Bringing embedded chips to smart devices

Microchips are found in every modern electronic device from mobile phones to kitchen appliances. But as users demand more processing power on an ever smaller scale, bulky chips mounted on printed circuit boards are running out of room.

Embedded chips offer a solution, and EU-funded researchers are helping to bring them to full-scale commercial production with a range of manufacturers.

Embedding components into the circuit board makes them more robust and allows more functionality to be built into the same space, leading to smarter phones, cars and other consumer devices.

Take apart a computer, a mobile phone or a television and the first thing you will probably notice are the microchips, mounted inside plastic casings with metallic feet protruding from the sides. Surface-mounted chip packages, as the beetle-like components are known, have long been the backbone of the semiconductor industry.

But they may not be for much longer.

A fast-emerging technique in which the chips are embedded inside the printed circuit board (PCB) rather than mounted on top of it, offers many advantages - and not just in terms of size. Embedded chips are already in commercial production on a small scale and they are likely to be used much more widely in the future.

Research into embedded chips began more than a decade ago but has advanced in leaps and bounds in recent years, thanks particularly to EU-funded research projects.

‘Hiding Dies’, an initiative funded under the EU’s Sixth Framework Programme (FP6), carried out the fundamental research and development needed to produce prototype embedded chips as thin as 50 µm, roughly the same width as a human hair.

‘High density integration by embedding chips for reduced size modules and electronic systems’(HERMES), a follow-up project to Hiding Dies, has now built on that research, improving the design of embedded chips and moving them towards large-scale industrial production.

“Embedded chips have many benefits compared to surface-mount devices. The main one is obviously size - embedded chips have a smaller footprint and are also much thinner,” said Johannes Stahr, who coordinated the HERMES project.

“In addition, on such an embedded PCB, you can also assemble on top - that means a second level of components on top in a reduced space, something you simply can’t do with a moulded chip package, for example.”

By moving the components to the inner layers, additional space is created on the outer layers so more active and passive components, such as transistors and capacitors, can be placed on the same footprint.

This also opens the door to 3D systems-in-package in which chips and other components can be stacked on top of each other to deliver increased functionality in less space.

Because all the components can be placed so close together, short interconnects between them reduce signal distortion and improve thermal performance. It also ensures that the modules are considerably more robust and reliable because there is no soldering or bonding involved.

Because the components are located between the PCB conductor layers and not on top of the PCB, contacts can be made from either side, allowing increased design flexibility for different applications.

“Hiding Dies focused on a so-called ‘face-up’ process for embedding in which the chip is glued to a core material and then a special material - a resin coated copper foil - is laminated onto the assembled core.

“However, this results in a relatively difficult lamination process because the resin has to be softened and the lamination has to be carried out all together so as not to break the chips,” explains Stahr, who is also group technology manager at AT&S, Europe’s largest PCB manufacturer, in Austria.

This process works in the lab but may not be the optimum solution for manufacturing chips on an industrial scale. In HERMES, the researchers adopted a different approach.

“We focused on a ‘face-down’ process: we print a dielectric onto a copper foil and then assemble the components face down into the adhesive,” said Stahr.

The advantage is that the process can better handle embedding components of different sizes anywhere from 1 x 0.5 mm for passive components to 8 x 8 mm for silicon dies and external components can then be assembled on the top side of the PCB to create very high density modules.

Because the component is visible to the assembly-machine camera, unlike in the face-up process, it can assemble the components with more accuracy. This is crucial for large-scale production where the slightest defect can cost millions of euros to correct.

The HERMES team was recognised at the ICP APEX Expo in Las Vegas, Nevada, in 2010 when their project was given a ‘Best International Conference Paper’ award for a paper focusing on the technical options for chip embedding and reliability.

Using techniques developed in the project, the HERMES team has built hundreds of embedded-chip demonstrators for three end-user partners.

“We have not set up a prototype production line, but rather a full-scale industrial production line for real volume production,” said Stahr.

For project partner Bosch, the supplier of motor vehicle components, the team is producing a complete motor control unit for the motherboard of diesel cars and trucks, containing an 8 x 8 mm embedded processor along with memory and other passive components.

Though a car, given its overall size, may not seem to face the same space constraints and hence demand for miniaturisation as a mobile phone, there are very good reasons why Bosch and other motor vehicle companies are interested in the technology.

“They have a limited space for the motherboard. With this module, they can keep the same simple motherboard but have a more complex motor control unit for better performance,” said Stahr.

Because embedded chips are more robust, there is less risk of faults developing due to the extreme conditions encountered in a moving vehicle, from large temperature fluctuations to intense vibrations.

“AT&S made a comparison between embedded components and SMD components and we saw excellent reliability from embedded components. In drop tests, our component withstood 15,000 drops before the testers had to stop - they simply couldn’t break it,” said Stahr.

For project partner, Infineon, the team is producing functional demonstrators of power modules that could be placed into a range of consumer products from air-conditioning units to washing machines.

These power MOSFETs have better current flow and thermal conductivity compared with SMD components due to having contacts on both sides and shorter interconnects using copper-filled microvia.

For Thales, the team is working on highly complex modules for secure communications, embedding almost 400 components, including five chips, into a 10-layer construction on a single PCB.

“There is an enormous range of applications in many sectors for this technology and we are now on the frontline of producing embedded chips on a commercial, industrial scale,” said Stahr.

The HERMES project received research funding under the EU’s Seventh Framework Programme (FP7).

ATXS Austria

www.ats.net/en/