Automated optical inspection

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Automated optical inspection (AOI) is an automated visual inspection of a wide range of products, such as printed circuit boards (PCBs), LCDs, transistors, automotive parts, lids and labels on product packages or agricultural products (seed corn or fruits). In case of PCB-inspection, a camera autonomously scans the device under test (DUT) for variety of surface feature defects such as scratches and stains, open circuits, short circuits, thining of the solder as well as missing components, incorrect components, and incorrectly placed components. Agricultural inspections might check for variations in part color, perhaps to find ripe fruit. AOI is a type of white box testing. It is commonly used in the manufacturing process due to the fact that it is a non-contact test method. AOI is able to perform all the visual checks performed previously by manual operators, and far more swiftly and accurately. AOI systems are implemented at many stages through the manufacturing process. They are used for inspecting parts that have limited and known variations. For defect or flaw detection, the AOI system looks for differences from a perfect part.^[1] There are systems capable of bare board inspection, Solder Paste inspection (SPI), as well as inspecting the component placement prior to reflow, the post-reflow component conditions, and post-reflow solder joints (see Reflow soldering). These inspection devices all have some common attributes that affect capability, accuracy, and reliability.

In this way AOI can be used to catch problems early in the production process. With faults costing more to fix the further along the production process they are found, it is essential to notice any problems soon after they occur. For example process problems in the solder and assembly area of a PCB can be seen early in the production process and information used to feedback quickly to previous stages, avoiding the production of too many boards with the same problem.^[2]

Low costs and programming efforts make AOI a practical and powerful quality tool for both prototypes and high-volume assembles. It is often paired with the testing provided by boundary scan test, in-circuit test, and functional test. In many cases, smaller circuit board designs are driving up the demand for AOI versus in-circuit test.

Contents 1 Process 1.1 Light source 1.2 Capturing an image 1.3 Programming 1.4 Data collection 1.5 System memory 1.6 System speed 1.7 System accuracy 2 Types of AOI machines 2.1 Stand-alone and inline AOI systems 2.2 Closed- and open-top AOI systems 2.3 AOI and combined AOI/AXI systems 3 Inspection in surface-mount technology 4 Bare PCB inspection 5 Related technologies 6 References

[edit] Process

A machine vision or an AOI system can take millions of data points (pixels) in a fraction of a second. These data points are used for visual inspection and precision measurement.

AOI visually scans the surface of the PCB. The board is lit by several light sources and observed by a scanner or by a number of high definition cameras. This enables the monitoring of all areas of the board, even those hidden in one direction by other components.

[edit] Light source

Lighting preprocesses the image to amplify features that need to be inspected and suppress noise. Advances in lighting have improved the capabilities of vision systems, in part by reducing the computation required by the vision computer.

First, the test engineer has to choose which light source to use for the lightning: a LED or fluorescent light (see also Fluorescent lamp). This depends on 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, on the other hand is excellent for inspection of component placement pre-reflow but is typically used for location inspection only. The problem with it is that because of the frequent light changes the fluorescent lamps are ruined fast. The adoption of standard LED-based lighting has improved AOI systems because it is very stable and easily controlled when compared to the older incandescent and fluorescent lighting solutions. LEDs are not also perfect: they become darker with time but this can be easily adjusted.

Another lighting method projects a pattern of light on an object, often by using a laser with a holographic lens. The distortions of this structured light pattern can be measured and processed to recover the object’s 3D structure (see 3D scanner#Structured light). AOI systems using structured light can, for example, compare complex objects such as engine blocks or PCBs to the designed shape in CAD files.^[3]

The position of the light sources inside the machine is also important. It is possible to position the light source directly above or on one side of the PCB.

[edit] Capturing an image

If a scanner is used it has to scan the surface of the PCB from above only once. If image cameras are used, one has first to decide how many cameras are needed. There are systems with only one camera which scans the DUT from above and systems with couple of cameras from all sides. To be able to scan the device from all points of view, the cameras should be able to move in both X- and Y-direction controlled by software. In order to program this software the test engineer has to have the CAD data.

Then the type of image cameras has to be chosen. There are several types in use 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 Charge-coupled 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.^[4]

As the DUT slides down the track, it interrupts a laser beam. A sensor detects this interruption and signals the vision computer that the part is in a known location. The vision computer then triggers the camera/cameras to simultaneously acquire images of the device. When sensor cannot be used to trigger image acquisitions, the vision computer must detect when a device is present by analyzing the images, and this slows down the system.

