Design, simulation and fabrication of liquid cooled LTCC devices utilizing integrated channels

Beside the possibilities low temperature co-fired ceramic (LTCC) offers for electronic packaging it also enables the fabrication of micro fluidic elements like channels and embedded cavities. Hence, LTCC facilitate the realization of complex and integrated micro fluidic systems. However, for many power applications it is necessary to have a short thermal path between the power semiconductor and the heat sink. The poor thermal conductivity of LTCC necessitates an opening in the ceramic, to bond the chip directly to the heat sink. The focus of the presented paper lies on the heat dissipation by liquid cooling. For this purpose two cooling methods utilizing liquid cooling are investigated. In method 1 a power chip is placed on a LTCC substrate with an integrated fluidic channel. A coolant is pumped through the channel in order to cool down the device. Thermal vias are added in the ceramic and the fluidic channel to optimize the heat transfer to the coolant. In method 2 the power chip is placed directly in the channel and coolant is pumped through an opening over the chip in order to realize jet impingement cooling. The designs are simulated using ANSYS CFX to estimate the thermal resistance of the devices and to evaluate the influence coolant flow rate. The designs are fabricated using the DuPont 951 tape system in combination with high purity carbon tape to form the fluidic channels inside the LTCC and silver metallization. The jet impingement cooling design is fabricated in two separate LTCC modules, one containing a flip chip mounted thermal test chip (TTC) and one containing the fluidic elements. The two modules are bonded using an aluminum filled epoxy. The TTC is used to evaluate the thermal resistances, which are 3.1 K/W and 1.1 K/W for method 1 and method 2, respectively. Advantages of the presented cooling methods are the low thermal resistance and the good embedding capability in the co-fire LTCC process.