Thermal Control of Interfaces for Microelectronic Packaging
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Mat. Res. Soc. Symp. Proc. Vol. 515 ©1998 Materials Research Society
F]
Heat sinkor heat pipe
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conductive Adhesive
Heatsinkorheatpipe
Conductive Adhesive Heat pipe or Heat Spreader
~Heatsinkorheatpipe conductive Adhesive
~~-Heat pipeorHeatSpreader awesome •Non-adhesive
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interstitial material
Fig. I Cooling designs for FC-PBGA Technology
Fig. 2 Warpage of the silicon device and its carrier Warpage of Flip-chip PBGA Package The warpage of FC-PBGA package is produced by a CTE mismatch between the chip and organic carrier and a coupling through an underfill layer. In a high performance FC-PBGA package where a heat sink is usually required, the warpage as well as the absolute displacement of the top silicon surface becomes an important design parameter for optimum thermal solution, especially when non-conductive interstitial materials between the heat sink and the silicon are employed. If the silicon warps and displaces in the direction normal to the surface during operation, non-uniform distribution of apparent interface pressure over the interstitial materials will be developed, which will produce non-uniform thermal conductance. Warpage contours of the front surface of FC-PBGA package and the total absolute movement of the chip center was measured by Verma et al. [1], which is shown in Figs. 2-3, respectively. The change in magnitude for the total absolute displacement from room temperature to 100°C is approximately 1.6 mil, which is a significant portion of the thickness of a non-adhesive elastomeric interstitial material. This large displacement will adversely affect the thermal performance of the microelectronic package if a heat sink is rigidly attached to a motherboard or an enclosed heat pipe system is utilized. Removal of heat from microelectronic packages is a problem faced by electronic engineers. To a large extent interstitial materials such as elastomeric gaskets and epoxies have replaced 216
Area of interest
PCB
Encapsulation Silicon Die
150 °C
Room temperature
r 100 °C
200 °C
Fig. 3 Warpage contours of the front surface of FC-PBGA package, Verma et al. [1].
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60
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50 40 30 20 10 0 25
60
100
150
200
Temperature (°C) Fig. 4 W displacements as a function of temperature, Verma et al. [1]. silicone greases and mica pads as a means for enhancing heat transfer across a component junction. In order to shed some light on the thermal joint resistance of interstitial materials, which would prove beneficial to electronic packaging engineers, a summary of published experimental data for thermal performance of various interstitial materials (i.e., metallic, nonmetallic, and polymeric) is presented.
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Metallic Materials Fried and Kelly [2] investigated the contact resistance for a pair of contacting stainless steel surfaces with one surface coated with vapor-deposited magnesium or aluminum. The experimental data indicated an order of magnitude decrease in thermal contact resistance when compared to the bare interface junction. The authors stated that the significant imp
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