What is a conformal coating Automated Optical Inspection (AOI) system?

An AOI system for inspecting conformal coated circuit board is to replace a manual operator who physically has to sit and inspect every single board at the end of a line. Conformal coating inspection is a critical and often neglected stage of the conformal coating process. It is the last area of the assembly line that is 95% manually operated around the world yet it is one of the most critical factors.

There have been several challenges to be met in AOI for inspecting conformal coating. The fact that the coating is transparent so everything under the coating can be seen , the coating fluoresces under UV light and the PCBs are 3 dimensional means that the process offers a lot of challenges.

However, companies like Modus have 25 years of experience with image processing and were pioneers at developing scanner based industrial image processing systems. The solutions Modus provide today are complete software and hardware development from one source with specific long time experience in the automotive area.

SCH Technologies are working as a partner with German company, Modus, to provide an automated conformal coating inspection system for automatic inspection of conformal coatings and defect detection.

Conformal coating inspection board from modus.

Why do I need a conformal coating AOI system? I have an expensive selective robotic system so this should be fine.

Conformal coating inspection is a critical process yet is the last part of the coating line that is generally not automated. The general industry still relies on an operator visually inspecting a circuit board by eye under UV light. However, it is known that operator fatigue leads to errors in inspection and PCBs that are coated incorrectly will get past the inspectors.

Further, anyone who uses a robot system knows that the system can go wrong. The selective robot dispenses a liquid. This process can go wrong in a variety of ways including

  • The coating drying out in the valve leading to a blockage or misdirected coating onto the board
  • The material running out and not enough deposited
  • The PCB put in the wrong way or the wrong PCB for the program put in the machine
  • A component out of alignment leading to a collision of the valve with the board
  • The wrong viscosity put in the machine or the viscosity changing with temperature
  • The material curing in the machine causing a blockage

It is possible to engineer many of these problems out with options purchased at the time for the selective robot. But, ultimately, no manufacturer of material or equipment will guarantee no problems with their process. So, an inspector is needed. Use a human operator and you can introduce inspection errors.

Therefore, an automated conformal coating inspection system can be used to remove the operator error in the line and improve the process.

What is parallax and why is this important in my inspection of conformal coated circuit boards?

Parallax is defined as a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

How this relates to conformal coating AOI is that with a fixed camera or a standard scanner the camera is at a fixed point and the circuit board with its 3D components may have “shadows” where the camera cannot see down the side of the components. This can be seen in the two photos below.

Parallax free image for conformal coating inspection

Parallax in image for conformal coating inspection

Figure 1: Camera directly above image with the image showing all detail down the side of the components

Figure 2: Camera slightly to the side. Hidden areas behind the components are now shown clearly.

How do I avoid parallax in my conformal coating process?

To achieve parallax free processing you have to use a specialist lens which removes the parallax issues from the scanning process and eliminates shadows down the sides of high components.

Fixed camera solutions or scanners with a fixed camera system at the centre can suffer from parallax where high sided components can cause shadows in image processing.

Modus have made unique technological advances in parallax-free image capturing for conformal coating processing. They use a specialist lens on their scanner based system that adjusts the lens focus so the image appears to be viewed from above at all points along the scan.

Click Automated Conformal Coating Inspection System for further information on the Modus system.

What is the maximum width of the pallet that I can inspect?

What is the maximum width of the pallet that I can inspect?

We will configure the system whether this is for the cameras or scanner based technologies for this width. So, if you require a 300mm conveyor width and inspection window we would provide the camera or scanner solutions to this request.

What is the maximum height of components on the PCBs?

This can be critical so important to know due to the length of the machine required and the imaging technology used in the conformal coating AOI.

The first question to ask is are the PCBs 3D in nature or very flat? This helps determine whether we use camera or parallax free scanning technology?

Take a look at our conformal coating AOI parallax link here to understand the issues more.

What standards can the conformal coating AOI systems inspect to?

The systems have a variety of software options right up to inspecting TRUE IPC Class III standards. The reason we state true standards is that most people are not inspecting to this level even though they think they are. This is due to two reasons:

  1. It is very difficult to inspect to IPC Class III standards as it is under magnification, is time consuming and challenging for operators.
  2. Also, it is very difficult generally for customers to produce conformal coating finish to this level wihout creating lots of defects.

The key to all inspection, manual or automatic, is to decide what you are looking for.

What type of conformal coating defects are you looking for?

Are you looking for bubbles? Are you looking for foreign bodies? Or is it just coating in the right place and coating not in connectors? These questions are related to the inspection criteria you desire and it is very important to define exactly what are you actually looking for.

The software we use can measure all of these factors. The key is to understand whether your production process can stand up to the level of inspection / interrogation that the machine will provide?