Evaluation of compatibility of thick-film PTC thermistors and LTCC structures

• Published in: Microelectronics and reliability Vol. 45; no. 12; pp. 1924 - 1929
• Main Authors: Belavič, Darko, Hrovat, Marko, Kita, Jaroslaw, Holc, Janez, Cilenšek, Jena, Golonka, Leszek, Dziedzic, Andrzej
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Oxford Elsevier Ltd 01-12-2005; Elsevier
• Subjects: Applied sciences; Electronic equipment and fabrication. Passive components, printed wiring boards, connectics; Electronics; Exact sciences and technology; Semiconductor electronics. Microelectronics. Optoelectronics. Solid state devices; Thermoelectric, pyroelectric devices, etc; Thermistor; Temperature coefficient; Thick film resistor; Sheet resistivity; Interdiffusion; Microstructure; Ceramic materials; Electrical characteristic; Crack
• ISSN: 0026-2714; 1872-941X
• DOI: 10.1016/j.microrel.2005.03.002

The electrical and microstructural characteristics of 1
kΩ/sq. thick-film thermistors with high positive temperature coefficients of resistivity, i.e. PTC 5093 (Du Pont) fired either on “green” LTCC (low temperature cofired ceramics) substrates or buried within LTCC structures, were evaluated. The active phase (ruthenium oxide) in the PTC thermistors is not present in the dried films but is formed during firing due to the decomposition of the ruthenate phase. Because of interactions between a glassy LTCC material and thermistors electrical characteristic, i.e. sheet resistivities and noise indices of thermistors fired on the surface LTCC substrates, changed from values obtained on alumina substrates. The differences in the measured electrical parameters were attributed to the interactions between the thermistor layers and the glassy LTCC substrates. The inter-diffusion of oxides, mainly PbO and Al
2O
3, was confirmed by microanalysis. In the case of buried thermistors, presumably due to the incompatibility of both materials, the structure de-laminated during firing. Cracks between the buried PTC films and the LTCC substrates as well as cracks in PTC films resulted in high sheet resistivities and high noise indices.