Yang-Fei Zhang

Simulation of fluid flow and heat transfer in microchannel cooling for LTCC electronic packages

The hydrodynamic and thermal characteristics of microchannel networks are investigated by finite element analysis with commercial software Fluent. The simulation model is based on fabricated thick film LTCC substrate with 3D cooling microchannels. A comparison of the cooling performance among fractal-shaped microchannel, parallel microchannel, serpentine microchannel, spiral microchannel is also conducted numerically based on the same heat flux and the same mass flow rate. It is found that fractal-shaped microchannel facilitates the lowest fluid pressure drop and the most uniform temperature distribution over the substrate, and that the spiral microchannel network enables the smallest temperature rise.

Process research of LTCC substrate with 3D micro-channel embedded

Along with the development of the electrical technology, the power of the circuit rises rapidly, heat dissipation becomes a key problem in the design of the circuit. The 3D microchannel coolers made with LTCC (low temperature co-fire ceramic) technology can absorb the heat of the chip and pass it to the external environment by the liquid circulation. In this paper, the critical manufacturing processes of LTCC multilayer substrate with 3D micro-channel embedded: lamination and sintering are studied mainly. The special lamination sacrificial layer technique can prevent the 3D micro-channel from collapsing and deforming during the lamination. In addition, the sintering profile is optimized which helps avoid the crazing and delamination of the multi-layer substrate. In conclusion, the intact LTCC substrate with 3D micro-channel embedded can be fabricated using the optimized lamination and sintering process parameters, which makes the following heat dissipation experiment and design optimization more conveniently.

Simulation on heat transfer of microchannels and thermal vias for high power electronic packages

The heat transfer characteristic of cooling microchannels and thermal vias integrated in low temperature co-fired ceramic (LTCC) multilayer electronic packaging substrate have been investigated by finite volume simulation method, including straight, spiral and I-shaped fractal microchannels. The width of microchannel is 200 microns and the height is about 186 microns. The side length of Cu thermal via is 0.1, 0.2, 0.5 and 1 mm with the amount of about 100. The water mass flow rate at the inlet was controlled at 45 ml/min and a high power chip of 100 W was supplied at the center of LTCC surface. The heat transfer behaviors were characterized by the decrease of maximum working temperature and temperature distribution of heating chip surface. It is found that the spiral microchannel with 1 mm thermal vias has the best cooling ability and the maximum temperature is reduced to 74.84°C. The adding of cu vias has great enhancement on the heat transfer by reducing the maximum temperature over 10%. The heat distribution of heating area is significantly improved due to the high speed of heat transfer from the chip to the cooling microchannels by high thermal conductive of Cu via.

Investigation of a unified LTCC-based micromachining and packaging platform for high density/multifunctional microsystem integration

3D system-in-package has recently been considered a major enabler for high density and heterogeneous microsystem integration. We hereby proposed the concept of a unified micromachining and packaging platform based on LTCC (low temperature cofired ceramic) material system and process, which is implemented by first enhancing an existing LTCC hybrid IC fabrication line and then integrating different LTCC micromachining process modules one by one. Hence, the unified process flow can be accomplished within just one single package-test house. The platform has been capable of micromachining basic 3D MEMS (micro electromechanical system) microstructures into LTCC laminates and using them as a packaging substrate for mounting IC/MEMS from other process platforms, realizing self-contained and versatile microsystems of high density. The 3D microstructures formation process consisting of green tape machining, lamination and cofiring are demonstrated. The designing, analysis and fabricated samples of various micro functional structure enabled by the platform are illustrated, including embedded cooling microchannels (capable of lowering substrate temperature by more than 50K), microaccelerometer for harsh environment, micro Pirani gauge for in-situ vacuum level monitoring and THz (tera hertz) vacuum microelectronic devices. Samples of overall packaged MEMS and IC chips with micromachined LTCC substrate are displayed, showing ultra-low leakage (<; 5×10-11 Pa·m3/s) vacuum packaging capability and significantly enhanced device performance. In addition, the platform has demonstrated the potential of stacking several laminates with mounted chips into a 3D frame-like microsystem. In comparison, 3D integration purely based on Si micromachining, e.g. anodic-bonding based in-situ wafer encapsulation, may only support a very limited spectrum of devices/materials and integration density and is somehow too expensive for many MEMS researchers.

