Heat Creates Sticky Problem

Closeness and warmth are good things in most affairs of the heart, but that's not true in the electronics of a heart scanner. When GE Healthcare was designing a heart scanner it says is the fastest imager available, chips that were fast enough were so power-hungry that they drove temperatures high enough to hamper reliability.

Few engineers would have thought that a silicone adhesive would become a critical element in the project's success, helping dissipate heat and solving the problem. When they started scouting for heat-reduction schemes, GE's SilCool LTR silicone provided a solution to their cooling problems.

One of the crowning products for GE Healthcare, a $15 billion business unit, is the LightSpeed VCT (Volume-Computed-Tomography) scanner. The scanner rotates in just 0.35 seconds, with coverage of 40 mm. That's fast enough to perform a true volumetric scan of the heart in only five heartbeats.

The speed also makes it possible to scan patients who have severe chest pains to non-invasively determine whether they show evidence of heart attack, pulmonary embolism or aortic dissection. Those are the three most life-threatening causes of chest pain, and they're sometimes difficult to diagnose quickly using conventional techniques.

The additional speed is a positive both for patients who spend less time in the confined space and for hospitals that can do more scans in a day. But faster processing also brings challenges. As microprocessors and other semiconductors run faster, they also generate more heat. Increased heat generation is a universal problem, but it is particularly acute in the scanner.

That's because the extra heat has a negative effect on a diode that's critical for quick, precise imaging. Engineers wanted to put the controller board close to the diode, minimizing interference by shortening the transmission lines between them. "Putting the electronics close to the diode helps reduce noise," says Ashutosh Joshi, senior development engineer, GE Healthcare.

Proximity and heat generation were not good things in this matter of the heart. "Our heat load doubled, and the electronics were much closer," Joshi says. Previously, controllers were more than a foot from the diode. Now, they're only a few inches away. And if left uncooled, the scanner's electronics would generate enough heat to exceed the diode's optimum temperature range--which tops out around 40 C.

Sticky subject

One of the key elements GE engineers used to control this temperature is an adhesive silicone that links the heat sink and the chip it cools. "Contact between the heat sink and the chip is very critical," Joshi says. SilCool also holds temperature sensors in place, giving them a more accurate reading of the board's surface temperature.

SilCool, which GE's Advanced Materials business produced, finds wide use in electronics that run at high temperatures. "This material is also used in power semiconductors or on any other board-level products that generate a lot of heat," says Gail Riley, a marketing manager at GE Silicones.

The electronics, which include eight ASICs housed in micro BGA packages, are mounted on a conventional FR-4 circuit board. That substrate provides low costs, but doesn't provide the cooling capabilities of ceramic or other more costly substrates.

The shift to adhesives brought benefits beyond enhanced cooling. It also simplified manufacturing and improved reliability. In earlier scanners, GE used silicone-based gap pads that were much harder to attach. "We needed a large press, and that exerts pressure that can warp the board," Joshi says.

Warping isn't good for any solder contacts, but it causes the greatest problems with some of the tiny passive components, which have extremely small amounts of solder linking them to the circuit board's traces. SilCool has mechanical properties that help even after installation. "It's flexible so it doesn't stress the chip," Joshi says.

The adhesive also helps cut production time. GE started with a material that took six to eight hours to cure at room temperature. SilCool can be cured at higher temperatures, taking only two hours at 100C or just an hour at 150C.

Not a breeze

Though SilCool helped transfer heat to heat sinks, the problem didn't stop. Engineers still had to remove the heat from the system. This part of the design was a difficult task.

Engineers explored a wide variety of cooling options, including heat pipes and chillers, but finding space for them would have been an issue. "This is a rotating system, so every inch counts," says Joshi. Heat sinks and fans, when combined with the SilCool's contribution, proved to be the most effective and compact thermal management strategy.

However, the unique attributes of a rotating system mean that the fans aren't running all the time. When it's actually scanning, LightSpeed's rapid rotation creates a lot of air movement that rushes over the electronics. That huge influx of ambient air can cause too much cooling, making it difficult to stay within performance tolerances. "When the gantry starts to rotate, we slow the fans down," Joshi says.

That kind of careful thermal management helps keep the sensitive system from drifting even as electronics heat up during scans and airflow changes during rotation.