Liquid Cooling Is Coming To Chips and Boards

March 26, 2008
Online columnist Lou Pechi argues that the now-popular techniques for cooling chips and pc boards are becoming inadequate as levels of power dissipation continue to rise

In a previous column, I tried to predict future trends in on-board power distribution, and the handling and distribution of input power to the various IC devices on the pc board. As each one of the devices keeps increasing in capability and performance, it also generates more and more heat. This article, will address methods of removing and dissipating the heat.

Currently, most systems use an inefficient airflow method to remove heat from board-level devices. Improving heat transfer involves adding heatsinks with multiple fins to increase the surface area. With the increasing number of devices and their heatsinks sticking up in the air, great effort needs to be taken when laying out components on the pc board to prevent airflow “shadowing” by adjacent components. When shadowing occurs, downstream devices—instead of getting cool air—get the hot air left over from the hot upstream components.

Some applications take the heat out of the ICs from the bottom using thermal “VIAs.” These vias are plated-through-holes that conduct the heat to other layers of the circuit board. Unfortunately, this method wastes valuable routing space on multilayer boards.

So what are some alternative cooling solutions?

One possibility is liquid cooling. Just like an automobile engine is cooled by circulating coolant through the hot engine and then dissipating that heat in the radiator, so too, a liquid (rather than air) may be circulated across a pc board to remove the heat from its components. But one of the main issues with liquid cooling on pc boards is that electrons and liquids do not mingle well. So solutions to prevent the comingling of the two will be required.

Presently, there are applications where coolant is circulated through pipes placed close to the edge of the circuit board and away from the ICs. Here, copper extensions reach the ICs to remove the heat. This concept is described in the US Patent 4791983 abstract:
“A frame assembly for liquid-cooling, consisting of a frame and an array of heat dissipating surfaces, which can be positioned to overlie several IC packages…”

Many of these solutions almost appear as an afterthought, just as a window air conditioner solves the overheating problem in a single room. To use the analogy of the building air-conditioning, what we need on the circuit board, is a central air-conditioning system. Such a system must be designed at the beginning of the pc-board design to solve heating problems before they appear.

As the power density of high-performance ICs increases, device cooling is becoming a more significant concern. Conventional forced air-cooling techniques will be unable to meet the needs of future power-hungry devices – especially 3-D multichip modules that pack more processing power into less space. Cooling high-power electronic devices dissipating more than 300 W/cm2 at the die is beyond the capability of most conventional air- or liquid-cooling solutions.

A new technique for fabricating liquid cooling channels onto the backs of high-performance integrated circuits could allow denser packaging of chips while providing better temperature control and improved reliability.

Some liquid cooling techniques are already in production or at a research stage, circulating liquid through separate cooling modules attached to the integrated circuits, or through micro channels fabricated onto the back of chips using high-temperature bonding techniques.

One such cooling method developed at the Georgia Institute of Technology includes polymer pipes. These pipes allow electronic and cooling interconnections that can be used in automated conventional microelectronic manufacturing processes without damage to the integrated circuits. This compact solution transfers cooling liquid directly into a giga-scale integrated (GSI) chip, and is fully compatible with conventional flip-chip packaging.

Future printed circuit designs, in addition to the electronic board layout discipline, will require a new, presently rare, fluidic layout specialist discipline. For their part, IC manufacturers will also need to develop devices that can interface with the on-board fluid distribution systems to provide fluid circulation within the integrated circuits.

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