High Density Hybridisation Using ENIG Bumping and Anisotropic Conductive Films

TL;DR

This paper introduces a scalable, low-cost flip-chip hybridisation method combining ENIG bumping with ACF bonding, enabling fine-pitch interconnections for high-density microelectronic assemblies without wafer-level processing.

Contribution

It demonstrates a novel ENIG-ACF hybridisation process for fine-pitch interconnections, including process development, experimental validation, and interface geometry optimisation through simulation.

Findings

Successful hybridisation of Timepix3 ASIC and sensors at 55 μm pitch.

Extended process to 25 μm pitch with aligned-particle ACFs.

Identified interface mismatch as a key limitation, leading to process optimisation guidelines.

Abstract

Fine-pitch hybridisation processes are essential for next-generation pixel detectors and high-density microelectronic assemblies. Conventional bump-bonding techniques, although reliable, remain costly and difficult to implement for single-die applications. In this work, we present flip-chip hybridisation results combining Electroless Nickel Immersion Gold (ENIG) bumping with Anisotropic Conductive Film (ACF) bonding, both developed in-house. This method enables fine-pitch interconnections without requiring wafer-level processing. The feasibility of the ENIG-ACF process for functional devices was demonstrated by hybridising functional Timepix3 ASIC and Ti-LGAD (Trench Isolated Low-Gain Avalanche Detector) sensors with 55 um pitch. The approach was then extended to test structures with 25 um pitch, using aligned-particle ACFs. The analysis revealed that the main limitation originated from the mismatch between ACF thickness and bump height, underlining the importance of interface geometry optimisation. To address this, a dedicated simulation program was developed to model the isotropic growth of ENIG bumps and to determine the optimum bump height as a function of ACF thickness and pixel-matrix geometry. The results provide valuable guidelines for future high connection density assemblies and demonstrate the potential of the ENIG-ACF process as a scalable, low-cost alternative to conventional bump-bonding techniques.