In theory there is no difference between theory and practice. In practice there is.
In this post we'll discuss some of the "best practices" for implementing capacitance sensing widgets and control surfaces on your printed circuit boards. We've done a mash-up of several tasty tracks from different vendors and technologies to give you a mastered mix-down...
You've got questions:
- How big do my Touch Widgets need to be?
- How thick of a covering Overlay can I use?
- How do I account for Manufacturing Variations?
- How close together can I place my sensor Widgets?
- How do I characterize the Signal to Noise Ratio (SNR)?
- Will it blend?
Touch Widget Design
In the Touch Widgets Library for Eagle post we covered the Anatomy of a Capacitive Sensing Touch Widget, and much of what went into the construction of the Widgets themselves; but here are a few notes on issues that you may not have noticed :
- Connection Pads are placed on the edge of the Widget (not the center) to reduce trace routing lengths, to minimize the parasitic capacitance (Cp) of the Widget and increase the Signal to Noise Ratio (SNR).
- The Top Restrict (tRestrict) and Via Restrict (vRestrict) layers are spaced to keep other signals a suitable (+1.0mm) distace away from the Widget. In particular, make sure to keep any high-frequency clocks or other signals away from the Widget as much as possible to increase your SNR and reduce any false triggering.
- The parasitic capacitance (Cp) of the Widget (as measured from the Widget to the surrounding ground hatch pour) tends to level out after about 1.0mm of spacing, however the SNR of surrounding ganged sensors (for sliders and radials) will start to drop off if the ganged sensors are too far apart. For that reason we have made 1.0mm (~40.0Mils) as our standard spacing between ganged sensors. If you use our individual elements (i.e. single slider element) to build up your own ganged sensor, you will need to factor that into your placement.
- You'll notice that we used rounded corners on our Widget designs, both to reduce radiated emmissions and to avoid SNR spikes on the (sharp) corners of the Widgets. Squares are not your friends (neither are triangles); be cool, what goes 'round comes 'round, and that best for your designs.
- We've provided 8mm, 10mm, and 12mm Widgets, which are generally good for plastic overlays that are <4.0mm thick. However the dialectric constant of the material used does make a difference, so we'll show a table of those later on.
- In general, you should leave >1.0mm of empty space between all sides of Widget, with an Isolate of 12Mils.
- Some controllers require a resistor in series with the Widget, place the resistor close to the Controller, not the Widget. If using a Widget with an LED underneath, place the LED current limiting resistor close the Controller, not the Widget.
- Best practice is to keep the Widget trace on the Bottom (back) side of the PCB, with a minimum of vias before connecting to the Controller. If possible, place the Controller on the Bottom (back) side of the PCB. If you must place the Controller on the Top side, equalize all the trace lengths and number of vias for all sensors, especially for ganged sensors like sliders.
- Minimize all layout traces, make them as short (and equalized) as possible, <100mm is a good metric. If you must have long traces consider using the larger Touch Widgets (12mm), and use a thinner (<2.0mm) Overlay.
- Long Traces with Small Widgets and Thick Overlay = #CapSense #Fail
Compilation of Touch Widget Best Practices
This is a compilation/distillation of best practices for our Touch Widgets Library for Eagle
|Round Widget Size||8 Mils||10 Mils||20 Mils||Larger diameter increases your SNR|
|Widget-to-Widget Spacing||1 mm||1 mm||8 mm||Larger if Ground Pour Isolate >12 Mills|
|Sawtooth Widget Size||1.6 mm||2 mm||5 mm||Make sure to round the edges on the sawteeth|
|Sawtooth Widget Spacing||0.6 mm||2 mm||5 mm||Note 1 - Overlay thickness effects this|
|PCB Thickness||0.5 mm||1.6 mm||2.5 mm||for FR4 material|
|PCB Via Hole Size||10 Mils||14 Mils||26 Mils||Our library default is 14 Mils|
|Trace Via Count||1||1||2||minimize Vias where possible|
|PCB Trace Width||7 Mils||7 Mils||20 Mils||7 Mil trace/space is pretty typical, we often relax to 10 Mils for higher yields|
|PCB Layer Stackup||1||2||4||Widget, Traces, Ground, Traces|
|Top Ground Pour Spacing||30 Mils||45 Mils||60 Mils||7/10 Mil trace, 45 Mil Space, 12 Mil Isolate, Hatched|
|Bottom Ground Pour Spacing||55 Mils||70 Mils||90 Mils||7/10 Mil trace, 70 Mil Space, 12 Mil Isolate, Hatched|
|Widget to Top Pour Spacing||0.5 mm||1 mm||4 mm||Note 1 - Overlay thickness effects this|
|Overlay Thickness||0 mm||1 mm||4 mm||typically 1-2 mm, plastic, glass, kapton|
|Overlay Adhesive Thickness||0 mm< /td>||0.1 mm||0.4 mm||additive with overlay thickness, use 3M/467/468MP|
All materials vary in their composition and thickness during manufacturing to some degree . There are really two ways to address this, classical engineering analysis and/or adaptive algorithms. In classical engineering analysis you need to account for all combinations of variations (min & max), and design in a guard band to account for them, typically this boils down to achieveing a minimum signal-to-noise (SNR) ratio given all extremes(combinations) of manufacturing variations.
The second method is to use an adaptive algorithm that continuously adjusts for several factors, including temperature and background interference, and we highly recommend this. However, it is not the panacea for all ills, and you should really do both the classical engineering analysis AND use a controller with adaptive algorithms. Trust me, you don't want to get THAT call at 4am from Asia when the consumer product you designed is failing on the manufacturing floor!
Characterizing Signal to Noise
This is where many novice designers #Fail when using capacitive sensing for the first time, they forget to leave a back-channel communication mechanism with the Touch Controller device to measure and catalog the performance of the sensor Widgets during development and deployment. Or it's "hey, we only need this during development, we can omit it in production, right?" So, when your contract manufacturer in Asia changed the material for your Overlay, AND the glue that attaches it without telling you; when everything is failing in production, what are you going to do to characterize the problem? Get on a plane? Trust us here, leave in an I2C or other backchannel communication mechanism for debugging problems. You need to be able to read back the raw counts from the sensors so that you can characterize the SNR of the sensor Widgets. Otherwise, "Abandon all hope ye who enter here". As a general rule of thumb you want to have an SNR of at least 5:1 minimum, and 9:1 for robust operation (across temperature and manufacturing variations).
Customers have many different requirements when it comes to overlay and mechanical package design. Some need to be concerned about electrical isolation and safety issues, so they want an overlay with 5kV to 12kV (yes, that's 12,000 Volts) or more of isolation to any bare metal Touch Widget. As a general rule, the thicker the overlay covering the larger the sensor Widget area needs to be. If you need something special, contact us, and we can make it up for you. The most common overlay materials are Polycarbonate and ABS plastic,with Acrylic coming in a close third. Lamps and smart LED's with touch control will often use glass.
|Material||Breakdown [V/mm]||Min. Thickness @ 12kV [mm]||Notes|
|Air||~2000||10||hard to control...|
This best practices post is dedicated to our customers, who always keep us on our toes by asking us the tough questions. If you see something we've missed, or have questions of your own, comment below, or send us a tweet!
PatternAgents Eagle CAD Libraries - Files and Documentation
TouchWidget_HowTo Example Project - Files and Documentation
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