Head in Pillow Solder Joint Failures
A Supplier of Networking Infrastructure products was experiencing random intermittent electrical failures with a complex networking chip. The failure mechanism was isolated to a number of BGA balls on one Assembly, and the failure mechanism was identified as Head in Pillow by cross-sectional destructive failure analysis.
Head in Pillow solder joint failures exhibit a distinctive visual signature, where the BGA ball mass and the PCB pad solder deposit interface forms a “waist” rather than a smooth interface. The “waist” indicates the delineation between the two masses that denies the intimate contact required for a typical BGA ball to PCB pad solder deposit interface.
The use of a destructive analysis method is clearly not a valid identification strategy, so a non-destructive test method would be desired for identifying Production occurrences of this failure mechanism, especially since the failure mechanism was shown to be often electrically intermittent under testing, and could possibly pass through typical tests unchecked.
A specific X-Ray technique was identified as a possible non-destructive test method, and a failure to X-Ray signature correlation exercise was conducted.
A BGA perimeter ball failure was positively confirmed as an electrical open, this in turn was identified under Ersascope visual inspection to possess the visual signature of a Head in Pillow failure, and was finally inspected under X-Ray. The “signature” under X-Ray was identified as largely unique, and proved to be an extremely reliable test method for identifying Head in Pillow in a non-destructive way.
This screening method was then utilized as a stop-gap quality checkpoint, while the root cause of the failure mechanism was identified and eliminated by process improvements.
Head in Pillow was found to have a number of contributing factors, including :
- PCB site coplanarity. Note that industry standard PCB flatness criteria are not necessarily stringent enough, especially as the size of a BGA increases.
- Component and PCB warpage under heat excursions, especially relative warpage, where opposing warpage directions increase separation distance.
- Temperature profile used for reflow, particularly where extended profiles are implemented to reduce delta T, peak temperature.
- Solderpaste Flux activity, especially at critical points in the reflow cycle.
- Contamination and oxidation of mating surfaces.
The optimization and thus impact reduction of some or all of the above, implemented on a case by case basis has been found to reduce and even eliminate the occurrence of Head in Pillow failures.
To see the resume of the expert associated with this case study, see the link below.
