POP package (Cavity BGA) warpage improvement and stress characteristic analyses

Abstract

In recent years, miniaturization, lightening, high performance, high reliability and low cost have been demanded intensely for electronic products, especially in the rapid growth of portable cell phone domain. Furthermore, multiple functional demand induces advanced package developments, such as system-on-chip (SoC) and system-in-package (SiP). System-on-chip (SoC) is an ideal package to integrate multiple functionalities in the chip level. However, the design and testing are very difficult, high cost and low manufacturing yield, these reasons drives multiple functional integration technology toward system-in-package (SiP) development gradually.Package-on-package (PoP) is a multi-package stacking structure of the TFBGA onto a BGA top that aims to reduce the placement and routing areas on board. This paper would introduce a new BGA structure with cavity for die placement and encapsulated by glob top. Unlike the regular PBGA as POP bottom package with the constraint of solder ball height, the package Cavity BGA would be more flexible The cavity BGA package has some other benefits such as the use of glob top to get rid of spending specific molding compound tooling cost and less warpage issue. The warpage performance and glob top material properties are closely related. The warpage Improvement criteria for glob top material properties are concerned and provided in the paper. This study is to investigate the cavity BGA packages warpage performance improvement and stress characteristic analyses as various liquid compound materials.The 3D finite element models using were commercial software ANSYS 9.0 constructed to analyze the warpage and stresses of the cavity BGA packages. For bottom warpage simulation, 15x15 mm2 cavity BGA with die size of 9.9x9.6 mm2 and thickness of 75 mum are utilized and the thermal loading was performed from 175 to 25degC. Four kinds of glob top materials were utilized in bottom warpage simulation. The warpage performance of the package would be decided by glob top material properties such as glass transition temperature (Tg), coefficient of thermal expansion(CTE), Young's modulus (E). Besides, the geometry dimension designs of package structure would have effects on warpage performance. They include die size, die thickness, core thickness, solder mask thickness, glob top thickness, and number of substrate. The foregoing various parameter effects on bottom warpage would have discussion in the paper. For Stress Characteristic issue, ball stress comparison with various TFBGA die size and TFBGA die stacking number are contained.The studies conclude that the cavity BGA with liquid compound (higher Tg= 135degC ) generates the smallest bottom warpage (6.5mil), since higher Tg causes smaller warpage. When the core thickness of cavity BGA increases from 60 mum to 100 mum, the bottom warpage improves 10%. The cavity BGA with 6 layers substrate generates bottom warpage 25% smaller than that with 4 layers. For warpage improvement of glob top properties, the simulation analyses demonstrated the high Tg is prefered to acquire the better warpage performance. The properties beyond Tg effect on warpage performance can be neglected within a specific range adjustment. The further direction would recommend with (1) higher CTE before Tg, (2) higher Young's modulus before Tg, (3) higher Tg. Also, the positions of the ball stress concentration and higher ball crack risk will be presented and discussed in the paper. The influences of the structure designs and material property options on the warpage behavior are investigated to establish useful design guidelines.