Forwarding Flexible PCB Design

By Mark Forbes, Altium

The wearable electronic device market continues to grow at breakout pace. IDTechEx calls for the $30 billion 2016 market to triple to $150 billion in 10 years. One of the areas that wearable devices are making a huge impact is in the medical industry where small, unobtrusive “patches” have replaced monitors that were worn on the patient’s belt or required a shoulder strap. Not only has the quality and variety of monitored data improved, but importantly, the patches are easily covered by clothing, protecting the patient’s privacy.

Wearable technology company MC10, founded in 2008, develops a variety of wearable medical devices that can monitor a wide range of data. Much of the use of their wearable sensors is in medical research, where the devices can collect complex physiological data with electronics that are virtually invisible, conformal and wearable. Its technology platform combines conventional electronics and novel mechanics to enable a new generation of thin, orthomorphic electronic systems that can stretch, bend and twist seamlessly with the human body.

The flagship product for MC10 is theBioStampRC, a wearable sensor patch that can be placed anywhere on the human body to gather deep insights into physical issues, such as muscular disease.

The BioStampRC wearable medical device is not much bigger than a bandage, yet it contains electronic and mechanical technology that is only possible with flexible PCB technology. Photo courtesy of MC10.

As an industry pioneer that is reshaping electronics for use across a wide range of new digital health, consumer and medical applications, MC10 needed to find an advanced PCB (printed circuit board) design software solution that would allow the company to push the boundaries of traditional electronic design in what is becoming a flexible, foldable world.

PCBs That Bend and Twist?

Producing a product like the BioStampRC required more than that standard, rigid PCB. The obvious answer is flexible circuit technology integrated with small rigid PCBs: a rigid-flex solution. While flex circuits have been around for decades, it’s only the past few years that they have taken off with the advent of wearable technology.

Designing the rigid-flex circuitry required and making it conformal to human limbs and muscles required a PCB design tool that provides complete flex circuit design and the ability to create and use non-standard shaped boards.

Boards designed for the BioStampRC are small and dense, with a lot of board cutouts, odd geometry and strangely shaped polygons with very accurate radii. Typically, a 3-5 cm board might have up to 300 custom drawn polygons. And, the board has to be readily manufacturable by a variety of PCB fabricators.

The Design and the Secret Sauce

Several of the design engineers at MC10 employed Altium Designer previously in their career, and wanted the unified environment it presents, having all necessary tools integrated into a single environment.  Plus, it has advanced features that make designing rigid-flex products much easier. Thus, selecting their PCB design tool was one of the easier challenges in the project.

Bryan McGrane, a senior electrical engineer at MC10 and veteran user of Altium Designer, did much of the design. After putting together the schematic, he began putting together the layer stackup. With Altium Designer, you can view the entire stackup—both rigid and flexible circuitry. The image below illustrates a typical layer stackup for a rigid-flex combination. “This was very important to us,” says McGrane. “Our product consists of many ‘islands of rigid PCBs’ connected by flexible circuits.”

It’s critical to be able to see the entire stackup, both rigid and flexible segments, when designing complex wearable technology. Image courtesy of Altium.

Once McGrane had a stackup model in place, he used the 3D capability of the software to model the bending and twisting that the product will experience. “I can build a model in about five minutes that does practically everything a sophisticated mechanical engineering modeling tool can,” he says.

“That was extremely important because our ‘secret sauce’ is the geometry and folding techniques used for the flex circuit serpentines. Those serpentines bend and stretch to accommodate the forces exerted on the patch by the body. Without being able to quickly model them, this project would have taken a lot longer,” he explains.

The flex circuitry is also part of the way the sensor couples to the patient. Unlike sensors that connect to the skin indirectly, such as a blood pressure monitor or pulse oximeter, the BioStampRC connects directly to the skin via electrodes on the reverse side. It can also be charged on a pad, without any other external connectors.

The flexible, self-contained sensor is charged without using connectors. Photo courtesy of MC10. Image courtesy of Altium.

“Our device creates much better quality data because of the direct connection to the patient’s skin. We measure the changing electrical potential on the skin, called electromyography. With that high-quality data, very accurate kinematic models of limbs, torso and back can be constructed,” says McGrane.

Manufacturability

Rigid-flex PCBs have traditionally been looked at as more difficult to manufacture than an all rigid board. While this is true to an extent, the common use of flex circuits has pushed PCB fabricators and contract manufacturers to become experts, meaning that there are many more quality options than in the not-so-distant past.

During the design, MC10 took pains to ensure that the product could be easily manufactured with high quality, while still maintaining a reasonable cost. They took a number of precautions on the flexible circuitry including using teardrop shapes around pads and vias, avoiding right angles and traces near the edge of the flex, and ensuring that traces were offset between top and bottom. Plus, the added some stiffening to the flex to ensure utmost reliability.

Altium Designer was used to create flexible circuitry. Image courtesy of Altium.

“Once the manufacturers are assured that those serpentines are not as difficult as they appear” says McGrane, “they have found they are relatively easy to manufacturer. And it turns out that they are relatively cheap.”

The Outcome

By using Altium Designer, MC10 has been able to push the envelope of rigid-flex circuit design, specifically the flex and stretch abilities. The capabilities of Altium Designer let the MC10 team harness their imagination, creating advanced, future-looking electronic designs, without back-end complexities.

• Altium
• MC10