The term ‘popcorning’, within the electronic manufacturing industry, is often defined as a catastrophic failure created by the presence of moisture within a component.   Common ‘popcorning’ will occur when a small amount of moisture is heated during the wave solder/reflow process and turns to steam. As electronic manufacturing service providers transition to lead-free processes which require higher reflow temperatures, the potential for ‘popcorning’ increases substantially.  Less common and more difficult to trace is ‘popcorning’ failures during power-up of the circuit board, as the failures may be intermittent. When ‘popcorning’ occurs, regardless of the contributing conditions, the component damage is internal and may not be recognized by visual inspection procedures.

X-Ray inspection permits a non-destructive inspection of components and presents a clear image of the component internal construction.  However, cracks in the die are difficult to see since silicon is transparent to x-rays and provides little density or contrasting differences. A common method of identifying ‘popcorning’ is to x-ray the solder joints of the device; as the moisture within expands, the package swells and a high force is applied to the component leads and subsequent solder joints.  The gull-wing leads of a QFP package, as an example, will distort from the internal expansion of the molding compound, forcing mis-alignment of the leads and disruption of the soldering joints during the reflow process.

X-ray inspection of the solder joints will readily discover a potential intermittent failure caused by ‘popcorning’.  Component leads may lift out of the paste during reflow yet remain in contact when the board cools.  When the board is tested or when power is applied, the component and substrate will expand, creating an open circuit.  Conditions of ‘popcorning’ may occur during repair and re-work of circuit board assemblies as any device will be at risk to ‘popcorning’ if it was not properly handled and stored.  X-ray inspection of re-work and circuit board repairs should be implemented as a normal course of inspection and quality assurance with special attention to BGA package types.
The ‘popcorning’ of BGA’s is often recognized during the x-ray inspection process with the appearance of a bridge-like appearance.  This physical distortion, caused by the internal expansion of the package during reflow, deforms the molten solder balls underneath.  When the solder paste becomes liquid, the distorted solder balls will bridge with other solder balls.

The x-ray inspection of BGA solder balls is best accomplished with an oblique angle view as the difference in solder ball diameter between inner and outer balls is very clear and distinct.  X-ray equipment offering solder ball measuring and solder ball comparison provides the best opportunity for the operator to identify conditions of ‘popcorning’.

BGAs as with many other components, will continue to be at risk as industry migrates to lead-free materials and processes.  To mitigate such issues, industry must recognize, develop and implement new measures within the manufacturing environment to prevent components from absorbing moisture. Many manufacturing centers have implemented a pre-bake process of the board substrate to reduce moisture. Additional information and recommendations for pre-baking can be obtained by contacting the Helpline at helpline@empf.org or telephone 610-362-1320.

For additional information on the above article or to schedule a demonstration of the Dage X-Ray equipment located at the EMPF, contact Robert N. Berta, 610-362-1200 ext 253 or via e-mail at rberta@aciusa.org.