Mechanical Meets Electrical

By Ed Sperling
For the first part of the 20th century mechanical engineering dominated almost everything in technology. For the second half, once the transistor and the integrated circuit became well entrenched, those two disciplines largely divided up the tech market.

More recently, however, they are being forced to collaborate in teams that historically had nothing in common. While the combination of electrical engineering with software has raised questions about how to trade information back and forth, mechanical and electrical engineering arguably are even further apart. But there is at least one consistent element throughout the most recent combination—power.

Power and heat
One of the biggest changes in engineering is that power is global. Physical effects such as heat, electrostatic discharge and leakage current can affect many other levels of a much larger system. That larger system could be a car, an airplane, or a data center.

“Inside of an engineering organization, someone near the top has to worry about the entire system,” said Larry Williams, director of product management for the electronics business unit of Ansys. “They have to think about boundaries between systems and subsystems, or between mechanical engineering and electrical engineering, because many firms are organized that way. When building a system, the optimum design can be found by considering the system as a whole, and additional margin is often found at those boundaries.”

He said that at a meeting within one defense contractor, he actually introduced the mechanical and electrical engineering teams, who had never met even though they worked on the same projects. Those silos have since begun breaking down, in part because systems demand power efficiency, better reliability and lower cost. Things that used to be done as purely mechanical engineering may be mixed together as part of a bigger system.

But the perspective of each is different. Consider thermal budgets, for example. Electrical engineers focus on turning off as much of a chip as possible when it’s not in use, and running what’s in use as efficiently as possible—even going so far as to weigh whether specific operations use less energy when they’re run at maximum speed for short periods of time or slower speeds over longer periods of time. Mechanical engineers, meanwhile, focus in the other direction—cooling the devices as close to the heat generation as possible. In the past, that meant simply drilling holes into metal and adding heat sinks and fans.

“As density has increased it is no longer possible to thermally manage a device around the PCB,” said Robin Bornoff, FloTherm product marketing manager in Mentor Graphics’ Mechanical Analysis Division. “It’s gotten to the point where the mechanical perspective cannot be a separate discipline. We’re now seeing representatives of the thermal design teams showing up right from the beginning in meetings with the system architect. They have to work together.”

That discussion becomes even more critical in 3D stacking, where heat can get trapped between two die. And it’s not just the stacked die that needs to be considered. It’s what’s around it, as well.

“Heat doesn’t obey existing design discipline barriers,” said Bornoff. “The heat will spread into the air, the chassis, the room, and out from there. How hot the silicon gets affects everything, sometimes even outside the building. That’s why you’re starting to see water-cooling in space applications and in data centers. It’s 1,000 times denser than air and 1,000 times better at removing heat.”

The challenge is to get that cooling as close to the source of heat as possible. So rather than just cooling a server cabinet, for example, the liquid is pumped around the processors producing the heat. There is even research under way in microfluidics to pump liquid around the chip itself in a stacked die. Bornoff noted that initial approaches tried to squeeze the fluid through very narrow channels, which required massive pressure. He said the latest research uses piezoelectric fans and pumps, whereby vibration creates movement in the fluid.

Fig. 1: Microfluidics. (Source: Imperial College of London)

MEMS and energy harvesting
Another confluence of mechanical and electrical engineering skills has been the MEMs world—microelectromechanical systems—which are growing in importance in markets ranging from touch screens to smart sensors and analog signal conditioners. There are even micromotors with gears attached to semiconductors.

“Electronics is relatively young compared to mechanical engineering,” said Cary Chin, director of technical marketing for low-power solutions at Synopsys. “But the next big rev of the market is pointed toward electromechanical systems. A lot of these are being looked at for technologies that will start to solve the power problem. With a mechanical system there is no leakage. And for devices that don’t require a really high level of performance, they may be able to power a system forever.”

Think about biomedical devices such as a pacemaker, for example. An energy scavenging system that includes semiconductor technology and mechanical energy harvesting can be used to provide enough power just from a person’s own heartbeat to both keep a steady pace, detect when there is an irregularity, and even act as a defibrillator for one or two stored duty cycles.

Fig. 2: Mini motors. (Source: Sandia National Laboratories)

“The challenge for the tools world will be to rethink optimization,” said Chin. “With power we already had to make significant changes for implementation and verification. Now what we may be looking at is support electronics, where the heavy lifting of computing is moved into the cloud.”

The future
The so-called Internet of things is another big driver in this whole shift to fuse together electrical and mechanical engineering. Within this scheme, systems will be defined as much collectively as individually, much as they are from subsystem to system, with the actual location of computing as distributed along the lines of the Internet.

Within this scheme, there will be many places that mechanical and electrical engineering cross paths, many driven by power, heat, signal integrity and new applications that are just now on the drawing board. For that there will also be new opportunities for tools that can explore tradeoffs of something done mechanically versus electrically, just as those types of tradeoffs are now made for the best kind of IP and processor cores within a given power budget. And as the silos break down, the possibilities are mind-boggling.