A Study of Organic Chip Carrier Fatigue Cracking

Chip carriers made of organic materials provide significant advantages in manufacturing cost and electrical performance. However, compared with its ceramic predecessors, the organic chip carrier laminates encounter additional thermomechanical challenges, particularly for larger chip applications. For flip chip packages, the temperature fluctuations in field operation cause repetitive thermomechanical stresses in the dielectric resin leading to fatigue failure. Such a failure is termed as dielectric resin cracking. Stresses causing resin cracking in an organic chip carrier laminate are due to both global deformation and local stress concentration from variable density of wiring features in the chip carrier. The package deformation is primarily driven by the thermal expansion mismatch between the silicon chip, the chip carrier and the heat spreader. The local circuit features, on the other hand, generate significant stress concentration, which greatly undermines the fatigue strength of the laminate. This paper focuses on a parametric study of the geometric features that accelerate laminate resin cracking. Flex fatigue experiments on standalone laminates with different design features will be described. In these experiments, the resin cracking stresses are simulated with 4-point bending. The cyclic effect is imitated by repetitive mechanical loading and unloading. The resin cracking failure is tracked by monitoring the electrical resistance of sensor nets in the laminate. This paper will be discussing the experimental results for test vehicles made of different material sets, both tested in ambient and in low temperature environments. A thermomechanical modeling methodology is outlined and verification of simulations by experimental results is presented.