Reliability of via-in-pad structures in mechanical cycling fatigue

Abstract Increased packaging density, along with large-scale implementation of ball grid arrays (BGAs), chip scale packages and DCAs in portable products has resulted in the need for innovative techniques to increase the I/O density in printed circuit boards. With the development of high density interconnect (HDI), via-in-pad has emerged as one of the key enabling technologies for increasing the I/O density. Via-in-pad permits the use of sub-surface layers for fan out and consequently, smaller packages with higher I/O can be utilized in the design. Additionally, since traces no longer need to be routed between pads, the solder joint pitch can also be decreased. The reliability of via-in-pad under mechanical bend fatigue is examined in this study. Mechanical cycling fatigue reliability is especially critical for portable products where keypad actuation often induces repeated bending in the printed circuit board. HDI boards manufactured by both the photovia and the laser via processes were examined. A three dimensional, non-linear, parametric finite element model was developed to predict failure mechanisms. For the bend fatigue experiments, globtop BGA packages were mounted on HDI boards containing via-in-pad structures. The number of cycles required for material fatigue was obtained as a function of applied bending loads. Failure analysis was conducted to determine the failure modes. Experimental results were correlated with finite element predictions.