TECHNICAL BRIEF VOLUME 4 - DFM Tips for Miniature Biomedical Sensor Circuits Design-for-Manufacturability (DFM) Tips for Miniature Biomedical Sensor Circuits

Introduction

Advanced foundry processes have enabled a new age of sensor circuit engineering, and designers in many fields from RF communications, embedded vision systems, and medical devices, are actively exploring new ways to employ them. One such foundry process, micron-level thin film technology, is becoming especially intriguing to designers of biomedical sensors. Thin-film circuits can be applied to flexible polyimide substrates making them capable of being bent and shaped considerably without impact on circuit performance or reliability. As such, the combination of flexible material and thin-film circuit geometries is leading medical designers to consider how far they can go in an attempt to enhance their sensors for ad- vanced procedures, and improved patient care. TECHNICAL BRIEF DESIGN-FOR-MANUFACTURABILITY (DFM) TIPS FOR MINIATURE BIOMEDICAL SENSOR CIRCUITS

For medical designers pivoting from one scale to another, however, there is often an immediate challenge of resolving their engineering ideas using dramatically less real estate. Many come from companies whose devices have been traditionally limited by the line and spacing constraints of single layer thick-film or tradi- tional flex circuit design on Kapton (which typically stops at around 2 mils). Considering that micron-scale thin film allows for a reduction in lines and spaces by over 100%, and also allows for multilayer techniques, a certain amount of education is required for designs to be production-ready. This Tech Brief aims to outline the top 7 considerations biosensor circuit designers should pay close atten- tion to early in the earliest stages of prototype development.

Further Understanding the each layer. This process keeps layer thickness Circuit Design Challenge stuck at around 1-5 mils. Thin-film flex cir- Though a medical device designer may be cuits rely on custom spun polyimide polymer an expert in the specific application and films on glass, and this approach enables sensing technology employed in a device, miniature flex circuits up to six layers, with this expertise does not necessarily translate layer thicknesses as thin as 10 microns or to building that technology on a large scale less. Trace lines and spaces for this technol- with stringent reliability constraints. More- ogy can be as small as ten microns, and via over, because the variables and potential hole diameters can be as low as 15 microns, combinations are difficult to define, so to are or .00059 mil. The significant increase in standard design guidelines. circuit density and proximity of circuit traces and features, naturally leads to electrical and Conventional flex circuits use rolls of Kapton, mechanical considerations more akin to a Dupont™ polyimide film pre-clad with met- integrated circuits (ICs) than prior flex circuit al, and are bonded with adhesives between technologies.

PAGE 2 www.metrigraphicsllc.com TECHNICAL BRIEF DESIGN-FOR-MANUFACTURABILITY (DFM) TIPS FOR MINIATURE BIOMEDICAL SENSOR CIRCUITS

1. Become a Maestro of Micro-circuit Traces, design. A related constraint is the spacing Spaces, Vias, and Crossovers. between vias, and pads. For extremely dense Like a conductor moving from a symphony interconnects, the capacitive and inductive to a quartet, circuit designers moving dra- parasitics are more of a concern, as these matically down in scale from millimeters to parameters have a greater impact on closely microns, while still having to create the same spaced leads, pads, and vias. kind of performance, need to become re- Both capacitive coupling and inductive sourceful and creative. But with limited time coupling are a function of distance. For very and capability for numerous trials and errors, sensitive biomedical sensor circuits, noise it’s important to enter into prototyping with and interference can be coupled in from “For extremely dense new knowledge. What’s also important to close adjacent conductors. With extremely interconnects, the recognize is that the typical practice of exag- narrow and complex routing sensor leads, it capacitive and in- gerating circuit features to simplify charac- is often challenging to avoid these parasitics, ductive parasitics are terization, is exponentially compounded due and it is advisable to consider their impact to the difference in scale. While making test early in the design phase. Many traditional more of a concern, as points large and accessible often allows for rule-of-thumb approximations for parasitics these parameters have simplified early stage evaluation, it can be a may not apply at miniaturized geometries, a greater impact on pitfall to count on the features around them and more detailed and exact simulations ever being anything near their position or may be required to more closely approxi- closely spaced leads, size at a submicron level. mate real-world performance. pads, and vias.”

