PFF stack-up cross section and a dual-layer sensor pattern.įigure 2. 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.įigure 1. Substrate material can play an equally important role as well. Usually a glass overlay doesn’t encounter this problem as much since glass has a lower heat transfer coefficient than plastic. This process may last for several minutes until the signal falls below threshold, or even becomes negative. The signal begins strong, but slowly degrades as the finger remains present. When a relatively warmer finger comes in contact with a colder sensor with about a 10-15☌ temperature difference, this can be large enough to trigger the problem. Mutual capacitance in PFF stack-up consists of both parallel plate and fringing capacitance. The RX electrode is on the film layer closest to the plastic overlay, while the TX electrode is underneath the RX. For PFF, plastic overlay is optically bonded to two layers of ITO film. Fringing or side wall capacitance is the mutual capacitance component. Transceiver (TX) and receiver (RX) electrodes are on the same layer. For PF stack-up, ITO film is optically bonded to the plastic overlay. Each has two kinds of structures using a plastic overlay, cross section, and associated sensor patterns for each stack-up. Plastic-film (PF) and plastic-film-film (PFF) are illustrated in Figures 1 and 2, respectively. To fully understand this subject, we begin with an introduction on stack-ups and sensor patterns under plastic overlay. A key property difference between plastic overlay and glass, which contributes to the finger-heating effect, is its much higher heat transfer coefficient. Hence, there is a growing interest with applying plastic overlay on touch sensor stack-up.

Plastic film overlay, except for a little less transparency, is lighter, lower cost, and has no durability issue as with glass. Especially for the tablet size, drop tests always have been a challenge for glass overlay. On the down side, in addition to cost, durability is another concern. Optical transparency is the major advantage. Touch sensors with glass overlay prevail on the current smartphone and tablet market.

Metal touch sensor bounce