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Advance Board Level Modeling Wafer Level Packages
In this paper it is determined that the effective flexural modulus depends on the stackup and it often does not correlate with the effective tensile modulus.
Technical Paper
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Authored By:
Tiao Zhou, Ph.D.
Southern Methodist University
Dallas, TX, USA
Zhenxue Han, Ph.D.
University of Texas at Arlington
Arlington, TX, USA
Tingge Xu, and Hongbing Lu, Ph.D.
University of Texas at Dallas
Dallas, TX, USA
Kenna Wang
Creekview High School
Carrollton, TX, USA
Summary
In finite element analysis (FEA) of board level temperature cycling (TC) or drop test (DT) for wafer level packaging (WLP), the printed circuit board (PCB) is often simplified as a homogeneous material. The PCB effective elastic modulus is one of the key properties required for FEA. It is obtained from tensile test, bending test, or calculation. The tensile and flexural moduli however, often have significantly different values. The FEA results thus can be inaccurate if PCB modulus is not chosen properly.
In this paper, the effects of PCB stackup, metal contents and metal trace orientations on effective tensile and flexural moduli are studied. It is determined that the effective flexural modulus depends on the stackup and it often does not correlate with the effective tensile modulus. Observations are made to assess if the PCB deformation is tension/compression dominant or bending dominant during TC and DT. Guidelines for effective elastic modulus calculation from tensile and flexural moduli are given in order to minimize the error in FEA of WLP board level TC and DT.
Conclusions
PCB effective elastic modulus obtained from different methods are presented. The effect of choice of elastic modulus on FEA results is investigated. The following conclusions are made:
1. The values of tensile modulus and flexural modulus are different. The flexural modulus is stackup dependent while the tensile modulus is not.
2. Smaller trance angle on routing layers results in higher effective modulus. Using Cu content alone and ignoring trace routing detail overestimates the effective modulus.
3. When simplifying the PCB as a homogeneous material, using tensile or flexural modulus alone as the effective elastic modulus in both TC and DT FEA results in error. It is recommended that:
a. Use the greater of the tensile and flexural moduli as the effective modulus for TC
b. Use the flexural modulus as the effective modulus for DT
Initially Published in the SMTA Proceedings
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Comments
Interesting observation backed with substantial data. Thanks for sharing.
Pankaj Shrotre, Sierra Circuits
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