The basics of waterproofing capacitive touchscreens

Touchscreens get wet from time to time. It is frustrating to a user if the touchscreen controller cannot handle water appropriately.  It’s important to understand what waterproofing really means for capacitive touchscreens, how the physics of capacitive sensing is affected by water, and why your choice of touchscreen controller makes a difference.

What Does Waterproof Mean, Exactly?

There are international standards that cover waterproofing in detail.  The International Electrotechnical Commission (IEC) standard IEC-60529, for example, contains definitions for Ingress Protection (IP) ratings.  One of the highest ratings a product can have is IP-67, meaning it can handle a significant amount of dust (dust rating of 6) and can be immersed in water up to 1m (water rating of 7) without damage.  Few mobile devices can meet this requirement, however, and the IP rating is not widely used for capacitive touchscreen product requirements.  Waterproof requirements for touchscreens usually revolve around the user experience and how the product behaves in the presence of water, rather than by using destructive tests.  Although not formally standardized, two subset definitions of waterproofing are becoming widely used in the industry: Water Rejection and Wet Finger Tracking.

Water Rejection

Water Rejection is the ability to reject false touches from occurring while liquid is on the touchscreen and completely recover after the liquid is removed from the touchscreen.  As an example, if you spill your coffee all over your phone, you don’t want it to automatically make a phone call or send an email.  In fact, you wouldn’t want it to do anything while you are desperately trying to get the spill cleaned up.  Once dry, you expect the phone to work as it did before you spilled the coffee.  Water Rejection is the most common and important requirement for waterproofing because liquids will get on the touchscreen surface and the touchscreen must recover without false touches.  Water Rejection does not cover what happens when you touch the water on the surface; Wet Finger Tracking covers this.

Wet Finger Tracking

Wet Finger Tracking is the ability to track the position of fingers on the touchscreen in the presence of water.  Water on the touchscreen surface produces error in the capacitive measurement, which degrades touch accuracy.  A Wet Finger Tracking requirement should always contain an accuracy number.  1-2 mm is normally a good amount of tolerable error in the presence of water, as it allows a good margin for critical features such as dialing a phone number or sending an email.

For both Water Rejection and Wet Finger Tracking, the characteristics of the liquid are important and should be specified.  Several characteristics that apply to the formation of liquid on the touchscreen surface include:

• Condensation--A thin layer of condensation formed on the touchscreen in high humidity or quick temperature changes.
• Water Droplets--Raindrops, sweat, or any type of liquid droplets that fall on the touchscreen.
• Thin Film--A larger volume of water that covers the entire touchscreen surface with a thin film of liquid.  A coffee spill might fall in this category.

Although the above is not an exhaustive list, it covers most use cases involving liquid.

For Water Droplets, it is important to specify the size.  A typical small droplet is normally less than 3 mm in diameter (measured after dropping it on the touchscreen and spreading out).  A large droplet is typically between 3 mm and 18 mm.

All three forms of liquid create different capacitive error profiles that the touchscreen controller must handle.  It’s important to note that the same form of liquid will create different results for self-capacitance sensing (self-cap) versus mutual-capacitance sensing (mutual cap).  (For readers that have no prior knowledge of self-cap vs. mutual cap, you only need to know for now that mutual-cap allows for true multitouch user input while self-capacitance does not.) 

Some touchscreen controllers make use of both at the same time to catch difficult heuristic problems associated with rejecting false touch detections due to liquid.  To understand these problems, we first need to understand some basics of capacitive sensing physics in the presence of water.

Capacitive Sensing Physics with Water

Capacitive sensing works because humans are conductive.  Impure water, such as tap water or coffee, is also conductive and this causes error in the capacitive measurement.  Figure 1 provides a simple self-cap physics model with electric field lines representing capacitance.