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Electronic joint breakdowns prompt TWI investigation

Connect, no. 159, March/April 2009, p.3

Optical image of BGA joint microsection showing overall diameter and diameter of void
Optical image of BGA joint microsection showing overall diameter and diameter of void

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

The electronic failures were occurring during thermal cycling tests. The joints were part of a ball grid array (BGA) 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, that had already been micro-sectioned by the customer, using optical and scanning electron microscopy techniques.

The schedule of work devised by TWI involved evaluation of the joint shape, size, level of voiding, solder microstructure and the quality of the wetting 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 assessed 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 co-planarity 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.

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