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Evaluation of Low-Pressure-Sintered Multi-Layer Substrates for Medium-Voltage SiC Power Modules

Fig. 1: C-SAM images of (a) nano-silver paste sample before (left) and after (right) 300 thermal cycles from –40 °C to 200°C, and (b) a nano- silver preform sample before (left) and after (right) 300 cycles. Cross-sections are cut and imaged at the dotted red lines after 300 thermal cycles.
  Direct-bonded aluminum (DBA) multi-layer substrates have been fabricated using low-pressure silver sintering. AlN DBAs are used instead of DBCs and silver sintering instead of solder to improve the reliability of the multi- layer structure. These multi-layer substrates can be used to reduce the peak electric field strength, screen common- mode noise, and improve mutual inductance cancellation. These benefits are particularly important for medium- voltage (MV) silicon carbide (SiC) MOSFETs due to their high operating voltages, and fast switching speeds. The voiding content and defect density of the sintered bond is critical to the thermal performance and reliability of the power module. Scanning acoustic microscopy (C-SAM) images were used to examine the uniformity and quality of the sintered bond of the fabricated multi-layer substrates. Two large-area silver sintering methods were evaluated: 1) screen-printed nano-silver paste, and 2) stamp-transferred nano-silver preform.
   The reliability of the multi-layer structure was evaluated through thermal cycling tests from -40 °C to 200 °C. Fifteen substrate stacks were fabricated for reliability testing. Four samples were bonded with nano-silver paste using 1 MPa pressure, and 11 samples were fabricated with nano- silver preform and either 1 MPa or 3 MPa pressure. The nano-silver preform sintering time is 90 seconds, while the nano-silver paste must be sintered for 1 hour. The samples fabricated with the nano-silver preform and 3 MPa sintering pressure were the most uniform of the population, according to the C-SAM images. Fig. 1 shows the location of cross-sections taken on samples after 300 thermal cycles. The C-SAM image Fig. 1(a) suggests voids in the silver bond line for the nano-silver paste sample. The nano-silver preform sample imaged in Fig. 1(b) showed more uniformity, and no abnormalities are present in the C-SAM image after 300 thermal cycles. The samples shown in Fig. 1 were cross-sectioned, polished, and imaged with a digital microscope. No obvious failure in the bond layer of either sample is shown in either cross-section. However, small cracks are observed at the edges of the bond of the nano-silver paste sample (Fig. 1(a)). Cross-sections of additional nano-silver paste and preform samples will be imaged after 300, 400, and 500 thermal cycles to better identify the onset and propagation of voids and cracks.

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