Project achievements against state-of-the-art

In the Microscan project an in-line inspection system comprising four different inspection techniques has been developed and a proof in principle has been demonstrated. This is the first time that this has ever been done. There are a number of achievements in this project that can be considered novel against the state of the art:

• The combination of the inspection techniques offers the capability for 100% defect coverage.
• The inspection prototypes have been designed in such a way that they can be used either independently or together.
• A generic software framework has been developed that allows defect detection algorithms to be tested across different inspection techniques including AOI, X-ray and SAM.
• It is the first time that a water irrigation probe, used to minimise water contact with the PCB, has been included in an in-line SAM for PCB inspection.
• Defect detection algorithms for detection of poor wetting of balls have been implemented.
• The use of leaded tunnels enclosing the entry and exit conveyors of the X-ray inspection module, allowing the X-ray source to be continually on and thereby increasing the maximum inspection throughput.

In recent years the complexity of PCB assemblies has been increasing, leading to ever increasing demands placed on NDT examination for quality assurance. Currently, with the exception of AOI, most NDT is applied off-line in a batch situation, generally using only one inspection technique. If a serious defect is found, it is possible that the whole batch will be scrapped. Once a fault is found with one technique, it is frequently necessary to confirm this with optical or scanning electron microscopy and metallurgical sectioning. This type of methodology adds greatly to the cost of each PCB batch. At present when utilising all four inspection prototypes of the Microscan system it is suited for high-return PCBs, particularly PCBs manufactured for products in medical use and aerospace, rather than high throughput production work. Nevertheless, depending on the number and type of components present, the total inspection process for a PCB can take approximately a minute. Already at the start of the project it was expected that the space and cost savings obtained by combining all of the technologies in one comprehensive machine, could be outweighed by the compromises the combination would bring to the performance of each separate inspection technology. For example when all the inspection prototypes are used together a compromise may have to be made with respect to inspection throughput. In addition depending on a given stage in the manufacturing process it may not be necessary or convenient to apply all inspection techniques. These compromising factors depend to an extent on the given PCB configuration and complexity that is to be inspected and may or may not pose an issue to the PCB manufacturer. Since each of the prototypes has been designed around a common electrical, mechanical and software communications infrastructure it enables the inspection modules to be used separately, together, interchanged or in different stages of the production process to give the best results in terms of inspection coverage and inspection throughput. Given the ever increasing complexity of PCB assemblies, and the trend to increase the population density of components driving the move to more complex surface mount assembly technology such as BGA and other small foot print devices, there will be increased demands on NDT examination for quality assurance. It is key that two of the inspection prototypes in the system cater for inspection of BGAs.

An in-line inspection system comprising four different inspection techniques has been developed and a proof in principle has been demonstrated. The adoption of the SMEMA standard means that not only can the prototypes operate with each other, but they can be immediately incorporated into most PCB surface mount manufacturing lines with little modification.

Through the course of the development some new and novel techniques have been introduced and applied to PCB inspection, including a water irrigation probe for SAM inspection and new algorithms for detecting BGA wetting. Further work is required in order to refine throughput and commercialise the system.

TWI is seeking partners to collaborate and continue with this work.