die attach

**Die Attach and Thermal Interface Materials** is **the process and materials used to mechanically bond a semiconductor die to its package substrate or heat spreader while providing an efficient thermal conduction path from the active junction to the heat sink** — die-attach quality directly impacts device reliability, thermal resistance, and long-term performance in applications ranging from consumer electronics to automotive power modules. - **Epoxy Die Attach**: Silver-filled conductive epoxy is the most common die-attach material for low-to-moderate power devices. Dispensed or stamped onto the substrate, it cures at 150–175 °C with bond-line thickness (BLT) of 15–30 µm. Thermal conductivity ranges from 2 to 25 W/m·K depending on filler loading. - **Solder Die Attach**: For higher thermal performance, solder preforms (AuSn, SAC305, or high-Pb) are reflowed at 280–320 °C, yielding BLT of 10–25 µm and thermal conductivity of 30–60 W/m·K. Solder voiding must be kept below 5% by flux activation and vacuum reflow. - **Silver Sintering**: Nano-silver or micro-silver paste sintered at 200–250 °C under pressure (10–30 MPa) creates a porous silver joint with thermal conductivity exceeding 200 W/m·K and melting point of 961 °C. This enables reliable operation at junction temperatures above 200 °C, ideal for SiC and GaN power devices. - **Thermal Interface Materials (TIMs)**: TIM1 fills the gap between the die and an integrated heat spreader (IHS); TIM2 fills the gap between the IHS and the heat sink. Materials include thermal greases (3–8 W/m·K), phase-change materials, gap pads, and indium foil (80 W/m·K). Lower thermal resistance requires thinner BLT and higher intrinsic conductivity. - **Thermal Resistance Stack**: Total junction-to-ambient thermal resistance Rθja = Rθjc + RθTIM1 + Rθspreader + RθTIM2 + Rθheatsink. Die-attach and TIM1 often dominate Rθjc, making their optimization crucial for thermal management. - **Voiding and Delamination**: Voids in the die-attach layer create local hot spots and stress concentrations. X-ray inspection and scanning acoustic microscopy (SAM) are standard inline screens. Delamination during thermal cycling is tested per JEDEC moisture-sensitivity-level (MSL) protocols. - **Wire-Bondable vs. Non-Wire-Bondable**: Die-attach materials for wire-bonded packages must withstand ultrasonic energy without cracking. Flip-chip die attach may combine underfill with solder bumps, serving both electrical and mechanical functions. - **Automotive and High-Reliability Requirements**: AEC-Q100 Grade 0 (−40 to +150 °C) applications demand die-attach materials with matched CTE, minimal voiding, and robust adhesion after thousands of thermal cycles. Die-attach and TIM selection are pivotal engineering decisions that balance thermal performance, manufacturing processability, reliability, and cost across an enormous range of semiconductor applications.