Tom’s Circuits – Automated Optical Inspection for PCB Quality Control

February 20, 2018 , in Blog, Tom's Circuits

Automated optical inspection (AOI) finds defects in both bare PC boards and assemblies. With small components, these defects are often invisible to the naked eye. 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.

The picture on the left is from the AOI machine, and the picture on the right is taken with a conventional camera with a close-up lens. The picture on the right is a more natural-looking view, because human vision understands perspective and shadows. While humans perceive these distortions as natural and understand straightness, machine vision algorithms are more accurate and faster when they don’t have to contend with shadows and distortion.

The machine vision goes much further than this flat view. It makes precise measurements of the components, including height and rotation. The AOI machine used photogrammetry to determine component heights with 2 micron resolution and 10 micron precision.

The software extrudes the 2D image into a 3D view using the height measurements, and measures the planarity of the top of the IC. A tilted component is a problem that is not always easy to see.

To AOI customer orders, Tempo Automation uses the Mirtec MV-6 Omni.

These images from Mirtec show a slightly lifted resistor that is not making contact. The Mirtec software starts with the 2D view on the left, adds the height information with photogrammetry, and creates the 3D view on the right.

It’s Not a Webcam and Stepper Motors!

Printed circuit boards have small components such as 01005 resistors, 0201 capacitors, and BGA packages spread across a large PC board area. Making accurate measurements at production speed requires advanced equipment and software. I will focus on two of the hardware technologies: telecentric lenses and structured light.

Telecentric Lenses

A telecentric lens reduces distortion in AOI images by creating a flat image from parallel rays of light. This is called an orthographic view and it means that the size of objects in the image are independent of the object distance from the camera.

The easy way to make an orthographic picture is to take the picture from far away using a telephoto lens. There is not enough room for this in a factory. To construct an AOI machine, a telecentric lens creates the same effect. There are limitations, though. Since the magnification is independent of distance, the lens can’t see an object larger than the size of the lens. It would take a very large telecentric lens to see a forest! To image a whole PC board, the board is conveyed with motors and multiple images are stitched together. In this system, it is easiest to hold the board in the fixture when the board design has two parallel board edges with no overhanging components. Otherwise, a custom fixture needs to be configured.

Mechanical CAD programs provide orthographic views to enable precise drawing. Here are some CAD renderings of cubes of the same size:

The views in the top row are the normal perspective view. The block that is further away looks smaller.

The bottom row shows the orthographic views. They look less natural. An optical illusion may cause the back of the cubes to appear wider than the front. The blocks in the lower left appear to be the same size, even though one is further away. Or perhaps the rear block looks a little larger, since the eye attempts to enforce perspective.

To dispel the optical illusion, the rulers show that the lengths are the same.

For measuring the dimensions of parts with a camera, it is worthwhile to put up with the limitations and cost of an orthographic lens.

Structured Light

To extract the 3D profile of the parts, the AOI machine uses a projector to illuminate the parts with a scale similar to the rulers in the picture above. This precisely structured light shines in different colors and patterns, and the software uses the resulting images to measure component height.

The measurement of distance uses triangulation. By knowing the length of the baseline ℓ, and the angles θ and ϕ, it is possible to find the distance d to the object being measured.

The angle θ is created at P1 by a structured light source called a line generator. For simplicity, the structured light source is drawn below as a single slit.

In reality, the light source is more complex and has many adjustable lines of different colors. The goal is to have a source of light coming from point P1 at a known angle θ. Furthermore, the light is coded so that each point on the object is identifiable by the camera at P2. Each pixel on the camera image corresponds to an angle ϕ. Since the baseline length is known, this enables calculation of the distance d.

The Mirtec MV-6 AOI machine has are four structured light sources, and each generates a series of lines of a different color. The line spacing changes to enable accurate measurement of both tall and short components. Software combines multiple images and constructs the height map.

Clean Boards, Great Images

The AOI process can generate excellent PC board photographs for marketing, documentation, troubleshooting, board modification, and repair. These nearly distortion-free orthographic images feature straight lines. Accurate coordinate dimensions can be calculated using a pixel ruler and the datums visible on the design.

To make PC board traces and vias visible, use a translucent solder mask color. Black and white are the most opaque solder mask colors and do a good job of hiding traces and vias.

Optical Inspection at Tempo Automation

The trend toward ever-smaller surface-mount components creates demand for AOI that goes beyond the capabilities of the human eye. To keep up, Tempo Automation uses the best technology available to meet the demands for speed, precision and throughput to deliver high-quality, quick-turn manufacturing. Tempo also uses X-ray inspection for PCB assembly quality control.

Images of your PC bare boards and assemblies are available from Tempo by special request. Reach out to your Tempo Account Executive, or email the team for help with your AOI questions.


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