Date of Graduation

7-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Electrical Engineering

Advisor/Mentor

Naseem, Hameed A.

Committee Member

Spiesshoefer, Silke A.

Second Committee Member

Huitink, David

Third Committee Member

Ware, Morgan E.

Keywords

Ceramic synthesis; Die-attach process; Electronic Packaging; High-temperature; Integrated Electronics; Wire bonding

Abstract

This dissertation research focused on the synthesis and application of ceramic paste for high-temperature applications. An alumina paste material comprising aluminum dihydric phosphate and alumina powder was developed for high-temperature electronic packaging. Nano aluminum nitride and nano-silica powders were embedded to promote the paste curing process, limit the grain growth, and increase its bond shear strength. The chip-to-substrate bond strength was enhanced and met the MIL-STD requirements for die-attach assembly. Its encapsulation property was improved with fewer cracks compared to similar commercial ceramic encapsulants. The die-attach material and encapsulation properties tested at 500°C showed no defect or additional cracks. Thermal aging and thermal cycling were carried out on the synthesized paste. XPS analysis revealed a higher oxygen bonding percentage for the 10% nanosilica ceramic sample than other samples. XRD peak broadening is largest for the 10% nano-silica ceramic which indicated smaller crystallite sizes. The smaller crystallite size for the 10% nanosilica sample introduces a larger microstrain to the alumina crystal structure. FTIR revealed the presence of alumina-silicate bonds on these samples with the largest amount present in the 10% nanosilica samples. SEM and EDX results showed a uniform bond line for the 10% sample and uniform material distribution.

An electronic packaging technology that survives the Venusian condition was developed. Alumina ceramic substrates and gold conductors on alumina were evaluated for electrical and mechanical performance. The most promising die-attach materials were found to be thick-film gold and alumina-based ceramic pastes. Alumina, sapphire, silicon, and silicon carbide dice were attached to the alumina substrates using these die-attach materials and exposed to the Venusian condition for 244 hours. The devices on the packaging substrates were encapsulated by a ceramic encapsulant with no significant increase in cracks and voids after the Venusian simulator test. Wire pull strength tests were conducted on the gold bond wire to evaluate mechanical durability before and after the Venusian simulator exposure test with about 30.8% decrease which satisfied the minimum requirement for the MIL-STD-885 method. The overall wire-bond daisy-chain resistance change was 0.47% after the Venus simulator test, indicating a promising wire bond integrity. A titanium package was fabricated to house the ceramic packaging substrate and a two-level metalized feedthrough was fabricated to provide electrical interfaces to the package.

A double-layer ceramic electronic packaging technology that survives the Venusian surface condition was developed using a ceramic interlayer dielectric with gold conductors. A 60-µm ceramic interlayer dielectric served as the insulator between the top and bottom gold conductors on high-purity ceramic substrates. Test devices with AuPtPd metallization were attached to the top gold pads using a thick-film gold paste. Thermal aging for 115 hours at 500°C and thermal cycling from room temperature to 450°C were performed. Dielectric leakage tests of the interlayer ceramic layer between the top and bottom gold conductors revealed a leakage current density of less than 50  10-7 A/cm2 at 600V after thermal cycling. The die shear test showed a 33% decrease in die shear strength after thermal tests but still satisfies the MIL-STD method.

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