Stable interconnections for LTCC micro-heater using isothermal solidification technique

Abstract

Purpose

The purpose of this work is to explore the forms of intermetallic phase compounds (IMPCs) in Pt/In/Au and Pt/In/Ag joints by using isothermal solidification. This lead-free technique leads to formation of IMPCs having high-temperature stable joints for platinum-based micro-heater gas sensor fabricated on low temperature co-fired ceramic (LTCC) substrate.

Design/methodology/approach

Proposed task is to make an interconnection for Pt micro-heater electrode pad to the silver and gold thick-films printed on LTCC substrate. Both Pt/In/Au and Pt/In/Ag configured joints with different interactive areas prepared at 190 and 220°C to study temperature and contact surface area effects on ultimate tensile strength of the joints, for a 20 s reaction time, at 0.2 MPa applied pressure. Those delaminated joint interfaces studied under SEM, EDAX and XRD.

Findings

IMPCs identified through material analysis using diffraction analysis of XRD data are InPt3, AgIn2, AgPt, AgPt3, Au9In4 and other stoichiometric compounds. The interactive surface area between thick-films and temperature increment shows improvement in the formations of IMPCs and mechanical stability of joints. These IMPCs-based joints have improved the mechanical stability to the joints to sustain even at high operating temperatures. Elemental mapping of the weak joint contact interface shows unwanted oxide formations also reported. Physical inter-locking followed by the diffusion phenomenon on the silver substrate strengthen the interconnection has been noticed.

Research limitations/implications

Inert gas environment creation inside the chamber to isolate the lead-free joint placed between heating stamp pads to avoid oxide formations at the interface while cooling which adds up to the cost of manufacturing. Most of the oxides at a joint-interface increase minute to moderate resistance with respect to the level of oxides took place. These oxides contributed heat certainly damage the micro-heater based gas sensors while functioning.

Practical implications

These isothermal solidification-based lead-free solder joints formation replace the existing lead-based packaging techniques. These lead-free interconnections on ceramic or LTCC substrate are reliable and durable, especially those designed to work for heavy-duty engines, even at severe environment conditions.

Originality/value

Platinum micro-heater-based gas sensors handles over a wide-range of temperatures about 300 to 500°C. The specific temperature level of different oxide films (SnO2) on the micro-heater is capable of detecting various specific gases. This feature of platinum based gas sensor demands durable and mechanically stable joints for continuous monitoring.

Citation

Suresh Kumar, D., Suri, N. and Khanna, P.K. (2020), "Stable interconnections for LTCC micro-heater using isothermal solidification technique", Soldering & Surface Mount Technology, Vol. 30 No. 1, pp. 35-41. https://doi.org/10.1108/SSMT-04-2017-0012