AOI inspection: Why we hardly ever miss a faulty solder joint
Jens Brinkmann explains how the AOI system detects faults
As an EMS service provider, we manufacture assemblies for various customers and industries. Our core business is SMD assembly. The assemblies that we manufacture are used in the agricultural industry, for example, but also in aeroplanes or protective helmets for the police. Safety and durability are therefore among our most important quality factors. Recognising manufacturing defects early and completely and then preventing them is the supreme discipline for us in EMS.
AOI: automatic optical inspection - a definition
Automatic Optical Inspection (AOI) uses several high-resolution cameras to automatically check PCBs for faults and report them. The 2D camera system photographs the assembly from above and from the side at a 45-degree angle. During the three-dimensional inspection, a "3D scanner" also determines the height of the components. The inspection software digitally compares the images with the defined reference values (classification thresholds). If possible, the system is set up using a flawless reference board.
Automatic optical inspection is a non-contact inspection method that we use on a random basis during the production process. The typical time of use for AOI systems is directly after reflow soldering. This is the perfect time for troubleshooting. The later we find faults in the production process, the more expensive it is to rectify them. In addition, we can recognize process problems during soldering and assembly early on in the production process and use the information for rapid feedback to earlier stages. Once production is complete, we check every single PCB.
These AOI systems are used at A+B Electronic
- AOI system Viscom 3D-AOI S-3088 Ultra: With the latest camera technology, eight inclined cameras ensure precision, combined with an integrated fringe projector. Image quality is so crucial because the components on the PCBs are getting smaller and smaller.
- AOI system Viscom 2D-AOI S-3088 flex: Effective assembly inspection and convenient operation ensure fast and reliable defect detection.
- X-ray inspection system YXLON Cheetah
- 7 Vision Lyn EVO stereo microscope
That's why we hardly ever let a faulty solder joint slip through
Automatic optical inspection (AOI) is one of the most important and effective steps in our quality assurance. In AOI, we work with standardised processes - for example, in program creation to generate digital reference boards. As soon as the program has been written, a self-check is carried out before we perform a team check. Are all the components that are to be assembled really noted in the software? Each component has its own geometry. Normal chip components, for example, are rectangular and have component caps on the right and left. We define the geometry in the test pattern. We set up so-called match patterns. These determine the exact positions of the component and the subsequent test windows. All of our processes are routine - this enables us to avoid errors right from the program creation stage.
When the AOI system analyses an image of a PCB, it looks for a variety of specific features:
- PCB reference points
- Presence of components
- Component placement (displacement)
- Polarity
- Reflectivity of the solder joints
- Labelling patterns (such as barcodes)
Once production on one of our SMD lines has been completed, we check the batch as a whole. We have already spot-checked for gross errors beforehand. If the AOI system still finds faults, our specialist staff classify these faults in the revision and process them.
Pseudo errors and real errors in the AOI
If the AOI system indicates an error, a specialist in the revision department checks which error it is: a pseudo error or a real error. Up to this point, the system simply says that there are deviations from the reference. If it is a real error, we process it manually. Pseudo defects occur when the system does not recognize a certain location one hundred percent. If our inspection team classifies the component as a pseudo defect, the PCB is immediately returned to the production cycle. We analyze the data of the pseudo defect statistically in order to eliminate this defect in the future. The aim of AOI programming is therefore to generate as few pseudo faults as possible, but to find every real fault.
4 typical errors that AOI filters out during SMD assembly
If the AOI system has recognized an error and this has been classified as a real error, there may be various reasons for this. Four typical errors are, for example:
- Components missing or misaligned
- Component polarity reversed
- Solder joints are defective
- Pins are bent
The AOI system detects defective solder joints by color matching the light reflection. The software recognizes different shades of grey and can thus indicate the fault. In IC components, the pins run downwards in a staircase. As a result, they can easily twist or protrude upwards during the production process. During reflow soldering, components can "blur" together and cause short circuits due to fine hairline bridges between the solder joints.
X-ray inspection system for BGA (Ball Grid Array)
The camera systems of the 2D and 3D AOI can detect components whose solder joints are visible from above or from the side. Components with concealed connections on the underside are called ball grid arrays (BGA for short). They can therefore only be inspected for correct alignment using an AOI camera system. We use our YXLON Cheetah X-ray inspection system to inspect these concealed solder joints on multilayer PCBs. This allows us to X-ray the boards for defects and check that the solder points are connected correctly. A separate department at A+B Electronic, which is specially trained in X-ray inspection, works in a secure area that protects against X-rays. The X-ray inspection system massively improves our quality processes.
Miniaturisation and increasingly complex components require seamless quality management. Our core business is PCB assembly. We supply everything from a single source - from conformal coating, testing and complete assembly to logistics.
