Mechanical Characterization of Low Temperature Co-Fired Ceramics for Microelectronic Applications

Clemens Krautgasser

Research output: ThesisDoctoral Thesis

Abstract

Low temperature co-fired ceramics consist of ceramic grains in a glass matrix. They are used in modern electronics as substrate material for ceramic based printed circuit boards (e.g. RADAR-, Bluetooth- or WLAN- antennas). The reliability of brittle components depends strongly on their structural integrity. Therefore the characterization of mechanical strength and the resistance to crack propagation (i.e. fracture toughness) depending on loading, environment and time is very important. Strength tests were performed at elevated temperatures in air and argon showing a combined influence of temperature and humidity on the strength. Two mechanisms (i) thermal activated hydrolysis of bonds at the crack tip and thermal bond breaking on the one hand and (ii) chemical potential for hydrolysis on the other hand are dominating the subcritical crack growth. In air (room temperature up to 125°C) a counterbalancing of both mechanisms was found. Existing knowledge on uncertainties in SCCG parameter determination using constant stress rate experiments was expanded using the Monte Carlo simulation technique. Different influences from material (e.g. natural strength distribution) and experiment (e.g. measurement uncertainty of the strength, sample size, ...) were studied. In addition a Mathematica code to estimate confidence intervals for performed experiments is discussed. For three LTCC materials the fracture toughness was determined with special focus on the environmental influence (air, water, argon) and the measurement technique (single edge V-notched beam, chevron notched beam and ball on three balls test with artificial flaw). Testing conditions (slow loading rate, humid environment) were chosen to provoke SCCG during testing. The influence of the glass phase condition – amorphous or mainly crystalline – on the sensitivity to SCCG could be shown. In addition the influence of environment (dry to humid) compared to the influence of the loading rate (fast to slow) on the determination of a correct toughness value was presented. A significant improvement of strength, and therefore an improvement of structural integrity, can be achieved by residual compressive stresses in the surface layer. A case study on LTCC laminates showing different values of residual compressive stresses on the surface was performed and the influence of residual stress extended strength on the SCCG behavior was characterized. The potential of reliability enhancement using tailored residual stresses was discussed indicating a possible way for future ceramic based printed circuit boards and electronics.
Translated title of the contributionMechanische Charakterisierung von LTCC-Keramiken für die Mikroelektronik
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Danzer, Robert, Assessor A (internal)
  • Roosen, Andreas, Assessor A (external), External person
  • Bermejo, Raul, Co-Supervisor (internal)
Publication statusPublished - 2015

Bibliographical note

embargoed until null

Keywords

  • Ceramic
  • LTCC
  • SCCG
  • humidity
  • subcritical crack growth

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