Investigation of Moisture-Induced Warpage of Chip-on-Wafer in 2.5D IC Package

2.5D multi-chip packaging technology places multiple SoC dies and chiplets on a silicon interposer wafer so that better interconnect density and performance can be achieved. Individual chips are bonded through micro-bumps on a silicon interposer forming a chip-on-wafer (CoW) module. Excessive moisture-induced warpage of CoW die could lead to C4 bump failure in on-substrate assembly process. The risk is even higher in a case where a CoW module has stacked dies like HBM or SoIC, which are built with several kinds of polymer materials, on top of the silicon interposer. A two-step finite element modeling was performed: 1) moisture diffusion within polymer materials in a CoW module was simulated; 2) then hygro-thermomechanical simulation was performed to predict the warpage during reflow. Absorption / desorption test and CHS measurement were performed to characterize the moisture properties of the polymetric materials in a chip-on-wafer module. Different HBM structures, material overflow of the polymer which the stacked dies were built with, and storage humidity condition were simulated and discussed.