[edit] Programming

The AOI system takes time to "learn" the board. The system needs to be able to add the board to its memory. It has to "remember" the different types of components, their positions and also to check the quality of the soldered joints. It must be able to recognise and adapt to differences in the appearances of the board that result from the normal production process variations. However, it must be able to recognise any that affect performance. To achieve this it is normal to run a number of good boards (golden boards) through the system before full production starts so that the system can "learn" the board. Nowadays the best AOI machines can be set up relatively quickly and then they are reliably able to inspect boards.^[5] This can be done by offline programming. By doing so production downtime is reduced and the programmer has 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.

In case of automotive parts inspection for example, the system should know what features to measure or for sorting agricultural products the color of ripe fruit should be known.

[edit] Data collection

To obtain the necessary data one can use 3D software imaging or LED light measurements, which 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. The vision computer and its software analyze data (images) and calculate statistical process control (SPC) results in these areas. The results of the inspection are used to reject defective parts.

To reduce the computation required by the vision computer so that parts are quickly inspected it is a good idea to use a type of mechanical restraint known as staging or fixturing (see Fixture (tool)). The part is positioned in a known location and variability in where the parts are and how they look is decreased.^[6]

[edit] System memory

System memory is also 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.

[edit] System speed

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 the production line can be influenced by the AOI system. Since one hundred percent inspection is not always necessary, valuable time can be saved by dialing in the appropriate inspection level.

[edit] 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 accuracy of the program supplied by the user.^[7]

[edit] Types of AOI machines

[edit] Stand-alone and inline AOI systems

When using Stand-alone machines the PCB has to be inserted into and after the test taken out of the machine manually. Inline systems on the other hand are part of the production line so the PCBs move into and go out of the machine automatically.

[edit] Closed- and open-top AOI systems

Inside Open-top machines the artificial light is distracted by light sources different than the ones used by the system itself (eg. sunlight). Closed-top machines eliminate any light pollution because they are closed from all sides. In this way only artificial light is available inside the machine making these type of systems much more efficient.

[edit] AOI and combined AOI/AXI systems

Today's AOI systems have the capability to inspect visible solder joints on capacitors, resistors, and other components. However, without additional capabilities such as Automated x-ray inspection (AXI), ball grid array (BGA) and "J" leaded components are limited to polarity, missing, and placement error detection.^[8] That is why Combined AOI/AXI systems can provide the necessary mixture for high-performance and are becoming more and more popular.

[edit] Inspection in surface-mount technology

The growing demand for Surface-mount technology equipment is reducing the need for expensive rework and repair while increasing throughput. As PCB assembly manufacturers aim for a zero-tolerance regime, the demand for pre-solder paste and pre-reflow optical inspection equipment that detect faults such as poor quality solder joints, tombstoning, and other post reflow defects is expected to rise significantly. Technological improvements in AOI equipment have resulted in higher throughput, repeatability, and reliability as well as increasing quality and production yields for the PCB assembly manufacturers.^[9]

AOI's for a PCB board with components may inspect the following features:

• Area Defects
• Billboarding
• Component offset
• Component polarity
• Component presence/absence
• Component skew
• Excessive solder joints
• Flipped component
• Height Defects
• Insufficient paste around Leads
• Insufficient solder joints
• Lifted leads
• No-population tests
• Paste registration
• Severely damaged components
• Solder bridges
• Tombstoning
• Volume defects
• Wrong part

[edit] Bare PCB inspection

AOI for a bare PCB board insection may detect these features:

• Line width violations.
• Spacing violation.
• Access copper.
• Missing pad. I.e. a feature that should be on the board is missing.
• Shorts circuits.
• Cuts.
• Hole breakage. I.e. a drilled hole (via) is outside of its landing pad.

The triggering of a defects report may be either rule based (e.g. no lines on the board should be smaller than 50μ) or CAD based in which the board is locally compared with the intended design.

This inspection is much more reliable and repeatable than manual visual inspection.

[edit] Related technologies

The following are related technologies and are also used in electronic production to test for the correct operation of Electronics Printed Circuit boards

• AXI Automated x-ray inspection
• JTAG Joint Test Action Group
• ICT In-circuit test
• Functional testing (see Acceptance testing)

[edit] References