Saturday, June 23, 2018
Home/Current Issue >  Industry Articles >  Test and Measurement > 

How AOI Ensures Conformal Coating is Working
AOI system designed for UV inspection of conformal coating.

Everyday electronic devices such as mobile phone handsets are increasingly expected to withstand environmental stress while offering greater quality and longer lifetime; attributes that can be enhanced by conformally coating selected areas of their PCBs. In safety related electronics for automotive applications, or high quality industrial electronics, coating is usually regarded as essential. It provides protection against moisture, dust, chemicals and temperature extremes that could otherwise cause a complete hardware failure.

Increased demand for conformal coating has been matched over recent years by steady improvements in coating technology, which now allow effective and accurate results. For example Nordson ASYMTEK has patented a "Film-Coating" technology. This process allows a precise and repeatable closed pattern to be deposited within designated areas of the printed circuit board, with virtually no overspray. A platform such as Nordson ASYMTEK's Select Coat® system automates the selective conformal coating process and allows its easy integration into existing production lines. The Select Coat's built-in closed loop control ensures superior coating quality, while reliability, accuracy and throughput are maintained.

Automated Inspection Systems
A production line operator must have ongoing confidence in his conformal coating system's output. Just as for any other manufacturing process, traceability and quality assurance are essential. Therefore, any critical coating process includes checks to ensure the correct position of the coat, that the coating material hasn't strayed into any prohibited areas, critical parts are fully covered and that no splashes or drips have contaminated components such as connectors or headers.
UV image, angled front side.

Nordson YESTECH now supplies a fully automated system to provide this type of inspection and traceability — the Automated Coating Inspection (ACI) system, introduced in late 2009. The ACI system detects the fluorescence of conformal coatings in UV light. It draws on Nordson YESTECH's 15 years of experience developing and supplying Automated Optical Inspection (AOI) and Automated X-Ray Inspection (AXI) systems used for inspection of component placement and soldering on assembled PCBs.

US Military Application
Soon after its launch, the ACI system found its first application for a major US military manufacturer. This company was experiencing difficulties with the time taken for manual inspection of the coating and with human fatigue factors, allowing coating problems to evade detection. They needed a solution that could reliably detect the presence of the coating material and accurately find the edge position of the coated areas. Checking for sufficient coat on critical parts while ensuring that there was no stray material in other sensitive areas was also essential. The manufacturing process also called for the ability to generate programs quickly to cope with small batch sizes.

The system also had to demonstrate better repeatability and reliability than the human inspectors, and be capable of continued inspection, allowing for the fluctuations in fluorescence of the coating material due to fluorescent content, material thickness and curing times.

The ACI uses a 5 megapixel camera with white and red lighting for alignment of the board and barcode recognition, and an ultra-violet LED array to detect the coating itself. Special algorithms have been developed to detect the board's coated and uncoated areas, and programming can be easily done by self learning from a previously inspected and approved board or by manually adding special inspection areas.
Automated inspection relies on fiducials.

A sample known good board is loaded into the ACI and the programming wizard started. The wizard takes the programmer through the initial stages of setting the board size and creating a background picture of the board, which will later be used for failure reporting. The board alignment marks are then identified using the standard white or red lighting and a standard AOI algorithm. The board is then ready for programming. This is a simple process of defining the positions of the coated and uncoated areas. The system will then lay an inspection grid over each. The programmer can adjust the brightness and tolerance for the coating on and off position. The program is then ready for basic testing on the production run. All data can be stored for future use.

Precision Area Coating
If the board also has specific areas that must have no coating or drips, or areas that need to be precisely coated, the programmer can apply specific tests using information from the system part library. Libraries can be common across different board programs. In addition, barcode positions can be programmed using the system camera and lighting. These barcodes can be either one per complete assembly or one for each of the boards on a panel.
UV image showing a defect.

The Nordson YESTech ACI also allows users to focus in on specific components for critical coating inspection for bubbles, cracks/delamination and contamination. These inspections are similar in nature to those of standard AOI systems and are programmed by using the component reference designators, part and image libraries. By combining these features of the two inspection methods, users are able to inspect the boards and output data directly to both touch up stations and to SPC databases — all logged against the board serial number.

Fault Information
Fault information is passed directly to a rework station via a standard Windows network. The rework operator can access the data directly by scanning the barcode. Fault information showing the position of missing or excess coating is shown on the PC screen. Once rework has been completed, the operator moves on to the next board and the rework information is added to the SPC file. All rework and board status information is retained against the board serial number, for as long as the company requires.

Full pictures of the board and coating can also be kept automatically on file for future reference.

The SPC data is updated in real time with the status of the last board inspected and can be viewed from anywhere on the network. Standard outputs are to Microsoft Access, SQL or to a text file. Optionally, the data can be configured to communicate with factory automation software.

The demand for quality and resilience now routinely placed on electronics hardware means that conformal coating is becoming an essential part of mainstream PCB assembly lines. This automated deposition and inspection equipment allows the conformal coating process not only to become part of an electronics manufacturer's production line, but also to integrate into his QA system.

Contact: Nordson YESTECH, 2762 Loker Avenue West, Carlsbad, CA 92010 760-918-8471 fax: 760-918-8472 E-mail: Web:

search login