A publication of the National Electronics Manufacturing Center of Excellence December 2003

EMPF Director

Michael D. Frederickson
mfrederickson@aciusa.org


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Tech Tips... Automated Optical Inspection
I
t is crucial for Electronics Manufacturers and assemblers to have an efficient and cost effective method of inspecting Printed Circuit Boards during and after the manufacturing process. There are several different types of inspection systems in today's market that perform a variety of inspection functions. There are systems capable of inspecting paste, component placement prior to reflow, post-reflow component conditions, and post-reflow solder joints. These inspection devices all have some common attributes that affect capability, accuracy, and reliability. The purpose of the following technical tips is to help with the selection of the proper type of Automated Optical Inspection (AOI) system.

1. Light source
The decision to use a LED or fluorescent light for inspection of an assembly is dependent upon the area to be inspected. LED light measures post-print solder brick height more accurately than a fluorescent light source. This accuracy also makes LED light a good tool for post-reflow solder joint inspection. Fluorescent light is excellent for inspection of component placement pre-reflow but is typically used for location inspection only.

2. Image cameras
There are several types of image cameras used today. Streaming video 2D frame grabbers are common. They utilize a motion capture video camera that extracts one frame from a streaming video and creates a still image. However, the system sacrifices image quality for speed and efficiency.

A second type of camera imaging system is the Line Scan Still Image Camera. In this system, a still camera is placed relatively close to the target. Because of this, this system requires a very good lighting system. Unfortunately, the image can be distorted by subsystem imperfections such as transporter movement. This makes obtaining precise positioning and measurements difficult when compared to other types of systems. A benefit of the Line Scan Still Image Camera is the image acquisition speed, which is faster than a CCD camera.

The least common type of camera imaging system is the 2D Charged Coupling Device or CCD. The CCD is used for high-end and special applications such as space and military technologies. This system creates high precision still images in color that are more accurate than other systems.

3. Programming
Today's systems can be programmed offline using CAD data or golden board methods. This reduces production downtime and allows the programmer time to enter accurate parameters without stopping an assembly line. These systems also have a component database that saves data so that it does not have to be entered every time a new board is produced.

4. Data collection
3D software imaging or LED light measurements are common methods for measuring solder joint parameters. Typical systems use both methods to obtain accurate measurements. These systems use a directed light source or refracting light to measure height, area, and volume. They also use software to analyze data and calculate statistical process control (SPC) results in these areas.

5. Ball Grid Array (BGA)
Today's AOI systems have the capability to inspect visible solder joints on caps, resistors, and other components including toe fillets on gullwing devices (Figure 3-1). However, without additional capabilities such as x-ray inspection, BGA and "J" leaded components are limited to polarity, missing, and placement error detection.

6. Memory and Speed
System memory is important. Upgradeable systems that have the capacity for expanded memory are desirable. Databases expand every day particularly with the increase in lead-free components, solders, boards and assemblies that are rapidly entering the industry.

System speed is influenced largely by the size and complexity of the boards being inspected and the level of inspection. Control over the pace of your production line can be influenced by your AOI system. Since one hundred percent inspection is not always necessary, you can save valuable time by dialing in the appropriate inspection level.

7. System accuracy
The accuracy for finding defects such as missing components, skewed components, reversed components, or wrong valued components depends not only on the capability of the inspection system but also on the program accuracy supplied by the user.



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