As you’ll see, when DFM is considered for mi- Interconnect between the thin-film materials cron level designs, the circuit design will like- and components, as well as biosensor leads, ly undergo many changes in feature size and is another area for critical consideration. positioning for optimization. This includes There are standard thin-film attach methods, defining key relationships, such as line width also used throughout the microelectronics to metal thickness ratios, stitching vias to a industry, such as wire bond, solder, and pad width, stackup, and insulation/thermal epoxy attach. However, certain requirements, layer thicknesses, among other variables. such as higher conductivity and greater Among the most critical constraints to be bond strength, may require selectively plat- aware of, are the via to pad size ratios. The ing the pads to enable attach methods that minimum ratio for this depends on many meet specific criteria. factors, including insulation and metal thicknesses, which can be different for every

PAGE 3 www.metrigraphicsllc.com TECHNICAL BRIEF DESIGN-FOR-MANUFACTURABILITY (DFM) TIPS FOR MINIATURE BIOMEDICAL SENSOR CIRCUITS

2. Understand the Potential Ramifications of limit to the minimum metal and substrate Substrate Material and Metallization Choices. layer thicknesses, and this is what determines There are many DFM considerations the minimum circuit feature size. surrounding your choice in substrates and With the complex geometries of biomedical layering techniques. The first two critical sensor circuits, grounding, shielding, and choices are choosing a substrate type trace characteristics may require deeper con- and thickness. The type and thickness of a sideration than larger scale processes. This substrate impacts the physical, thermal, and is a product of a conductors grounding and electrical properties of the final micro-circuit. shielding effectiveness being a function of Thickness Measurement The third choice is choosing the material sheet resistance of a material at the frequen- cy and power of the intended and unin- Depending on the materials used and makeup of the various circuit traces and the desired circuitry properties, thin-film tended signals conducted. The same goes layering material. In addition to various metal and insulator layer thicknesses performance and repeatability variables, for traces, which makes deciding on a metal must be kept within a certain ratio for reliable processing. there may also be limitations of what metals feature’s size and thickness very important. and other compounds can be used in certain 3. Don’t Forget the Final Installation and medical sensor applications, especially those Usage Dynamics used in-body. Moreover, the circuits on each The number of layers, layer thickness, and layer will intrinsically interact with each other, circuit features all factor into the size, shape, making finite circuit layout as critical as func- and flexibility of a miniature and flexible thin tional electrical design. Improperly selected film circuit. Depending on the end applica- materials and geometries, and non-compat- tion, such as surgical implantation by hand, ible circuit layouts are the most common or integration into an automated manu- causes of circuit rejection at the production facturing process, the physical dynamics of stage. But what can be even more costly are the circuit may also be important factors to circuits that go undetected as problematic consider. until prototypes are run and tested and/or passed along to a customer for review and The overall circuit flatness, temporary adhe- then fails under use conditions. sion strength to a carrier, and final release strength may be dynamics that could be Typically, the metal thickness of a thin-film missed among the other details of biosensor circuit correlates directly with the size of the circuit design. These are additional factors circuit lead and pad features. Furthermore, could impact the yield, reliability, ease of the substrate, or insulation layer, is related to installation, and other important aspects the metal thickness, as the substrate must of end-use. Unlike standard flex circuits, completely cover the underlying metal. microminiature flex circuits are not designed Hence, the most critical circuit feature ge- to be constantly exercised, and installation ometry will likely decide the thickness of the in a motion application may result in circuit metal and substrate for that particular layer, degradation and failure. though thicknesses may vary from layer to layer, unlike many PCB processes. There is a

PAGE 4 www.metrigraphicsllc.com TECHNICAL BRIEF DESIGN-FOR-MANUFACTURABILITY (DFM) TIPS FOR MINIATURE BIOMEDICAL SENSOR CIRCUITS

