Microwave Journal
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metrigraphics

Miniature Sensor Circuit Fabrication

September 21, 2017

As biosensor circuits continue to get smaller and more complex, while also needing to be formed to fit in things like arthroscopic tubes―or provide higher sensitivity to epidermal bio-chemical interactions―the need for circuit designers to thoroughly understand both their breakthrough opportunities and manufacturing limitations, is critical to medical market success.

As a fifty-year-old, once government controlled laboratory, and now a contract manufacturer / thin film circuit foundry specializing in the smallest of rigid and flexible thin film circuits and electroformed components, Metrigraphics has been perfecting advanced circuit manufacturing for decades, and would like to share some tips with you as you explore your next-generation medical or biosensor circuit designs.

Issues such as material selection, design guidelines for traces and spaces, and multi-layer integrated functions can be daunting. So if after reading this brief article you have further questions, you’re invited to get in touch directly with the Metrigraphics technical support team any time you have a thin film circuit layout or fabrication process question.

Entering a new era in medical sensor circuit design and biomedical device material options

Intravenous and implantable devices, and surgical and cutaneous diagnostic products, are advancing at rapid paces and advanced materials and circuit manufacturing technologies are giving way to higher functioning device possibilities at sub-micron scales. Clear conductive films let you see through the product; biocompatible materials permit surfaces to go from barrier layer to active layer; new insulating materials are also offering electrical functionality; advanced photolithography on polyimides is providing high flex possibilities; and materials with thermal management properties are allowing for reduction of cooling elements and increases in power, just to name a few. But nothing is more critical than the electrical circuit layout itself.

You can’t make it smaller or better without stacking it up. Keys to high density design and sensor realization.

Almost every circuit designer is looking to either reduce the footprint of their finished device or increase its capability without increasing allotted real estate. This requires reducing feature sizes, re-orienting them, and moving certain functions to 2nd, 3rd, or 4th layers, or more. This is referred to as high density design or a multi-layer approach. And the discipline is often foreign to designers who’ve previously worked at bigger scales.

A number of factors are involved in making sure your thin film sensor circuit will perform as designed and can be manufactured at high yield , including line (or trace) density, line width and height, number of layers, and the number and proximity of various features such as coils and vias. Where features are located on the board may strongly impact performance factors such as a circuit’s electrical impedance, and whether features can be fabricated as part of the photo imaging process, or must be cut out with a laser as a separate step — which increases manufacturing cost, complexity and risk.

Mechanical and environmental questions to consider:

  • How much heat will the sensor circuit dissipate during operation?
  • How much shock and vibration must the sensor withstand?
  • What about acceleration loads?
  • Will the biosensor be subject to high humidity and/or liquid or bio-fluid emersion?

Assuming one familiarizes themselves with the variables needing comprehension at the outset of design, the next challenge becomes ensuring the finished circuit will be compatible with the steps in the assembly process, including packaging and testing. While sub-micron circuits can be achieved by partnering early with your manufacturer on design-for-manufacturability issues, these other critical issues are also very important.  One of the questions Metrigraphics asks, for example, that may not occur to a design engineer, is how the circuit will be handled after it leaves the foundry, such as during shipping, final integration, or in the sensor application itself. Based on that information you’re manufacturing partner might change the order or way in which they fabricate various features, perhaps to increase structural rigidity (such as by using thicker traces) or to increase the area of a bonding surface.

Getting to the medical market on time

Medical OEMs have an incentive to do all they can to speed things up and avoid time-wasting redesigns. That means selecting a proven fabrication partner, but also becoming more informed on other ways to make success routine.

Design the process, not just the circuit. A key aspect medical device designers sometimes overlook is that creating an ultra-miniature circuit is as much about designing the process recipe as it is about designing the thin film layout itself. That means that design, prototyping, and volume manufacture are all strongly interdependent. So having a relationship with a full-service provider can increase the quality and speed of each step. One of the ways that happens is by a continuous feedback loop that ensures issues that show up during manufacturing get fed back to the designer in real-time to improve yield. One of the time-tested corollaries of design/fabrication of thin film flexible circuits is that improving process design also improves product design and quality outcomes.

Vet your thin film circuit manufacturing partner thoroughly. Whether your circuit can meet design requirements, mechanical challenges, and environmental hazards has a lot to do with the capabilities of your foundry partner and their manufacturing equipment and processes involved in making the circuit. Those capabilities include state-of-the-art systems for computer aided design (CAD), photolithography, the layering of metals at an almost-atomic level, advanced laser cutting, and ultra-pure manufacturing equipment and clean rooms.

Communication is key. No one knows your product better than you. That’s why Metrigraphics recommends a face-to-face meeting at project kickoff with your circuit contract manufacturer. Rather than just sending a list of specifications the circuit must meet over e-mail, establishing personal rapport early is a must.

To learn more about thin flexible circuit design and manufacturing, and to request sensor circuit application support, drop Metrigraphics a line via their support page, or call 1-800-261-2557.

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