No abstract

3D chip-stack packages are more difficult to cool than 2D chip packages due to additional thermal resistances in the heat flow path. The additional thermal resistances are due to the presence of the C4 joins between the chips, the BEOL wiring layers in each chip and the silicon thickness of the chips in the stack. In this paper we present an efficient lid design for a 3D flip-chip package that allows contact, through a thin thermal interface material (TIM) layer, of exposed chip regions of the lower chips in the 3D vertical stack.The efficient lid was assembled on to 3D thermal test vehicle packages and its thermal advantage over standard lid 3D packages was experimentally demonstrated. The packages were cross-sectioned to ensure that the assembly process yielded the correct TIM gaps. A thermal conduction model was calibrated to the experimental data and the stacked chipchip and the efficient lid TIM thermal resistances were extracted from the model. A sensitivity analysis was then conducted to identify the important parameters controlling the thermal performance of the 3D package.

An emissometer was designed and constructed to measure the normal, spectral emittance of opaque solids over the spectral range from 1 to 8 /Jtm for temperatures ranging from 500 to 1500°C using an integral blackbody technique. The emissometer is gas-tight so that the gas environments surrounding the sample can be controlled and emittance data can be collected as a function of exposure time to a specific environment at a particular temperature. An FT-IR spectrometer collects the blackbody and sample infrared signals, which are ratioed to calculate the material's emittance. A computer code was developed to check the validity of the assumptions associated with the measurement technique. The emittance of silicon carbide in the form of a-SiC (Hexoloy SA) and TiB 2 -toughened SiC (Hexoloy ST) was collected over the spectral and temperature ranges given above. The emittance for Hexoloy SA was measured in the as-received condition and after sample exposure at 1300°C to carburizing, oxidizing, and low-pressure nitrogen environments. Emittance data for Hexoloy ST were collected for samples in the as-received condition and for samples exposed to oxidizing environments. The surface morphology and composition of the samples were characterized using SEM, EDX, and X-ray diffraction techniques. NomenclatureCi, C 2 , C 3 = correction factors for systematic errors, -d = layer thickness, imm E = emissive power, W/m 2 FJ_J = view factor, from A t to Aj H = irradiation, W/m 2 / = radiation intensity, W/m 2 sr n = index of refraction, -S = signal strength, mV § -unit direction vector, -T = temperature, K e -emittance 0 = polar angle, rad A = wavelength, /jum = azimuthal angle, rad Subscripts b = blackbody bg = background / = incident ra = measured s = sample ss = stainless steel drop tube A = spectral value, per unit wavelength

An analytically based approximate solution is presented for the thermal resistance of an axisymmetric heat source mounted on a conductive substrate with bottom- and top-side convective cooling of the substrate. The approximation closely matches an exact solution for bottom-side convective cooling and reference finite element solutions for top-side and both-side cooling over broad ranges of substrate thickness (10−4 ≤ t* ≤ 104 and 10−2 ≤ t* ≤ 102), substrate outer radius (1 ≤ b* ≤ 100) and convective Blot numbers (10-4 to 102). With bottom-side cooling, a minimum in the thermal resistance can occur over a wide range of substrate thicknesses. The approximate solution possesses simplicity and ease of computation as compared to exact or computational solutions for many microelectronic applications.

The effect on heat transfer of geometrically rearranging the surface area of a finned heat sink is investigated. Novel heat sinks with fluted and wavy plate fin configurations are designed and fabricated together with conventional longitudinal-plate and pin fin heat sinks. The experimental apparatus, consisting of the guard heater assembly, isolation chamber, wind tunnel, and data acquisition instrumentation, is described. The thermal performance of the novel and conventional heat sinks is measured and compared for the horizontal and vertical base plate orientations under natural and low-velocity forced convection conditions. Results, presented as the Nusselt number against the Rayleigh or Reynolds numbers, reveal that the pin fin heat sink generally outperforms the other heat sinks, when the heat sink surface area is held constant. A significant effect on heat transfer of the orientation of the forced flow with respect to the buoyancy flow is observed. Overall, the novel heat sink designs do not yield significantly better thermal performance than an optimized conventional longitudinal-plate heat sink.