Power, EMI and Thermal Management in One Unit
John Baliga, Associate Editor -- Semiconductor International, 9/1/2000
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The current method of delivering power to the microprocessor, and its off-chip cache memory depending on the design, involves a power regulator on the board, with wires running from the regulator, through the board and to the chips (Fig. 1). Upcoming microprocessor designs are calling for 1 V operating voltage, 100 A, 300 A/µsec slew rates, efficiency of more than 90%, regulation within 5% and voltage ripple less than 1%. This set of requirements is a significant challenge for the current design. The long path contains significant amounts of inductance and resistance, which robs some of the power and voltage on the way from the regulator to the devices. Also, with the currents required, those wires can act as antennas, carrying high currents at high frequencies, adding to the existing EMI problem.
The Incep design uses an enclosure that houses all the dice as well as the regulator. Power is delivered through screw-SMT-type coaxial contacts from the regulator module to the dice through coaxial contacts that run through the enclosure to the board (Fig. 2). The power is shielded through to the board, significantly reducing the losses typically seen. Also, the enclosure is grounded, providing a large ground plane for good power quality.
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In addition, the enclosure contains the EMI from the dice. The outer case of the computer does not need to block as much of it. The company notes that elaborate designs have been evaluated involving case design, and they are quite expensive. Since the outer case needs to block less EMI, larger air flow openings can be used while still meeting EMI requirements.
The air-flow component of thermal management is optimized by containing all the dice in one enclosure. Having multiple heat sinks in one area can lead to eddy currents and other interruptions of flow that can reduce the effectiveness of the air stream, as well as complicate the EMI issue. According to Incep, the improved air flow makes it possible to reduce the size of the outer case and still have effective cooling.
Since the enclosure must house multiple dice, there must be thermal contact between them and the heat sink in a way that is tolerant of height variations. Also, the thermal interface material between the dice and heat sink typically takes up 30% to 50% of the thermal budget, so making good contact is critical. The company has developed a corrugated metal interposer, or "thermal substrate" (Fig. 2, inset), to provide this contact. The interposer is designed to be under compression for the given set of die heights, and thermal grease is contained within it. Since the grease is contained, it is resistant to pump-out. •
