Jens Kokott, Manager of AOI/AXI systems at GOEPEL Electronic explores a revolutionary concept in layout independent fault detection by using AOI systems with high-end angled-view inspection
AOI systems are an essential element in the PCB production process in order to guarantee a reliable quality assurance. The user can choose from a variety of system options, which are characterised by diverse performance parameters so that a broad spectrum of quality demands can be satisfied.
The application of AOI systems is necessary for the inspection of the solder joints of PLCC components and furthermore, offers a higher inspection quality for pins of all kinds of IC components (e.g. SO, QFP, etc.).
Regarding the latter mentioned solder joint type, an orthogonal inspection is possible, however the layout at the solder joint has a crucial influence on the reliability of detection.
In this matter, EMS providers especially face the difficulty to not be able to control the design of the pads and arrangement of components; hence the consideration of layout guidelines will stay illusive.
To amend this, an angled-view can provide image information for additional analyses. However, this additional viewing direction should not be considered a remedy for increasing the quality of detection, because its application brings with it several boundary conditions, which also influence the overall process of the Automated Optical Inspection.
Besides an increased expenditure of time due to additional image capturing and the considerably smaller field of view, the arrangement of the components on the PCB can constrict the view on the inspected solder joint also with an angled-view inspection.
Taking into account these elements GOPEL Electronic has developed its ‘Chameleon’ rotating angled-view module, which allows an intelligent verify function, is layout independent and offers reliable fault detection.
Lifted Leads belong to the most critical faults in the production of PCBs. They result for example from lifted pins or from insufficient wetting (e.g. through oxidation of a pin).
In the electrical test (ICT, Boundary Scan, functional test) they may perfectly show an electrical contact to the pad on the PCB – however, in actual use there can be discontinuities and hence a malfunction of the PCB.
The detection of Lifted Leads is complex due to their diversified appearance. The following parameters have a significant influence on the quality of the solder joint/Lifted Lead:
The solder quantity
The width of the pad
The length of the pad
The solder flow at the pin foot
Flow behaviour on the pad
The height of the pin
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…
The angled-view is considered as ‘the’ remedy for maximum fault detection. However, a critical approach to the AOI systems is absolutely 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 tool, 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 regards to the height extension of the components. Consequently, it influences inspection quality and speed.
The sole evaluation of the front menisci at pin solder joints has only 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.
Components in an angled position other than the 90° steps require much higher efforts regarding the programming time, because the test samples that are defined in the library (test units, macros etc.) cannot be used and 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.
In developing the angled-view module, the company has compiled a catalogue containing all appearances of Lifted Leads and their combinations.
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 an extraordinarily big field of view (42mm by 42mm) with superior image quality and depth of focus, as well as a resolution of up to 10.5m/pixel.
The solder joint inspection close to IPC was paid special attention to. This 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, for example in SO-IC image capturing only one direction is necessary. Even for components with four pin rows, there are only two inspections necessary with one 90° rotation of the desired viewing direction.
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.
A modular integration into in-line and stand-alone AOI systems allows the module to be used for a significant quality increase even for the smallest of batches.
