Waterproof Solution for Capacitive Touch Display

Capacitive touch displays are commonly found today in household appliances, refrigerators, and other products in white electric appliances, one of these new challenges is operation in a wet environment. This article will discuss how to design waterproof solutions for CTP.

In recent years, touch screens have developed into the simplest and most direct way for people to communicate with machines.

The capacitive touch screen, in particular, utilizes the capacitance formed by the fingers of the human body, as long as the fingers touch, it will react. 

Touch technology provides a lot of flexibility for the terminal design, hence, it is widely adopted by commercial and industrial fields. 

However, due to the principle of its touch generation, the touch performance will inevitably be affected by external interference. For example, if the home appliance with the touch panel is accidentally stained with water drop, the user will no longer be convenient to operate, and in severe cases, the corresponding function cannot be effectively activated.

At present, there are many solutions in the capacitive touch application field, but most of them are aimed at improving the internal structure of the screen. We propose a terminal design solution that can be correctly identified and waterproof.

Concept of waterproof

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. which values the product protection level from dust and water. The highest rating of the product is IP-67, that is, it can handle amounts of dust (dustproof of 6), and can be immersed in water up to 1 m (waterproof of  7) without damage. 

However, the 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 the definition of waterproof has not been 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 refuse to touch by mistake while the liquid is on the touchscreen and completely recover after the liquid is removed.

Water is conductive, so it interacts with the same electric field when it lands in the active sensing area. This can lead to a report of a finger touch when water splashes onto the sensing surface even when no finger is present.

For example, if you accidentally spill coffee on the mobile phone, you don’t want it to automatically make a call or send an email, or even do anything when you are in a hurry to clean the phone. Once dry, you will expect to return to normal function.

It is the most common and important factor for waterproofing, because the liquid will stay on the touchscreen surface, and the touchscreen must be able to return to normal without any false touches. 

However, water rejection cannot support touch on the wet surface, which depends on the wet finger tracking.

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 errors in the capacitive measurement, which degrades touch accuracy. The wet finger tracking function ensures an accurate number.

In the case of water, there always 1-2 mm is normally a good amount of tolerable error. Key functions such as dialing a phone number or sending an email are sufficient to meet operational requirements.

Liquids or any conductive material will affect capacitive sensing signal and touch performance. As below table, the touch performance of the projected capacitive will be affected to varying degrees under different wet conditions.

Figure 1. State of water
Figure 1. State of water
FigureSituationEffectiveness performance of touch
aWet fingersSlight influence on sensitivity and linearity
bWater mist/moistureSome influence on sensitivity and linearity
cWater dropletsWhen fingers pass over water droplets, there will be misreport, disconnection or flicker
dWater dumpedWhen fingers pass over water mark, there will be misreport, disconnection or flicker
eWater accumulated on the surfaceWhen fingers pass over water, there will be misreport, disconnection or flicker
Table 1. The touch performance with various degrees 
under different wet conditions

Illustration of waterproof function on touch solution

A waterproof design implies system performance that is immune to the effects of water. 

For a water-tolerant design, water levels encountered in normal operation do not interfere with sensor operation. Splatters and spills on the touch surface are tolerated, but total immersion is not. 

In a water-tolerant design, only the touch of a finger produces a signal large enough to register as a “touch”. 

The touch solution supports a waterproof function, which can effectively resist signal interference caused by water.

When the controller detects that there is suspected water interference on the surface of the touch screen, it will automatically turn on the waterproof function.

In this mode, the touch function may be limited to maintain the stability of the touch. The following is the state diagram of the waterproof mode.

Figure 2. Mode state of waterproof
Figure 2. Mode state of waterproof

In Normal conditions, the controller provides a multi-touch function.

When water is detected, it will enter the “Water Condition” state. In this condition, the function will be degraded to single-finger or two-finger touch to meet the basic touch requirements. 

When excessive water is detected, to maintain the stability of the touch system, the touch function will enter the “Disable Touch” state, which means the touch function will be temporarily turned off until the water amount reduces and interference is improved.

If the waterproof requirement of the application is even higher, the firmware can also be directly adjusted to single-finger or two-finger touch to improve the waterproof effect, to avoid the potential errors from automatic detection and switching mode. The two-finger touch function under the water will be limited to simple two-finger gesture operations. 

Figure 3. Different touch gesture for operating
Figure 3. Different touch gestures for operating

Waterproof effectiveness is highly related to touch sensitivity. For certain highly sensitive applications, such as operating via a gloved hand or a tiny capacitor tip, the waterproof performance will be reduced or even worse.

Suggestions on Terminal’s Design for Better Waterproof

The following are the design principles for better waterproof effectiveness:

1. The touch system should be designed to prevent water from easily accumulating on the touch screen surface. 

The better waterproof effectiveness is a fully flat design. 

Figure 4. Design principles for better waterproof effectiveness
Figure 4. Design principles for better waterproof effectiveness

2.  The frame of the touchscreen should not be a conductor. 

Because when the water flows through, it will ground with the system through the frame, which results in a signal effect similar to finger touch.

In addition, if the user is connected to the touch screen through water, it will also cause a phenomenon similar to finger touch, resulting in false touch.

Figure 5. Waterproof comparison of different frame
Figure 5. Waterproof comparison of different frame

3. Applying waterproof coating onto the cover plate can reduce the water adhering to the surface, which is of great help in reducing the interference phenomenon of water.

Figure 6. Applying waterproof coating
Figure 6. Applying waterproof coating

4. A proper UI design can also reduce water interference in operation.

Water is easily accumulated on the edge area, hence, it is better to avoid designing the touch UI around the area.

Figure 7. UI design for terminal
Figure 7. UI design for the terminal

In fact, touching the touchscreen with a finger when there is water, the signal change generated by the finger is much larger than the water that generates signals in the opposite direction. Therefore, the fingers won’t be affected too much on the touch screen with water droplets, to realize effective waterproofness.

All in all, according to the above terminal design, the waterproof effect can be achieved better. Moreover, customized waterproof displays and designated IC during production can also enhance waterproofing. 

If the application of your terminal needs to be operated and touch-controlled even when disturbed by moisture, you can contact us, we can work on the waterproof touch display solution in light of your requirements.

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