Effect of high temperature cycling on both crack formation in ceramics and delamination of copper layers in silicon nitride active metal brazing substrates

Significance Statement

Power modules with silicon carbide have received a lot of interest because of their ability to operate at high temperatures, possess high breakdown voltage, and offer low switching loss. The high temperature modules find several applications as industrial down-hole oil and gas detection for well logging, in space exploration, electric vehicles and aircrafts. Metalized ceramic substrates are important building blocks for power modules because they provide electrically conducting paths with low resistivity, electrical insulation between circuit layers and heat sinks, and thermal paths between power devices and heat sinks.

Directly bonded copper substrates have been applied in silicon-based power modules owing to their operation and availability. Alumina is perhaps the most popular ceramic substrate, while aluminum nitride is normally used for high thermal conductivity applications.  However, a rise in the junction temperature at which silicon-based power devices operate, leads to significant reliability issues for the direct bonded copper substrate. There is a mismatch in thermal expansion coefficient between copper layers and ceramic substrates leading to large thermal stresses, and consequently fracture.

Silicon nitride with moderate thermal conductivity and excellent mechanical attributes such as toughness and high strength has been used as an insulating ceramic material. Silicon nitride active metal brazing has therefore been developed recently to offset the issues of wide range thermal cycling. Such substrates have superior resistance to the peeling off of copper layers. Dr. Hiroyuki Miyazakia and colleagues at the National Institute of Advanced Industrial Science and Technology in Japan investigated the detachment of copper layers from silicon nitride active metal brazing and aluminum nitride substrates in the temperature range of -40 to 250 °C implementing acoustic scanning microscopy imaging and residual bending strength measurements. Their work is published in Ceramics International.

The authors measured flexural strengths of rectangular aluminum nitride and silicon nitride samples applying the 4-point bending. They applied a compression load using an articulated 4-point fixture. The researchers also employed a modified edge-precracked plate method to investigate the fracture toughness applying aluminum nitride samples with 40x4x0.30 mm3 dimensions and 40x4x0.34 mm3 silicon nitride samples.

When they investigated the reliability of aluminum nitride and silicon nitride substrates under -40 to 250 °C thermal cycling, they did not observe copper layer detachment from the ceramic substrates for both silicon nitride active metal brazing substrates with 0.15mm and 0.3mm thick copper layers even after 1000 cycles. This was a demonstration of superior reliability as compared to aluminum nitride substrates, which exhibited copper detachment.

Acoustic scanning microscopy analysis revealed that for the 0.30mm silicon nitride-copper substrate, cracks were initiated in the ceramic fabric at the corner joint with copper layer after 100 cycles. The size and number of cracks increased with an increase in the number of thermal cycles. However, no crack was detected for the 0.15mm silicon nitride-copper substrate even after 1000 cycles.

The researchers also observed that the degradation of the residual bending strength for the silicon nitride substrates began from ten thermal cycles and proceeded gradually with the number of thermal cycles. However, the drop was much slower than that of aluminum nitride, and more than 65% of the initial bending strength was retained after 1000 cycles.

Crack depth in silicon nitride substrates with 0.3mm thick copper layers was 130µ after 1000 cycles while for the aluminum nitride substrate with 0.3mm thick copper layers, the crack depth reached 180µ after five thermal cycles only.

crack formation in ceramics and delamination of copper layers in silicon nitride active metal brazing substrates

About The Author

Hiroyuki Miyazaki is a Senior Research Scientist of Structural Materials Research Institute at National Institute of Advanced Industrial Science and Technology (AIST), Japan.  He earned his BS in inorganic materials engineering and MS in energy science from Tokyo Institute of Technology.   He completed his doctorate in inorganic materials engineering at the Tokyo Institute of Technology in 1997.

He engaged in the standardization project of mechanical testing for failure characteristic of advanced ceramics used in rolling bearing balls in 2005-2009.  This led to the development of reliable indentation fracture (IF) method.   The draft of International Standard Organization (ISO) was submitted to the ISO/TC 206 committee on the basis of results of the project.  He directed this standardization work item as a project leader with the help of experts from ISO member.  Finely, ISO 14627 “Fine ceramics (advanced ceramics, advanced technical ceramics) – Test method for fracture resistance of silicon nitride materials for rolling bearing balls at room temperature by indentation fracture (IF) method” was published in 2012.  To expand the scope of application, his new approach was testified in the round-robin tests using SiC, Si3N4 and Al2O3 in 2011-2014.   A draft of the improved IF method for general engineering ceramics has been submitted to ISO/TC206 in 2016 and is now under discussion among the expert.

Now his research also focuses on the standardization of reliable mechanical tests of ceramic thin substrates, that is, single edge-precracked plate (SEPP) method for determination of fracture toughness of ceramic thin plates and three-point & four-point flexural strength test of ceramic thin plates.  The draft of the former subject has been accepted as a committee-stage draft of ISO/TC206 and will be published within a couple years. The draft of the later subject will be submitted to ISO/TC206 this autumn.

 Reference

Hiroyuki Miyazakia, Shoji Iwakiri, Kiyoshi Hirao, Shinji Fukuda, Noriya Izu, Yuichi Yoshizawa, Hideki Hyuga. Effect of high temperature cycling on both crack formation in ceramics and delamination of copper layers in silicon nitride active metal brazing substrates. Ceramics International, volume 43 (2017), pages 5080–5088.

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