Conventional power electronic packages typically employ a direct bonded copper (DBC) substrate, two solder layers for bonding devices and base plates to DBC, and a thermal grease layer to connect the base plate to a heat sink as shown in Figure a. These multiple layers and their interfaces are the causes of concerns with respect to fatigue failure and delamination due to coefficient of thermal expansion (CTE) mismatch. Significant enhancement in durability can be achieved by eliminating the DBC and solder layers, so that the dielectric aluminum nitride (AlN) is directly attached to the heat sink with a low CTE as shown in Figure b.
- Devise a cost-effective, quick and easy transient liquid phase (TLP) bonding technique to produce a durable bond between AlN and AlSiC.
- Investigate metallographic, thermal, mechanical and dielectric properties of the bond material.
- Analyze the new package for thermal performance and demonstrate package level performance.
A novel transient liquid phase bonding technique was developed, which required no expensive surface deposition techniques, no custom gas atmosphere during bonding and brings the temperature of bonding down from 1070°C (temperature at which DBC is fabricated) to 565°C, which is below the melting temperature of aluminum.
Using Cu – Al thin foils cleaned using HCl, DI water and solvents kept in between AlN and AlSiC, graphite rigs holding the bonding stack under specified pressure was kept in a tube furnace, in which temperature is ramped to 565°C with a specified profile. After the bonds were made, they were taken out of the furnace and analyzed.
The resultant bonded samples have shown excellent mechanical and dielectric properties, whereas thermal conductivity of the bond material is found to be greater than 120 W/m-K, which is more than twice that of typical solder materials, such as Gold – Tin. The figure shows a sample SEM image of the bond, which shows that SiC particles migrate into the bondline and improve its thermal conductivity and rigidity. This makes the bond material unique in the sense that it has the malleability properties of Al, while SiC particles restrict its plastic deformation. Extensive fatigue and aging tests and package level demonstration efforts are ongoing and it has been found that the bonds show no change in their strength and bond void fraction after 1400 thermal fatigue cycles between -40°C and 150°C and 1200 hours of aging tests at 150°C, while none of the samples showed any failure or delamination. Successful demonstration of the benefits of this method is applicable to bond not only AlN to AlSiC, but also several other substrate – metal, substrate – substrate pairs in the electronic packaging field, such as CuSn bonding, AlN – CuMo/CuW etc.
More information about the bonding process and bond properties can be found here –
Pahinkar D. G., T. Puckett, S. Graham, L. Boteler, D. Ibitayo, S. Narumanchi, P. Paret, D. DeVoto, J. Major (2018), Transient Liquid Phase Bonding of AlN to AlSiC for Durable Power Electronic Packages, Advanced Engineering Materials, 1800039, DOI: https://doi.org/10.1002/adem.201800039
Thermal Performance – Computational Investigation
Due to removal of Cu layers in the conventional stack and replacement of those with AlSiC, heat spreading is affected as a result of lower thermal conductivity of AlSiC (40% of Cu). Therefore, investigation of efficient cooling techniques to improve the thermal performance of this new package and make it comparable to conventional power packages became necessary. I developed analytical models based on fundamental heat transfer and fluid mechanics to predict heat transfer coefficients (HTCs) and pressure drop (ΔP) values for single-phase liquid channel cooling, pin fin cooling, and spray cooling employed for cooling of the new package design for any values of mass flow rates and coolant pumping power. This study helped in optimizing the individual geometries for HTCs and ΔP. Next, I conducted computational fluid dynamics (CFD) and heat spreading analyses on the new package design with the optimized geometries to predict devise temperatures and showed a favorable performance over existing designs. Sample temperature profile is shown in the adjacent figure.
More information on this work can be found here –
Pahinkar D. G., Graham, L. Boteler, D. Ibitayo, S. Narumanchi, P. Paret, D. DeVoto, J. Major, Comparative Analysis of Single-Phase Cooling Techniques for Durable Power Electronic Package. ASME Journal of Electronics Packaging 2018, in review.
This research was sponsored by the U.S. Army Research Laboratory, Adelphi, MD and National Renewable Energy Laboratory, Golden, Co.