SMALL FORM FACTOR FORUM
Too Hot to Handle
BY COLIN MCCRACKEN
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Engineers everywhere are trying to beat the heat this summer. Some have contracts to fulfill, held up only by the thermal design and qualification. The 45 watt 2 GHz+ quad core processors have impressive performance specs, but try not to touch them with your finger unless a second degree burn is part of the cooling solution. You can’t ship with cooling spray, either. Perhaps some of us were mesmerized with the topless COMe (COM Express) module photo shoot and forgot to buy the massive fansink required to remove heat. Or maybe the active cooler is so bulky or heavy that it requires its own shipping container and simply hasn’t arrived yet. Ever wonder why the thermal solution is not included with the slim product photos on the website and datasheet?
Replacement modules (“upgrades”) are already in production with 3rd-generation Core i7 processors that knock 10W right off the top without sacrificing speed (still 2.1 GHz quad core). Tick tock, just like clockwork. Some are even available in the same late-lifecycle COMe Type 2 pinout as your existing carrier board for easy drop-in. Impressive. Those who research new product announcements will be rewarded. For everyone else, be prepared to spin your carrier to be able to access a lower power CPU with a Type 6 pinout.
OEMs who must always have the highest speed available are stuck with the highest power envelope as well, and they must put the pedal to the metal for cooling solutions that improve overall system reliability by channeling heat outside of the system more efficiently, rather than sharing that heat with other sensitive electronics inside.
“Caveat emptor” applies to OEMs looking at a quad core processor shoe-horned into the 95 x 95 mm Compact size. Evaluate these carefully, not just from the perspective of whether everything you need actually fits on the COMe module, but also according to how your system will dissipate the 45W through a smaller heatspreader than for other 125 x 95 mm quad core module alternatives. The long-term reliability issue might be hard to measure initially. The more metal mass closer to the CPU, the better.
Just as you would add up signal losses in your RF link budget or IR drops in your power circuit, it’s imperative to sum the temperature rises through the various components of the thermal solution. Air gaps or bubbles in the thermal interface material only get in the way. Inserting a very thick gap pad is a brute-force approach to the very wide Z-axis tolerance of processor packages themselves. But this type of pad comes with a high thermal resistance, and it is difficult to build up enough pressure as required by the processor manufacturers for proper heat removal. Instead, some creative approaches involve high-compliance metal blocks with springs and heat pipes and so on, although usually at a higher cost. Such heatspreaders with integrated heat pipes are available now, with or without fins and fans. The goal is to reduce the overall effective thermal resistance. Again, rewards are available to engineers who scour trade journals and websites rather than simply “drinking the KoolAid” of the first pretty datasheet atop the search engines. Each OEM can carefully choose its system’s total cost of quality destiny.
The recent launch of 4th-gen 22nm Core i7 “Haswell” processors offers some relief from the summer heat as well. Early samples are available now from COMe manufacturers for those who can test with a Type 6 carrier board. Whether for medical, communications or military applications, Haswell modules advance the performance possible at that highest power envelope, and even offer lower-power SKUs where power is a higher priority than performance. In this latter category, AMD has surprised the market with its recent introduction of eKabini SoC (system on chip) processors with up to a 2 GHz quad core rated at only 25W. Intel has good reason for concern, as AMD overcame significant odds to pull this off. Whether with Intel or AMD processor, COM Express offers multiple ways to beat the quad core heat through timely availability of the latest CPUs and advanced cooling solutions alike.