Modern electronic circuits are far more complex today than they ever were. A root cause of this complexity is the shrinking size of boards that results in thousands of soldered joints and parts being fitted into a compact space. Automated optical inspection (AOI) systems are needed to detect faults in the placement of these components and ensure the quality of a product.
By Baishakhi Dutta
With the large number of components packed onto shrinking boards, manual inspection of PCBs has become unfeasible. Currently, companies are looking at quicker time-to-market with high volumes, while maintaining high standards of quality in the manufacturing process. This requires reliable and fast production processes as well as verification methods. Automated optical inspection (AOI) systems detect faults at the early stages of the manufacturing process, as they are placed just after the soldering process in the production line. Detecting and fixing faults at the later stage of production is more difficult and costlier. AOI uses the following techniques to analyse a board’s quality:
- Template matching: This process compares the image of the board under production with the image from a ‘golden board’, and identifies the deviations.
- Pattern matching: Using this technique, the AOI system stores information of both good and bad PCB assemblies, matching the image of the board under production with these stored images to identify faults.
- Statistical pattern matching: Based on the data gathered by analysing the ‘good’ and ‘bad’ PCB assemblies, an AOI system statistically checks for various assembly results and failure scenarios. However, this technique can identify certain minor acceptable deviations from a production line and avoid flagging them as errors, based on the scale of the deviation.
Technological upgrades now enable AOI systems to find defects with high accuracy, so they are a very useful element in a manufacturing environment.
Choosing the right lighting source
Lighting is a key element in an AOI system, since the correct light source makes it easier to highlight the different types of defects. The advances that have been made in lighting technology in recent years have enabled the use of lights to enhance the images available. This helps the user to identify the defects quickly while also increasing the speed and accuracy of the production process.
Most AOI systems have defined lighting sets, depending upon the operations required and the product types to be tested. These have usually been optimised for the anticipated conditions but, at times, some customisation may be required. There are a variety of lighting types available.
- Fluorescent lighting: This is the most commonly used light source for AOI as it provides effective lighting to identify defects on PCB boards. However, one major concern is that fluorescent lights degrade with repeated use in the AOI application, so the quality of light changes constantly.
- LED lighting: With the advent of LED lighting, AOI systems are able to adopt a far more stable form of lighting. The degradation of an LED’s light output can be compensated for by increasing the current, apart from controlling the level of lighting. Compared to fluorescent or incandescent lights that were used earlier, LEDs are considered to be a better choice.
- Infrared or ultraviolet: Sometimes, in order to enable AOI systems to reveal certain types of defects, infrared or ultraviolet lighting may be required, and the right choice will enhance the productivity of the system.
The positioning of the lighting for an automatic optical inspection system is as crucial as the type of lighting source. The placement of the light source is important since it not only ensures that all areas are well lit, but also helps to quickly highlight the defects. However, careful adjustments may be needed for different assemblies.
Things to know before buying your AOI solution
While purchasing an inspection system for your manufacturing line, there are many factors that need to be considered. Since there are many products that seem to address your inspection requirements, how do you make the decision of which system to use? According to Praveen Madaan, country head, India (SMT division) Juki India Pvt Ltd, there are three crucial factors to think about when investing in an AOI solution. These are:
- Users must opt for a multi-platform inspection machine capable of both SPI (solder paste inspection) and AOI. This will ensure that the platform can be used for both SPI and AOI, without changing the software. You can utilise this machine effectively even when you need to change your production line.
- Easy program editing is an important factor—the ‘template mode’ can automatically generate inspection data of the component just by pasting the template to the area. Also, programming for the OK/NG inspection can be done very easily.
- The TOPSS or Trane Official Product Selection System saves manpower while ensuring high quality management. It offers centralised controls for OK/NG judgment, repair operations, quality traceability, the inspection results’ statistical analysis (SPC) and offline program editing.
Here are a few AOI models that have been launched in the market recently
|Model: VT-S530; Manufacturer: Omron Corporation|
|The VT-S7 series features 3D-SJI based on high-accuracy solder shape restoration.
Accurate quantitative inspection of solder bondability is now possible, ensuring that quality control levels conform to international standards such as IATF (ISO/TS) 16949. With
engineering specifications and a high resolution compatible with the semiconductor industry, the VT-S530 provides stable inspection of 0402/0603 microchip components. It supports dual-lane operations for higher production throughput. Dual-lane operations are also possible with PCBs of various sizes, up to 510mm (W) x 330mm (H).
|Model: KY-P3; Manufacturer: Koh Young Technology Inc.|
|The KY-P3 inspection systems quickly and accurately measure pin height, missing pins, potential pin offsets (pin centre to the CAD centre), shoulder height including absolute height and co-planarity, as well as distance (including feature-to-feature absolute distance, relative distance as well as inner and outer distance for fork pins).
They take 3D measurements and perform 3D automated inspections that are both on target and accurate, unlike in the case of conventional 2D inspections, company sources claim. Fast inspection condition settings and modifications are enabled with a new AI engine, which is optimised for GUI usability and easy programming.
|Model: RV-2-3D; Manufacturer: Juki Corporation|
|The RV-2-3D PWB visual inspection machine has many benefits, including saving of space and compact single-lane inspection.
Juki’s unique calibration technology adjusts the distortion of the XY axis and lens, lighting individual differences to avoid instrumental errors in the equipment. Stable inspection results with no machine differences can be captured even when you run the same inspection program in several lines, with high repeat accuracy.
The high-speed head unit features 3D projectors in four directions and a 2D high-speed colour camera (160FPS 160 images/sec). The Juki proprietary designs ensure quicker image-capturing and image-processing, along with a 34 times faster inspection time (0.41sec/FOV) compared to other systems, company sources claim.