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

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.