A Brief Analysis of Capacitive Touch Technology

Compared with the traditional resistive touch technology, which currently has the highest market share, capacitive touch technology brings many advantages to users, including: up to 97% penetration rate and more realistic color presentation, bringing us better visual enjoyment; the touch function can be realized by just touching it lightly or even without actual contact with the screen, bringing users easier and more flexible control; longer service life, the touch life of the capacitive screen is about 200 million times, which is 200 times that of the four-wire resistive screen (1 million times) and five times that of the five-wire resistive screen (40 million times).

　　The signal detected by capacitive touch technology comes from the tiny changes caused by touch. According to the different working principles, it can be roughly divided into surface capacitive touch technology (SCT, Surface Capacitive Touch) and projected capacitive touch technology (PCT, Projected Capacitive Touch). The former is commonly used in large-scale outdoor applications, such as public information platforms (POI) and public service (sales) platforms (POS) and other products, while the latter has become a hot topic due to the multi-touch mobile phone iPhone launched by Apple.

　　From the development process of touch technology, the earliest market to introduce touch technology was the industrial control field, whose purpose was to integrate the complex and large-scale mechanical equipment control panel into a single window, multi-page screen. At that time, medium and large-sized resistive screens were used. However, the shortcomings of resistive screens such as insufficient life and tolerance could not meet the needs of the industrial control field. Therefore, when medium and large-sized SCTs came out, high-end equipment and machines immediately switched to SCT solutions. It was not until around 2003 that small-sized resistive screens were used in portable products such as PDAs and GPS due to the reduction in manufacturing costs of resistive screens, and touch technology officially entered the consumer market. In 2006, the iPhone adopted a small-sized PCT, and its excellent optical properties and multi-touch function set off a trend, becoming the most watched touch technology in recent years.

　　From the above, it is not difficult to find that the current consumer market dominated by small size has only two touch technology options: resistive and projected capacitive. Although the former is low-cost, its poor optical performance and tolerance have long been criticized by the market; although the latter has many advantages, there are only a handful of suppliers who can truly mass-produce it, and the price is naturally quite expensive, so it is only seen in a few high-priced products.

　　The main reason why SCT is rarely used in the small-size market is the cost issue. SCT panel manufacturers have long lacked key optical coating technology and must outsource processing, while SCT touch ICs are controlled by a few technology manufacturers and are priced high. In addition, unlike resistive screens that can be freely matched with resistive ICs, SCT screens and ICs must have excellent compatibility to work stably. The above factors make the price of SCT in small-size consumer applications not far from PCT, so it is naturally difficult to be adopted by the customer base.

　　However, compared with resistive technology, SCT can significantly improve its defects; compared with PCT, SCT technology is more mature and stable and can be introduced in mass production. Therefore, we can reasonably infer that when the overall cost of SCT is greatly reduced due to the strategic alliance and technical resource integration of industry members, SCT will have the opportunity to become the best solution for small-size consumer applications.

　　The following will briefly introduce the basic principles of PCT and SCT, and compare the advantages and disadvantages of these two technologies.

　　Projected capacitive touch technology

　　PCT is built on the concept of matrix. In the touch screen manufacturing part, the ITO of the PCT panel is etched to produce a specific pattern, the purpose is to improve the SNR value of each touch point and enhance the accuracy of recognition. By replicating the aforementioned pattern several times in the X-axis and Y-axis directions (the number of times depends on the screen size), a PCT matrix similar to a keyboard is formed. Figure 1 is the most common diamond pattern currently.

　　

　　Figure 1: Diamond pattern layout of a projected capacitive screen.

The orange diamond pattern in Figure 1 forms the ITO wires in the X-axis direction (a total of m wires), while the green diamond pattern forms the ITO wires in the Y-axis direction (a total of n wires); the PCT controller drives these wires in sequence to detect whether there is a change in capacitance due to touch.

　　

　　Figure 2: PCT equivalent RC circuit and the detection waveform on the X2 wire before and after finger touch.

　　Take the simplest RC oscillation scheme as an example. We simplify the equivalent circuit of the X2 wire in the X-axis as shown in Figure 2, forming an RC circuit composed of n Rp and n Cp, where Rp and Cp represent the equivalent ITO segment internal resistance and the inherent capacitance of each PCT node (the intersection of the XY axis). When the finger approaches or touches the screen, a capacitance (Cf) will be added to the screen; for this RC oscillation circuit, the appearance of Cf means that the oscillation period becomes longer and the frequency decreases. By calculating the changes in the oscillation period and frequency on the X2 wire before and after the finger touches, the PCT controller can identify the touch location and even the distance between the finger and the screen (that is, provide Z-axis information).