A LTCC microsystem vacuum package substrate with embedded cooling microchannel and Pirani gauge

This paper reports the designing, simulation and initial experimental investigation into a LTCC vacuum microsystem package substrate acting both as a vital panel and a functional structure for compact system-in-package (SiP) integration. Design, validation and experimental results for microchannels with different planar axial shapes are presented. Experimental and simulated temperature distribution over the substrate demonstrate the effectiveness of microchannel design, with substrate temperature rise cut by over 70% compared with those without microchannels. The effect of vacuum on cooling is simulated and potential ways to enhance heat transfer are suggested. The structure and principles of a Pirani gauge integrated onto the substrate are displayed. This micro gauge is formed by wire bonded, instead of by micromachining, and is proved to be both simple and effective in in-situ vacuum measuring inside a compact package. Therefore, this substrate proves an promising option for SiP applications in defense, industrial and consumer domains demanding high packaging density and vacuum or airtight circumstances.

Microchannel water cooling for LTCC based microsystems

The heat dissipation of six different types of microchannel networks integrated in LTCC based microsystems has been investigated by experimental measurement and simulation analysis, including straight, serpentine, spiral and fractal-shaped microchannel networks of curve, I-shaped and parallel. The cross section of microchannel is 200 μm × 200 μm and the total length is about 200 mm. The water mass flow rate at the inlet was controlled at 7.5 ml/min by a powerful micro-bump and the heat flux of surface heating area was supplied from 0.2 W/cm2 to 1 W/cm2 by an array of chip resistors. The simulated maximum temperature rise by finite volume method agrees well with the experimental results. It is found that the spiral microchannel has the best cooling ability and reduces 86.8% of the maximum temperature rise at 1 W/cm2. The microchannels are proved to have little effect on the strength of the substrate by finite element analysis method due to the small size of cross section.

Nanoscale mechanical properties and microstructure of 3D LTCC substrate

With the development of three-dimensional low temperature co-fired ceramic (LTCC) packaging substrate, the internal microscale structures have great effect on the strength, toughness and lifetime of LTCC microsystem, where the study of local or nanoscale mechanical properties and microstructure is critical, especial for the prediction of the local failure behaviors. In this study, the nanoscale mechanical properties and microstructure of 3D LTCC substrate have been investigated. The nanoscale mechanical properties were measured by nanoindentation method on both the top and lateral faces and discussed according to the profile of the residual pit. The Youngs modulus and hardness measured by nanoindentation were compared for two faces. The microstructure was studied by using Scanning Electron Microscope and Scanning Probe Microscope. The results show that LTCC can be regarded as a macroscopically homogeneous isotropic material and a particle-reinforced composite at microscopic scales. The properties of the individual particle and matrix were successfully measured and discussed. Moreover, significant effect of the machining process of the embedded channels and cavities on the strength of the substrate and fluid flow in the microchannel are found and discussed according to the microstructures, including the defects found at the corner of the microchannel and impurities found inside the microchannel.

Microstructure and viscoelastic behaviors of graphene/PMMA nanocomposites

The microstructure and viscoelastic behaviors of graphene filled poly(methyl methacrylate) nanocomposites (graphene/PMMA) have been investigated. The composites were prepared by in situ polymerization method with graphene weight fraction from 0.1 to 1 wt.%. The microstructures were observed by transmitted polarization microscope (TPM) and scanning electron microscope (SEM). The viscoelastic behaviors characterized by storage modulus and loss tangent were measured by dynamic nanoindentation method. The effects of graphene content and force frequency on the viscoelastic behaviors were studied and discussed according to the features of microstructures and mobility of molecular chains. The graphene nanosheets with an average diameter of about 8.6 micrometers are found to disperse homogeneously in the PMMA matrix. The interface between graphene and PMMA is strong and few defects are observed in the impact fracture surfaces of the composites. The results indicate that the microstructure and viscoelastic properties of PMMA polymer are significantly improved by adding a low content of graphene.

The Effect of Cavities and Channels on the Strength of LTCC Substrate

The effect of cavities and channels on the strength of LTCC substrate has been investigated by finite element analysis with commercial software ANSYS. The simulation model is based on three-point bending test of thick film LTCC substrate. Axial tensile stress and shear stress among layers are characterized to describe the effect on flexural strength and inter-layer shear strength. X, Y-axial channels, Z-axial via-hole, and cavities of various amounts are formed in the middle or surface layers of the LTCC substrate. The simulation results show that the maximum tensile stress always appears on the midline of the substrate bottom and causes fracture. The shear stress concentration has been found on the sidewalls of the holes and cavities and enhances the interlayer stripping.