Why are companies taking such grand steps to curtail improper thermal management? “A lot of applications (systems) are getting smaller, e.g., Mac Air, and the thermal path is being both shortened and rearranged,” says Sara N. Paisner, senior microelectronics technology scientist at Lord Corp.
Generally, heatsinks are placed directly above the component. But the latest techniques move heat in alternative directions. “Now the heatsink may be behind the component, or heat may be dissipated through the board itself,” says Paisner.
Yet thermal management isn’t so simple anymore. “Casing material is acting as both an EMF (electromagnetic field) shield and a heatsink, as the casing itself has become part of thermal path,” says Paisner. A typical printed-circuit board (PCB) includes a built-in heat path, causing systems engineers to rethink their design strategy. Everything is shrinking, and now several components share cooling responsibilities while heat transfers to a larger area.
The preventative measures taken by Intel and AMD with respect to proper thermal design are interesting from a chip perspective. To start with, Intel indicates that “The processor requires a thermal solution to maintain temperatures within operating limits.” It uses thermal diodes, digital thermal sensors (DTSs), and the Intel Thermal Monitor to monitor die temperature.
Used in conjunction with the thermal sensor, the thermal diode can be used to calculate silicon temperature. The DTS is an on-die sensor that continuously monitors and outputs data on the die temperature relative to the maximum thermal junction temperature. Temperatures that will cause catastrophic conditions can be detected when a special bit is set in the DTS.
The Intel Thermal Monitor helps control the processor temperature by activating a thermal control circuit when the silicon temperature reaches the maximum. This, in turn, modulates the core clock as needed to keep the silicon temperature in check.
Also, the monitor generates an external signal (PROCHOT#) if the processor is above the thermal trip point. It can generate an interrupt signal as well. If the monitor is deactivated, a special signal (THERMTRIP#) will be asserted, indicating imminent failure if the core voltage isn’t switched off immediately.
AMD takes a similar approach. Its “Thermal Design Guidelines” whitepaper provides specifications such as the maximum length, width, and height of the heatsink, in addition to the heatsink and fan material requirements.
While CPUs are an easy target because they dissipate so much heat, other system components must not be overlooked. This is where some simple calculations come into play, as well as some basic thermalmanagement theory.
THE JUNCTION BONE’S CONNECTED TO THE SINK BONE
Thermal management moves heat from the semiconductor junction and into the surrounding ambient environment. Typically, heat is transferred from the semiconductor to the package, then to the heat spreader (sink), and finally to the ambient environment. Your design may not have a heatsink, or it may have more exotic technologies like fans and pipes.
Still, the general theory remains the same—spread heat from a small area to a large area. According to the basic theory of thermal conductivity, the rate at which heat conducts through a material is proportional to the area perpendicular to the flow of heat and the temperature gradient.
Junction temperature (TJ) is the operating temperature (typically in °C) of the semiconductor junction, where most of the heat is generated. Thermal resistance is the effective temperature rise (typically in °C) per unit of power dissipation (typically watts) of a designated reference point (such as junction or case) above an external reference point, such as the lead, case, or ambient air.
Thermal resistance is expressed as θLetter1Letter2 (e.g., θCA or θJA). Letter1 is the designated reference point and the letter typically represents the initial for the reference (e.g., C = case; J = junction). Letter2 is the external reference point and has a similar representation structure (e.g., A = ambient).
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