Jinghui Industry Ltd.
1. With the increasing requirements of performance for power modules, alumina (Al2O3) or aluminum nitride (AlN) ceramic substrates are no longer the best choice for the function of heat transfer and dissipation, and more and more designers have begun to consider using more advanced ceramic materials instead. For example, in the application of new energy vehicles (xEV), when the chip temperature rises from 150°C to 200°C, its switching loss will be reduced by 10% accordingly. In addition, new packaging technologies such as brazing and leadless modules also put forward more rigorous requirements on internal material parts.
2. Prolonging service life in harsh environments is another driving factor for the iteration of materials.
Like wind turbines, the expected service life of the wind turbine is 15 years without any failures in all environmental conditions. As a result, wind turbine designers have been also trying to improve substrate technology. The third actuator for improving ceramic substrate is the use of silicon carbide components (SiC). The first modules using silicon carbide with optimized packaging technology reduce losses by 40% to 70% compared to conventional ones, but the latter needs to be combined with new packaging material such as silicon nitride (Si3N4) substrates.
These trends reveals the limit the use of traditional alumina and aluminum nitride substrates in the future, and silicon nitride based substrates will become the preferred choice for high-reliability power module designers in the future.
Due to its outstanding thermal conductivity and thermal expansion coefficient close to the chip, as well as excellent bending strength and high fracture toughness, silicon nitride is very suitable for producing substrate products in the field of power electronics. The characteristics of silicon nitride ceramics and the detailed comparison of key values such as partial discharge or crack growth say that they have significant effect on the performance of the thermal conductivity and thermal cycle of the substrate.
When selecting insulation materials for power modules, the material characteristics that need to be considered mainly include thermal conductivity, bending strength and fracture toughness. high thermal conductivity is essential for fast heat dissipation of power modules. At the same time, the bending strength is very important for the processing and use of Ceramic Substrates in the packaging process, and that the fracture toughness is the key to predicting reliability.
96% alumina (Al2O3) Aluminum nitride (AlN) 9% Zirconia toughen alumina (9% ZTA) Silicon nitride (Si3N4) Thermal conductivity (W/m.K) Bending strength (MPa) Fracture toughness (MPa/√m)
24 180 280 90 450 450 700 650 3.8~4.2 3.0~3.4 4.5~5 6.5~7
B. The biggest advantage of aluminum nitride is that it has a very high thermal conductivity (180 W/mK), but its reliability is only moderate. This is because the fracture toughness of aluminum nitride is low, and the bending strength is similar to that of alumina.
C. In view of the increasing demand for higher reliability, zirconia toughened alumina (ZTA) ceramic materials came into being. These ceramic materials have higher flexural strength and fracture toughness, but their thermal conductivity is comparable to standard alumina. As a result, the former has limited use in high-power applications with the highest power density.
D. Looking at the table, silicon nitride perfectly combines high thermal conductivity and high mechanical properties. It has a thermal conductivity of 90 W/mK and the highest high fracture toughness (6,5-7 [MPa / √m]). Silicon nitride These characteristics will make it the most reliable metallization substrate option.
The reliability of various metallized substrates was tested by passive thermal cycling. All substrate combinations are listed in Table 2. All combinations use the same design, including the same copper thickness (d(Cu)= 0.3mm). Other design features such as pitting or gradient etching are not used to improve reliability. The detection conditions are as follows:
· Double cavity detection system
· Thermal conductivity =205 K (-55°C to +150°C)
· Exposure time: 15 min
· Tilt heating time < 10 s
In addition, different samples were examined by ultrasonic microscopy to detect stratification and conchoidal rupture: Alumina, 9% zirconia toughened alumina and aluminum nitride directly bonded to copper substrate: after 5 cycles per cycle
Silicon nitride active metal brazing (AMB) : after 50 cycles
Front surface copper (mm) Ceramic (mm) Back side copper (mm) Thermal cycle (1 cycle) Al2O3 DCB Substrate 0.30 0.38 0.30 55 9% ZTA DCB Substrate 0.30 0.32 0.30 110 AlN DCB Substrate 0.30 0.635 0.30 35 Si3N4 DCB Substrate 0.30 0.32 0.30 5000
A. Conch rupture is A typical failure mode in temperature cycling and has been detected in aluminum oxide, 9% zirconia toughened aluminum oxide and aluminum nitride directly bonded copper substrates. In general, the reason for the occurrence of conch rupture is that the coefficient of thermal expansion of copper and ceramics is different when the temperature changes.
