TWI Bulletin, January - February 2009

Case study: Electronic joint breakdowns prompt TWI investigation

Failures in soldered interconnect joints came under intense scrutiny recently when one of TWI's Member companies presented it with a thorny in-service failure problem.

The electronic failures were occurring during thermal cycling tests. The joints were part of a ball grid array that had been assembled onto a printed circuit board.

The objective of TWI's work was to investigate the solder interconnect of a BGA device, using optical and scanning electron microscopy techniques.

The schedule involved evaluation of the joint shape, size, level of voiding, solder microstructure and wetting quality to the PCB pads.

The thickness of the copper/tin intermetallic layer between the surface mount pad on the PCB and the BGA's solder balls was measured and this gave an indication of whether the correct manufacturing process conditions had been used.

The thickness of the copper/tin/nickel intermetallic layer at the component interface was also measured to ensure that this hadn't grown too thick during the assembly process. The stand-off height of the solder joints was also measured to help evaluate the coplanarity of the device. All this information, as well as any unusual features revealed within the joint were reported confidentially to the client and a judgement offered by TWI on the overall quality of the solder interconnect.