To develop features which allow the combination of in-line optical inspection with other NDT techniques including Micro-focus X-ray, acoustic microscopy and thermal imaging. This will require the development of hardware and appropriate communications software to allow the units to be transferred. The necessary software for automated control will be developed. This work-package also aims to improve the quality of the AOI image analysis system.

Background
Assembled PCBs can suffer from many defects and it is highly beneficial for these defects to be detected early on in the production stage, as the later the failure is detected in the PCB assembly line the more the rework/repair cost.

A modern PCB assembly line comprises at least three key stages: solder paste printing, component placement and reflow oven. In solder paste printing a stencil is placed in contact with the board and a wiper blade passes over the stencil thereby depositing the paste through the stencil holes. Optically inspecting the board immediately after this stage can highlight if the stencil has misaligned with the board, and if sufficient paste as been deposited. The next stage is component placement where the PCB is populated with the insertion of components. Immediately inspecting after this stage verify's the operation and accuracy of the pick and place machines. Typical defects that can be detected are missing, offset skewed and flipped components, and wrongly placed components. Once the solder paste has been deposited and the components placed, the last stage is the reflow oven. Reflow cooks the PCB and solders all of the pads simultaneously. Optically inspecting after reflow enables verification of solder joint quality, evidence of excess and insufficient solder, and component migration.

Since our AOI system works by comparing an acquired sample image to a defect free "gold standard" sample image, any causes of defects in the sample image can be inherently detected due to cosmetic differences. Therefore the same AOI technique lends itself to post testing to any of the three stages.

Specification
A Hitachi KP-F120 CCD camera with 1392 x 1040 effective pixels is used together with a 1:1.4/16mm lens. Typically the height of the camera is positioned at 27 cm above the PCB giving a field of view of approximately 120 x 120 mm. The distance was established empirically and depended on the type of components under inspection. In order for the algorithms to function reliably an area of at least 20 x 20 pixels is required to represent the component area under inspection. If for example a component having very fine pitch leg spacing was under investigation then the camera would have to be positioned closer to the component in order for the AOI system to reliably detect any defects then say compared to checking for missing resistor components.

Images are acquired from the camera by a Nexeon - PCI CameraLink, 80MHz pixel clock single input frame grabber with 32MB frame buffer mounted in a personal computer (PC).



Currently ambient lighting is used to illuminate the PCB during the inspections and lighting conditions were therefore not optimised. In the next phase, the intention is to mount a white LED ring light on the camera objective to give a uniform and shadow free illumination of the PCB, and to enclose the inspection area in light proof paneling.