Pad Cratering Evaluation of PCBs

Dongji Xie, Ph.D.
Dongkai Shangguan, Ph.D.
FLEXTRONICS, San Jose, CA

Helmut Kroener
Multek, Schongau, Germany

Pad cratering in the PCB is a new failure mode encountered in electronic assemblies, particularly in lead-free products. The failure mechanisms and root causes are not yet fully understood, and lack appropriate industry standard tests for PCB qualification with regard to pad cratering. This paper reviews major publications and research reports on various PCB materials from industry studies in this field. Various PCB tests, such as flexural strength test and pad strength test, have been studied. It is recommended that the qualification of the PCB can be done in two stages: PCB board level and PCBA product level. From those results, a new qualification method is suggested for screening out PCB pad cratering failures

Pad cratering occurs from the cracking of the laminate and/or prepreg materials. Several tests can be used at the PCB materials level, board level, and system level. The flexural strength is not sensitive enough for capturing pad cratering because the prepreg layer normally fails earlier than the core PCB. Three types of board level testing on the pad strength can be used: ball shear, ball pull, and pin pull. Among them, the pin pull test may capture more pad cratering modes than ball shear and pull
tests.

The pin pull test is similar to the peel test for BGA pads and can be used as a good indicator of pad health. The prepreg may crack even if the PCB strain is controlled under 500ue. Solder mask defined pads are preferred in the prevention of pad cratering as it provides more contact area and hereby reduces the stress in the prepreg layer. A new system level PCB qualification process is suggested to mitigate the pad cratering risk. The qualification test uses actual PCBA and simulates multiple reflows and board handling as well as stresses in the field.