Reworkable epoxy underfills with optimized curing and flow behavior for advanced electronic packaging

Purpose This study aims to evaluate the curing, thermo-mechanical and interfacial behavior of two commercial reworkable epoxy underfills for surface mount technology (SMT) applications. The work focuses on understanding the relationship between curing characteristics, viscosity and wettability, and how these parameters influence underfill flow, fillet formation, voiding and rework performance. By integrating Fourier Transform Infrared (FTIR), Differential Scanning Calorimetry (DSC), thermomechanical analysis (TMA), rheology, contact angle and microstructural analyses, the study seeks to identify an optimal underfill formulation that balances processability, reliability and reworkability which are critical factors for improving manufacturing yield, repair efficiency and sustainability in advanced electronic packaging. Design/methodology/approach The increasing complexity of SMT assemblies has intensified the need for reliable yet reworkable underfill materials. This study evaluates two commercial reworkable epoxy underfills (samples A and N) with respect to their curing, thermo-mechanical, flow and rework performance. FTIR and DSC analyses confirmed complete curing for both materials, with sample N showing higher reactivity while sample A exhibited controlled curing behavior favorable for reworkability. TMA and rheological measurements highlighted significant differences in thermal expansion and viscosity response at processing temperatures, with sample A maintaining more stable flow and wettability at 60 °C. Contact angle, cross-sectional and lapping analyses revealed that sample A achieved superior wetting, fillet formation and coverage, reducing void formation compared to sample N. Rework tests demonstrated that sample A could be removed cleanly with minimal Printed Circuit Board and component damage, while sample N left residues and caused surface scratches. Overall, sample A provided a more balanced trade-off between reliability, processability and reworkability, underscoring its suitability for advanced SMT applications and its potential contribution to cost-effective and sustainable electronics manufacturing. Findings Both underfills achieved complete curing, but Sample A exhibited controlled curing, balanced viscosity and stable wettability at dispensing temperature, enabling superior flow and coverage. Sample N showed higher crosslink density and adhesion, improving stability but hindering reworkability. Cross-sectional and lapping analyses confirmed that Sample A produced fewer voids and better fillet uniformity, while rework trials demonstrated cleaner removal and minimal substrate damage. Overall, Sample A offered an improved trade-off between reliability and reworkability, making it more suitable for high-density SMT assemblies where efficient repair and process stability are essential. Originality/value This work provides an integrated, process-focused evaluation of commercially available reworkable underfills, linking fundamental material behavior to actual SMT assembly and rework performance. Unlike prior studies that focus mainly on resin formulation or curing kinetics in isolation, this study correlates curing conversion (FTIR/DSC), thermo-mechanical response (TMA), flow behavior (viscosity/wettability), fillet formation, void distribution and rework cleanliness under realistic dispensing and rework conditions. The results highlight the trade-off between reliability and reworkability and identify an underfill formulation that supports both high assembly yield and clean component recovery, directly addressing manufacturability and sustainability requirements in advanced electronic packaging.