Defect-Free Cu-AI Interconnects: Enhancing Automotive Reliability via Dual-Metal Passivation

The relentless expansion of automotive electronics, driven by the convergence of Advanced Driver Assistance Systems (ADAS), autonomous vehicle architectures, and rapid electrification has fundamentally reshaped the reliability paradigms of IC packaging. Automotive standards, particularly AEC-Q100 Grade-0, now mandate a zero-defect philosophy, compelling components to sustain near-perfect performance under extreme thermal excursions, humidity, and electrical bias. While wire bonding retains its dominance as the primary interconnect technology due to its established supply chain and cost-efficiency, the transition from gold (Au) to copper (Cu) and palladium-coated copper (PCC) remains a complex challenge. Although Cu wire offers superior electrical and thermal conductivity alongside slower intermetallic compound (IMC) growth, it introduces a critical vulnerability: a significantly heightened susceptibility to corrosion and voiding in harsh operating environments. [1]

Fundamentally, the Cu-Al interface constitutes a galvanic couple driven by a substantial electrochemical potential difference. The risk of failure is exacerbated by the hygroscopic nature of traditional epoxy molding compounds (EMCs), which facilitate moisture ingress. When combined with mobile ionic contaminants, specifically chloride ions (Cl-) derived from environmental exposure or material outgassing, the interface functions as a galvanic cell [2]. In this system, the aluminum bond pad acts as the anode, undergoing accelerated oxidative dissolution, while the copper wire serves as the cathode, supporting oxygen reduction. The presence of Cl- is particularly deleterious; it attacks the native alumina (Al2O3) layer and acts catalytically to drive localized pitting, crevice corrosion, and eventually, catastrophic bond lift-off [3].

To mitigate this failure mechanism at its source, this work presents a novel, high-volume-manufacturing (HVM) compatible dual-metal passivation strategy. (Fig. 1) Existing solutions, such as Organic Solderability Preservatives (OSP) or organic inhibitors, typically target copper selectively, leaving the aluminum anode vulnerable. In contrast, this approach utilizes a two-step aqueous process to simultaneously passivate both the cathodic Cu wire and the anodic Al pad. A preliminary activation step generates a high density of reactive surface hydroxyl groups, which serve as anchoring sites for the subsequent deposition of a self-assembled, hydrophobic overlayer. This molecular film forms a continuous, covalent barrier that interrupts the corrosion circuit at both electrodes, effectively blocking ion transport and reducing surface energy.

Extensive reliability testing validates the robustness of this chemistry. Under accelerated corrosion protocols involving immersion in 100 ppm Cl- solution, standard unprotected PCC-Al devices exhibited a 23.3% ball lift-off rate within just two hours, highlighting the inadequacy of native oxides. Conversely, devices treated with the dual-metal passivation-maintained failure rates well below 1% under identical stress conditions (Fig. 2). Surface characterization provides further insight into the protective mechanism. Static water contact angles increased by over 40% on both Cu and Al surfaces, confirming the formation of a low-surface-energy barrier that repels aqueous electrolytes. Furthermore, FTIR spectroscopy identified chemical signatures consistent with covalently bound organic layers, while Atomic Force Microscopy (AFM) verified that the coating forms a uniform nanoscale film without altering the topography essential for subsequent adhesion. Concurrent adhesion testing is also in progress to validate the passivated material’s compatibility with standard epoxy molding encapsulation.

In summary, this research demonstrates a practical, scalable passivation chemistry that stabilizes the Cu–Al interface against aggressive environmental factors. By bridging the protection gap between dissimilar metals, this method offers a vital solution for next-generation automotive devices and holds significant promise for emerging advanced packaging applications in MEMS and Cu-to-Cu hybrid bonding.