Mechanical Reliability of Flip Chip Interconnection in Pb-free Systems


Yifan Guo

Skyworks Solutions, Inc. 

4311 Jomboree Road

Newport Beach, CA 92660





In the development of flip chip interconnect using Pb-free solders, the reliability of the interconnect system under the thermal cycle conditions need to be determined. The major concerns are solder fatigue and UBM (Under Bump Metallurgy) strength.  In this report, a methodology of evaluating the UBM reliability in lead free systems is presented. To ensure the UBM reliability, it is required that the UBM strength is greater than the UBM stress at any point of the thermal cycles. A simplified stress model is developed to determine the UBM stress in the thermal cycles. The die pull test is a common method which is used to evaluate the UBM strength. A model for the UBM strength evaluation is also developed for the die pull test. According to these models, the stress and strength of the UBM are determined under the die pull tests and the thermal cycle conditions for three lead free solder systems.  In order to calculate the stress in the UBM, material properties of the lead free solders were needed. The measurements of material properties of the three lead free solders are also discussed in this report.