Simultaneous Measurement of the Thermal Boundary Resistance and Thermal Conductivity of Die-Attach Materials in Sandwich Structures via Multiwavelength Transient Thermoreflectance

The thermal conductivity (k) of die-attach materials has traditionally been considered the primary factor governing the total thermal resistance (Rth) in chip interlayers, whereas the thermal boundary resistance (TBR) between the die and the die-attach layer (e.g., nanosilver paste) is often overlooked, leading to a significant underestimation of the overall Rth. Due to the embedded nature of die‑attach materials, the simultaneous measurement of both TBR and k in sandwich configurations presents an experimental challenge. This work demonstrates the feasibility of concurrently characterizing TBR and k for nanosilver paste in sandwich structures using a multiwavelength transient thermoreflectance technique integrated with a bidirectional heat flow model. Validation tests performed on sandwich samples containing nanosilver paste, as well as four common fluid materials, confirm the reliability of the proposed approach. Samples bonded with Au exhibit substantially lower TBR than those bonded with Al, which can be attributed to Au’s exceptional chemical stability and superior wettability during soldering with nanosilver paste. Monte Carlo error analysis indicates that Au‑bonded samples show uncertainties of approximately ±6% for the TBR between Au and nanosilver paste (TBRAu‑paste) and ±5% for k of the nanosilver paste (kpaste), whereas Al‑bonded samples exhibit higher uncertainties ranging from ±7−17% for TBR between Al and nanosilver paste (TBRAl‑paste) and ±12−19% for kpaste. Sensitivity analysis elucidates the origin of the greater measurement variability observed at Al‑bonded interfaces. The developed method thus establishes a practical and effective platform for the thermophysical characterization of die‑attach layers in advanced chip manufacturing.