Thermo-mechanical reliability evaluation and comparative fatigue assessment of 2.5D chiplet packages with viscoelastic C4 underfill

By Peisheng Liu ^1,2, Feiyu Qiang ¹, Yaohui Deng ¹, Jiajie Jin ¹, Jinlan Wang ^1,2
1 School of Microelectronics, Nantong University, China
² Department of Information Technology, Xinglin College, Nantong University, China

Abstract

2.5D chiplet packaging is increasingly constrained by thermo-mechanical fatigue under thermal cycling. This work presents a case-oriented sequentially coupled thermo-mechanical finite-element evaluation that combines heat conduction, temperature-dependent constitutive laws, and a JEDEC-based environmental thermal-cycling profile in a representative package with viscoelastic underfill. The objective is to compare the relative influence of C4-underfill condition and solder-alloy selection under consistent modeling assumptions, rather than to establish a package-specific calibrated lifetime-estimation method. In the model, micro-bump underfill is retained as a structural baseline, while C4 underfill is introduced as a comparative variable in the transient fatigue analysis. Thermal fields are mapped into structural simulations with a Prony-WLF viscoelastic underfill model and an Anand viscoplastic solder law, allowing consistent extraction of stress redistribution, creep, relaxation, and local fatigue-driving indicators at critical package interfaces.

Simulations show that heat removal is dominated by the upward cooling path, creating a vertical temperature gradient and pronounced thermo-mechanical sensitivity in peripheral C4 solder joints. Under cycling, the joints approach a near-stabilized later-cycle response in which the equivalent plastic strain range Δε_p and the inelastic strain-energy density increment ΔW_ave serve as comparative local fatigue-driving quantities. Applying the strain-based Engelmaier and energy-based Darveaux relations to the same later-cycle fields indicates that the outermost package-edge joint remains the dominant damage-sensitive location. In the transient fatigue analysis, introducing a viscoelastic underfill around the C4 solder balls reduces Δε_p and ΔW_ave at these joints to about 5–10% of their no-underfill values, leading to markedly higher comparative fatigue-life estimates under the present modeling assumptions.

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