Study of the thermo-mechanical behavior of glass interposer for flip chip packaging applications

The reliability of flip chip packages using either glass or organic substrate, and then mounted on printed circuit board (PCB) was investigated. This study is focused on the high stress concentration areas in the package structures and identified the stress mitigation solutions. Four-point bending test revealed that the fracture strength of the glass with through glass vias (TGV) can be significantly reduced by 10~30% with respect to the different via densities. The brittle nature of glass along with its low fracture strength, particularly with TGV, could have serious implications on the reliability of the packaging system using glass substrate. In order to increase its fracture strength, the glass surface was coated with a low CTE film and is shown to effectively strengthen the glass. The assembled glass substrate packages subjected to thermal stressing were simulated by finite element modeling. To reduce the high stress concentration and improve the stress distribution in the glass interposer system, the parametric factors on the structure, materials, geometry and shape were investigated. According to our simulation result, the CTE of glass substrate is critically important in affecting the stress levels in the Si die, BGA ball and glass itself. To minimize the stress levels occurring on either side of the package structure, glass substrate with a medium CTE (~8.3 ppm) is demonstrated as the optimal choice when sandwiched between the Si die (CTE~2.7 ppm) and PCB board (CTE~16 ppm) with C4 and BGA balls, respectively. Since the glass CTE can be tailored by adjusting its compositions, glass with an optimal CTE can be manufactured by the supplier.