4. What About Special Features? may have led to working models and small If certain special features used to enhance volumes of a circuit—are the same as those the competitiveness of your biosensor required to produce a circuit at high volume. or medical device over other products is (This is not limited to University research. required it’s important these be on the table Expert medical designers at major brands early with your manufacturing partner. This often make similar errors in their own labs). could be anything from bonding bumps, Teaming up with a thin-film manufacturing use of exotic metals, micro-circuit stiffeners, partner early in the design phase can help and extremely dense circuit patterns. All of to prevent development going down a road that later requires redesign when approach- Design Compromises Don’t Always which, need to be considered during the Equal Device Compromises ing manufacturing at volume. design stage to avoid unfortunate redesign Medical sensor circuit designers are as and revalidation. 6. Don’t Overspecify to the Point of Overkill. susceptible to over-engineering as any- one. Taking a team approach to design is Exotic materials may incur substantially It may be hard to imagine that a medical the best way to weigh trade-offs relative greater costs that will price them out of the device may be specified to be “too good”. to time-to-market and the true value of a design feature. market, and there may be a limit to how, But as any engineering team knows, it’s not or if, they can be processed. Some metals a designer’s nature to leave anything on the are unable to be processed in the presence table in regards to performance. Overspecifi- of others, as certain metals and processing cation occurs when a particular performance agents may interact chemically. For exam- parameter is overvalued, or there isn’t a ple, platinum can only be used on a gold precise understanding of a nominal range for circuit if it’s added to the metal stack during a parameter. This can lead to redesign, mate- the optimal process step. This is a result of rial swaps, and manufacturing trials that will the platinum metal etchant also etching all increase costs and time-to-market. Ensuring other metal types. This is somewhat similar that nominal parameters, and involving a to a solder hierarchy used in PCB and PCBA manufacturing partner early in the design manufacturing. process, can help prevent over-specifica- tion and improve the competitive position 5. Avoid the Pitfall of Assuming That if it Works in the Graduate Lab it Will Work at of your biosensor or medical device in the Volume. market.

It’s common practice for many medical 7. Properly Prepare your CAD Files. device companies to collaborate with ad- Though struggling through the minutia of vanced scientific and engineering Universi- correcting every design rules check (DRC) ties to develop next-generation technologies and layout versus schematic (LVS) error may and devices. But, a common pitfall of this be painful, it is necessary before handing off practice is that there’s a presumption that a design to a manufacturing partner. As the the prototype manufacturing tools and manufacturer may use different software equipment used in a University lab—that

PAGE 5 www.metrigraphicsllc.com TECHNICAL BRIEF DESIGN-FOR-MANUFACTURABILITY (DFM) TIPS FOR MINIATURE BIOMEDICAL SENSOR CIRCUITS

tools to interpret the CAD files, any unad- Conclusion dressable errors could be compounded when being prepared by the manufacturer. Industry disrupting breakthroughs using Another reason, is that there could be nested more compact and flexible micro-circuits errors that only reveal themselves after a par- in biomedical sensors and other devices ticular DRC or LVS error has been corrected. are enabling innovative medical sensing Without correcting these errors, unintended solutions with much greater performance, circuit performance is possible, and tracking capabilities, and accuracy than ever before. down and correcting these issues could However, these miniature, micron-scale elec- delay time-to-market. tronics often require another level of circuit design expertise, especially as compared to Next Steps: It is often more efficient to follow a system- traditional flex circuit or PCB design. This is Download our thin film circuit design atic design process that minimizes errors, especially the case when designing thin film guidelines than having to backtrack each time an error circuits for manufacturability in medium to Learn more from our insightful blogs is encountered, especially if correcting one high volumes. In addition to all of the consid- Read our FAQs and Submit a DFM error creates another. erations mentioned above, the application support question engineering and DFM expertise of your thin During a CAD file review, your thin film Request a Quote film manufacturing partner can also be a manufacturing partner will analyze every make or break decision when pioneering individual line and feature to ensure that is new devices in this highly competitive properly drawn. Ideally, a customer supplied market. email of the DWG or DXF file with accompa- nying properly dimensioned PDF drawing of the final product can help to ensure the efficiency of this process. The following are common errors that circuit designers often make: • Overlapping and/or incomplete lines • Inaccurate or missing labels of all layers • Missing features • CAD drawings that don’t exactly match the specification drawing • Revision control related issues

50 Concord St., Wilmington, MA 01887 Phone 1.800.261.2557 • [email protected] www.metrigraphicsllc.com