The NPL has announced initial results from its PCB assembly surveys to support a recently launched industry defects database. The NPL (National Physical Laboratory) is the UK’s national measurement institute and is dedicated to developing and applying the most accurate measurement standards, science and technology.
The aforementioned assembly survey revealed that open joint failure is currently the most common issue seen by industry. Open joints are the most difficult to detect in manufacture, and the root cause of these defects can be a simple process control issue or a more complex technical problem requiring laboratory analysis. PCB solder finish is seen as the second biggest issue, which is understandable given the number of different options and the need to change surface coatings for lead-free assembly.
“The reason for conducting these surveys is to allow comparisons between defects submitted online and the trends seen in industry,” said Davide Di Maio of NPL. “The database will take time to populate with defects, but with worldwide industry cooperation and the introduction of defect types from our own research studies, this will result in a unique resource.”
Defect database and videos
The NPL’s defect database (http://defectsdatabase.npl.co.uk) allows engineers to search through a range of defects covering components, printed circuit boards and assembly problems. It also allows engineers to submit defects online with full details and solutions to current problems, or request advice and possible solutions to process issues or failures. The aim of the project is to add more process, material and environmental defects each month to complement the online submissions from readers.
An example of the type of content one can expect to find in the database is shown in the accompanying image. At first glance, this might appear to be a case of missing conformal coating, but the situation is not as simple as it initially appears.
In fact, this is a satisfactory board assembly after coating. It has a Parylene coating which is around 15 μm in thickness on all surfaces of the board and components. Based on the process, the coverage and thickness is virtually guaranteed. It is difficult to see the coating due to the process and the type of material used, so it is not actually a defect. Most conformal coatings are fairly easy to inspect visually with UV light, however one of the best coatings in terms of performance is difficult to see. This coating is more expensive than most and the process is normally subcontracted.
It is sometimes possible to see the coating under higher magnification on the side of the components close to the board surface, normally when a thicker coating has been applied. Inspection can also be easier if a QC label is placed on the surface of the board prior to coating or if it is possible to compare the solder mask surface with and without coating.
Dataweek has partnered with the NPL to feature a new manufacturing defect each month, as well as a video which can be accessed at www.dataweek.co.za/video. Please note that the featured defect and the video will not necessarily cover the same topic. This month’s video covers solder wicking as it relates to reflow soldering.
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