B. In 35 thermal cycles, the reliability of aluminum nitride directly bonded copper substrate is the worst. Among all ceramic materials, aluminum nitride directly bonded copper substrates have the lowest measured fracture toughness (K1C=3-3,4 [MPa /√m]), which may explain the above findings. The results of aluminum oxide directly bonded to copper substrate after 55 cycles are very close to these results. Among conventional materials, 9% zirconia doped directly bonded copper substrates have the best performance, and their reliability is twice that of standard alumina materials (110 cycles).
C. No failure was detected in the silicon nitride active metal brazing sample after 5000 cycles. Compared with 9% zirconia toughened alumina directly bonded copper substrate, the reliability is 45 times higher. The excellent result of 5000 thermal cycles is due to the high fracture toughness of silicon nitride (K1C=6,5-7 [MPa / √m]), although its bending strength is slightly lower than 9% doped zirconia (650 MPa and 700 MPa).
These results show that the flexural strength of the Ceramic material used to fabricate the metallized substrate is not a key factor in determining the service life of the substrate. For reliability prediction, fracture toughness is the most important physical property of ceramic materials.
Figure 3: Failure Ultrasonoscopy of 9% ZTA DBC substrate
Figure 3: The main difference of failure mechanism between 9% zirconia toughened alumina directly bonded copper substrate and silicon nitride active metal brazing after multiple thermal cycles.
Figure 4: Failure Mode Ultrasonoscopy of Si3N4 DBC Substrate
After more than 5,000 cycles, the silicon nitride ceramic material remained intact.It can be seen in Figures 3 and 4 that after more than 5000 cycles, a conch-like rupture has occurred in 9% zirconia toughened Alumina ceramic material,while the silicon nitride ceramic material is still intact.
The thermal resistance coefficients (Rth) of five sets of metallized substrate samples were measured below. The measurement setup is shown in Figure 5.
Figure 5:Thermal resistance test result
Figure 5 shows the results of the thermal resistance test. All samples involved in thermal resistance analysis were flanked by a 0.3mm thick copper layer. As expected, the substrate using 0.63 mm thick alumina has the highest thermal resistance coefficient. This is due to the low thermal conductivity of alumina (24W/mK).
A. The thermal resistance coefficients of 0.32mm thick 9% zirconia toughened alumina directly bonded copper substrate and 0.32mm alumina directly bonded copper substrate belong to the same range.
B. Even if the thickness of the ceramic layer used is 0.63 mm, the thermal resistance coefficient of the aluminum nitride directly bonded copper substrate with the highest thermal conductivity (180 W/mK) is the lowest.
C. The thermal conductivity of silicon nitride is half that of aluminum nitride (90W/mK), which also explains why the silicon nitride active metal brazing with half the ceramic thickness has the same thermal resistance coefficient as the silicon nitride directly bonded copper substrate (silicon nitride is 0.32mm and aluminum nitride is 0.63mm).
High-strength silicon nitride insulation meets the growing demand for longer service life and higher thermal performance of power modules. A comparative investigation of the silicon nitride active metal brazing technology and the traditional 9% zirconia toughened alumina directly bonded copper substrate ceramic material shows that the reliability of silicon nitride is 50 times that of the latter. The superior mechanical properties of silicon nitride ceramic materials, especially the extremely high fracture toughness (K1), effectively improve their reliability. In addition, the higher flexural strength of silicon nitride allows it to be used in thinner cross sections, and its thermal properties are comparable to those of aluminum nitride.
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