Characterization of orthotropic CTE of BT substrate for PBGA warpage evaluation

Material characterization is a crucial part of BGA thermal warpage evaluation. In principle, the warpage behavior of BGA under temperature change is affected by the elastic modulus and the coefficient of thermal expansion (CTE) of the packaging materials. Therefore, the accuracy of material properties is critical for predicting the warpage trend and quantity. However, due to the rather thin BT substrate, it is difficult to measure the accurate orthotropic CTE in the planar direction using the conventional TMA method. This study proposes a new method to estimate the CTE of the BT substrate in PBGA devices. The required sample is cut into strips, which are bi-material beams containing the BT laminate and EMC of a PBGA. The concept of Stoney's formula is employed to establish the relations of the CTE, the curvature and other parameters of the sample. Through this method, the CTE of the BT substrate is measured using a dynamic mechanical analyzer (DMA) and calculated as a function of temperature, giving a more accurate description than the conventional temperature dependent CTE. A simulation study and the corresponding mechanism analysis are also given in this paper. With the estimated BT substrate CTE and other material properties (including elastic moduli and CTE of EMC), the warpage simulation is performed in the FEA model of the bi-material beam as a sub-model of the PBGA device to estimate the warpage behavior. From the comparison of the simulation and experiment results, the measured material properties and calculated BT CTE could lead to a reasonable prediction of warpage in both the qualitative and quantitative senses. The proposed methodology may be used for academic research and industrial applications.