Systems for the automated optical inspection (AOI) have become an essential element in the PCB production process for guaranteeing a reliable quality assurance. In order to control the PCBs independently of their layout and assembly parameters, a high-end angled-view inspection is indispensable. However, such a module is not easy to develop, as Goepel electronic knows from experience.

Lifted Leads belong to the most critical faults in the production of PCBs. They result e.g. from lifted pins or from insufficient wetting through the pin's oxidation. In the electrical test they may perfectly show an electrical contact to the pad on the PCB - however, in the actual use there can be discontinuities and hence a malfunction of the PCB. These Lifted Leads not only belong to the category of critical faults, moreover, they are very complex to detect due to their diversified appearance. Numerous parameters affect the quality of the solder joint and the Lifted Lead. These include length and width of the pad in the same way like height of the pin and the solder properties: the flow behavior, the quantity and the solder flow at the pin foot. All these criteria are combined in reality so that there are a variety of possible appearances. In the daily use there may be Lifted Leads that are detectable with orthogonal inspection, however, this is significantly dependent on the PCB layout and the applied solder quantity.

An Angled View Is not Enough
In the field of inspection systems, the angled-view is considered as the solution for maximum fault detection. However, a critical approach to the AOI systems is necessary as a range of parameters have different impacts on the performance:
The field of view (FOV) of the angled view is responsible for the inspection speed of the complete system. As a comprehensive use of the angled view is necessary for the inspection of complex PCBs with a high number of ICs, the FOV is decisive for the time of inspection.
The depth of focus and the image quality of the angled view is also decisive for the whole FOV on the PCB as well as in regard to the height extension of the components. Consequently, it influences the inspection quality and speed. The sole evaluation of the front menisci at pin solder joints has a limited informative value regarding the solder quality. An angled inspection of the IC pins enables an optical inspection that resembles an IPC-conform test much more.

Sufficient Resolution
Especially for the inspection of fine pitch ICs up to 0.3mm a sufficient resolution is necessary for safe fault detection. If there are higher components in front of the tested pins there might be overlaps in the angled field of view. One tries to evade this situation with design regulations, however, this is a question of practicability.
A deflexion of the PCB during the inspection process might cause a displacement of the inspection positions in the camera image of the angled-view. In order to counteract this effect adequate compensation algorithms can help.
Components in an angled position other than the 90°-steps cause higher efforts regarding the programming time, because the test samples are not defined in the library (test units, macros etc.). So, a manual adaptation is necessary.
Besides the image capturing using angled view, an intelligent verify function is necessary in order to reach a safe fault detection with minimum false call rate under the above named circumstances.

Supported by Customers
As a basis for the development of a powerful angled-view module, Göpel electronic prepared a catalogue that contains all appearances of Lifted Leads and their combinations. Based on this catalogue, all classes of appearances were backed up by real samples in close cooperation with customers. The result is a pool of thousands of Lifted Leads in different appearances. Based on this, a method of image capturing was identified by modifying the directions of viewing and illumination, which ensures the detection of all catalogued Lifted Lead variants.
According to the requirements of the image capturing, an angled view module was developed that is distinguished by a big field of view (42 x 42 mm) with a high image quality and depth of focus, as well as a resolution of up to 10.5 μm/pixel. On the basis of the images captured in this way, an algorithm was developed that enables the automatic detection of Lifted Leads under all circumstances. Thereby, the solder joint inspection close to IPC was paid special attention to. That is only possible due to an angled-view inspection of the IC pins. This approach, using an intelligent optical design, allows the simultaneous inspection of two opposite pin rows of an IC. Hence, there is a significant advantage in the inspection time as e.g. for an SO-IC image capturing from only one direction is necessary. Even for components with four pin rows, there are only two inspections necessary with one 90° rotation of the viewing direction.

Verification of the Algorithm
The final verification was successful: All available real Lifted Lead samples incl. the application in production are detected at 100% without increasing the false call rate. The application in critical assembly situations was also considered in the development of the angled-view module (e.g. covering by higher components) along with the inspection of components in any angular position. Due to these considerations, the rotatability in 1° steps over an area of 360° was integrated in the module.
The benefit for the user is demonstrated in the following points: For components in different angular positions (e.g. 20°) no adjustment of the inspection position or the parameters is necessary as the viewing direction has only to be adjusted to the respective angular position (e.g. 20°), and the inspection can be carried out using a standard library sample in 0° position. If the viewing direction is obstructed by components, it is possible to turn the viewing direction for a few degrees until the solder joints are completely visible. Consequently, a reliable fault detection is guaranteed.

Summary
For assuring highest quality demands in the production of PCBs a powerful angled-view inspection guarantees layout independent and safe fault detection. The size of the captured field of view influences the inspection speed of the AOI system significantly. The free rotation of the viewing direction in an area of 360° allows an easy inspection of solder joints that might be covered by components in front of them.
A modular integration into in-line and stand-alone AOI systems allows the module to be used for a significant quality increase even for smallest batches.