(This column, which originally appeared in Global SMT & Packaging magazine 6.9 (October 2006), is also available as a free PDF.)

Inspection of lead-free joints has been a hot topic over the last few years, and now with staff trying to come to grips with real world pressure of interpreting good from bad, urgent support is necessary. Standards do exist as a reference but often they are a collection of images with very little of the process information provided for reference.

As this is an obvious concern to the industry, the LEADOUT team developed a specific activity within the EU funded project to address these issues. Looking at boards soldered with one alloy can look completely different on any of the popular surface finishes like tin, silver, OSP copper or gold over nickel. Determining if a board is gold or copper OSP is fairly simple, but telling the difference between silver and tin can be more demanding, even for the experienced engineer.

Figure 1: A SOT package with a tin termination finish, reflowed with SAC alloy onto an immersion tin printed circuit board

Figure 2: A SOT package with a tin/lead termination finish, reflowed with tin/lead alloy onto a tin/lead HASL printed board

If it’s difficult for production staff, it can also be challenging for Automatic Optical Inspection (AOI) machines. Recent trials with four different surface finishes on flexible and rigid circuit boards illustrated how using different lighting can enhance the inspection cycle and reduce false calls in manufacture. Lead-free assembly can be successfully handled by modern AOI systems just like x-ray, but some process optimization is still necessary. Obtaining more information on the process and materials package helps the interpretation and, ultimately, corrective actions.

One of the key outputs from LEADOUT is a searchable photo-inspection library of solder joint images taken during assembly and reliability trials carried out in the project. The Photolibrary will be of particular use to inspection and quality staff as a source of reference, training and technical material. Importantly, the images have associated joint information such as the solder alloy, assembly method, component/joint type, printed circuit board surface finish and, where available, component finish. This extra data allows the library to be searchable, so that users can specify material and process combinations and locate the relevant pictures. Knowing more information about the assembly process allows a better assessment on the benefits that could be derived buy changing process materials or finishes to enhance inspection.

Keywords : Bob Willis

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