for a given PCB assembly.

This  type of camera has  also an impact on performance. Speed against accuracy is a balance that has to be struck and will impact on the camera type used .

Here we will discuss two types of image capture methods used in AOI system.

  • Streaming video:   In this case one type of camera are used for the AOI. It takes streaming video from which complete frames are taken. The captured frame then enables a still image to be generated on which the signal processing is performed. This approach is not as accurate as other still image systems but has the advantage of very high speed.
  • Still image camera system:   This is generally placed relatively close to the target PCB and as a result it requires a good lighting system. It may also be necessary to be able to move the camera under software control.

AOI light source

A proper lighting source has a great impact on the AOI process. As a proper lighting source makes it possible to highlight different types of defect more easily.

Most AOI systems have a defined lighting set depending upon the operation required and the product types to be tested.

Different types of  lighting available are :

  1. Fluorescent lighting:   The most widely used lighting used for AOI application is the Fluorescent lighting as it provides an effective form of lighting for viewing defects on PCBs. The main problem with fluorescent lighting is that the lamps degrade with time.
  2. LED lighting:   LED lighting  is a far more stable form of lighting for the AOI system applications. Although LED lighting does suffer from a reduction in light output from the LEDs over time, this can be compensated for by increasing the current. Using LED lighting, the level of lighting can also be controlled. LEDs are therefore a far more satisfactory form of lighting than fluorescent or incandescent lights that were used years ago.
  3. Infra-red or ultra-violet:   Infra-red or ultra-violet lighting are required to enhance certain defects, or to enable automated optical inspection to be carried out to reveal certain types of defect.

The positioning of the lighting for an automatic optical inspection system, AOI, is as important as the type of lighting source. The light sources require positioning not only to ensure that all areas are well light,  but also to highlight defects.

AOI system programming

There different methods that can be used to programme an AOI system are :

Use of “Golden Board” :

In this method, the AOI sytem is provided with a known good board as target board. This board is then processed through the system so that it can learn the relevant attributes such as the components, the solder profiles of each joint, and many other aspects. In order to provide the system with enough variance data several boards are often required.

Algorithm based programming :

In this method, PCB data is provided to the system and it then generates its own profile for the board. This scheme will also require real boards, but fewer are generally required.

It is really very important that any PCB manufacturing area should be able to check the quality of the boards coming off the end of the line. So that they can monitor the quality and when problems are detected they should be able to rectify the process so that further boards are not affected by the same problems.

aoi test

So, if you want to buy PCBs, you better ensure that they have been AOI Tested. One way to do this is by ordering PCBs from PCBWay.com, as they conduct AOI tests before manufacturing the ordered PCBs.