Finger-heating effect in mutual capacitive touch sensor design
A finger-heating effect has been observed on mutual capacitive touch sensors with plastic film overlay. The symptom is exhibited as serious sensitivity degradation when there is a greater than 10°C temperature difference between the finger and a touch sensor. An in-depth experimental and theoretical analysis was performed to understand its fundamentals, followed by a proposed physical model addressing the electric field lines coupling among substrate materials, mutual capacitive electrodes, and the human finger. Recommended sensor stack-up and pattern design solutions based on mutual capacitive sensing chips are presented.
Touch sensors with glass overlay prevail on the current smartphone and tablet market. Optical transparency is the major advantage. On the down side, in addition to cost, durability is another concern. Especially for the tablet size, drop tests always have been a challenge for glass overlay. Plastic film overlay, except for a little less transparency, is lighter, lower cost, and has no durability issue as with glass. Hence, there is a growing interest with applying plastic overlay on touch sensor stack-up. A key property difference between plastic overlay and glass, which contributes to the finger-heating effect, is its much higher heat transfer coefficient. To fully understand this subject, we begin with an introduction on stack-ups and sensor patterns under plastic overlay.
Plastic-film (PF) and plastic-film-film (PFF) are illustrated in Figures 1 and 2, respectively. Each has two kinds of structures using a plastic overlay, cross section, and associated sensor patterns for each stack-up. For PF stack-up, ITO film is optically bonded to the plastic overlay. Transceiver (TX) and receiver (RX) electrodes are on the same layer. Fringing or side wall capacitance is the mutual capacitance component. For PFF, plastic overlay is optically bonded to two layers of ITO film. The RX electrode is on the film layer closest to the plastic overlay, while the TX electrode is underneath the RX. Mutual capacitance in PFF stack-up consists of both parallel plate and fringing capacitance.
When a relatively warmer finger comes in contact with a colder sensor with about a 10-15°C temperature difference, this can be large enough to trigger the problem. The signal begins strong, but slowly degrades as the finger remains present. This process may last for several minutes until the signal falls below threshold, or even becomes negative. Usually a glass overlay doesn’t encounter this problem as much since glass has a lower heat transfer coefficient than plastic. Substrate material can play an equally important role as well. In one of our experiments, if the film substrate is replaced by FR4 in a printed circuit board (PCB), the finger-heating effect disappears – even with a plastic overlay.
Figure 1. PFF stack-up cross section and a dual-layer sensor pattern.
Figure 2. PF stack-up cross section and a single layer pattern.
Analysis and physical model
A simplified equivalent model of a TX/RX sensor is shown in Figure 3, where Cm is the intrinsic mutual capacitance between the TX and RX electrode, and Cfm is a finger-induced mutual capacitance. R is the resistance of sensor. Either Cm or Cfm has the following dependency on dielectric constant ε: length of the TX/RX perimeter L, and distance between the TX and RX electrodes. Also notice that dielectric constant ε is a function of temperature, T:
Physical understanding of this equation can be done only by examining the field line coupling in this capacitive system. Figure 3 illustrates the electric field distribution in both PFF and PF stack-up.
Figure 3. Electric field distribution in PFF and PF stack-up.