　　Surface capacitive touch technology

　　The SCT panel is a piece of ITO layer coated evenly. There is an outgoing line (UR, UL, LR, LL) at each of the four corners of the panel connected to the SCT controller. In order to detect the exact location of the touch point, the SCT controller must first establish a uniform electric field on the SCT panel. This part of the work is achieved by the driving circuit inside the IC charging the panel. When a finger touches the screen, a small amount of current will flow; at this time, the sensing circuit in the IC will analyze the current on the four lines respectively, and calculate the XY coordinates of the touch point according to the formula in Figure 3. In order to overcome the influence of interference, the calculated coordinate values ​​can be processed using hardware filters or software filters.

　　The biggest difference between PCT and SCT is that PCT has the opportunity to achieve multi-touch, while SCT can only achieve single-touch; in this sense, PCT seems to be superior to SCT, but in fact it is not entirely true.

　　The working principle of PCT is not complicated, so it is not difficult to provide a demonstrative prototype; however, when engineers tried to convert PCTDemoSet into a mass production plan and prepare for mass replication, various technical challenges came one after another. The main challenges include the following three aspects.

　　The signal to be detected is weak (made into a coarse sound) and is easily affected by the environment and becomes unstable, resulting in inconsistent sensitivity of the touch function, and even possible malfunctions. The capacitance change caused by touch is actually very easily affected by temperature and humidity. The current common countermeasure is to use timed automatic calibration to overcome this problem. In addition, other electrical appliances in use or other functions of the product itself (such as the RF of a mobile phone) will interfere with signal measurement. This part must be achieved by improving SNR through software or hardware; so far, IC designers still have room for improvement in this regard.

　　The yield of mass production needs to be improved (made into a rough Song). PCT technology has the inherent advantage of multi-touch. Therefore, since the advent of the iPhone, almost all the focus has been on PCT. The touch industry, including touch screen manufacturers, IC design companies and solution developers, has invested a lot of resources in development; but to this day, there are still not many PCT products on the market. The problem is that the overall production yield is still not high, which makes the cost high. The reasons for the low yield are: the existing PCT controller is still not mature enough, and the self-adjustment ability in different application environments is still insufficient, so the touch menu is unstable. In this case, the characteristics of the PCT panel (such as: the resistance and capacitance values ​​within the panel, etc.) have to be strictly limited to reduce variables; due to the capacity limitations of the PCT controller, the strict requirements for the consistency of the PCT panel characteristics are often the main cause of the low yield.

　　Patent issues (made into thick Song) Since many basic gestures and multi-touch functions have been patented and approved by some manufacturers, the multi-touch advantages of PCT products cannot be brought into play in many application fields (especially the main mobile phone market)! Overall, the PCT solution will definitely have a place in the market in the future because of its multi-touch advantages. However, judging from the current situation, it is still not a mature solution.

　　In contrast, although SCT technology also faces the problem of capacitive signals being easily interfered with in controller design, through the continuous efforts of IC designers, SCT solutions can be widely used in various environments. For example, the SCT touch IC (WT5750F) jointly developed by Weltrend Electronics and Wanda Optoelectronics can easily overcome the differences in various external environments and touch screen sizes through various adjustment mechanisms contained in the IC, and stably provide high-quality touch experience. In addition, in order to meet the needs of handheld devices for miniaturization of mechanisms, Weltrend Electronics' SCT touch IC has made great efforts to improve product integration, so it can simultaneously meet the two important demands of miniaturization and cost reduction.

　　In addition to IC, the touch screen structure of SCT is simpler than PCT, and it is easier to overcome the problem of noise interference in hardware. In addition, through process improvement and breakthroughs in key technologies, the overall SCT solution has a cost competitive advantage comparable to that of resistive type. Taking Wanda Optoelectronics' self-developed optical coating technology as an example, the light transmittance of the touch screen can be increased from 87% to 97%, and the reflectivity can be reduced from 12% to 2%. More importantly, the yield of optical processing can be controlled at more than 90%, and the step of outsourcing processing can be eliminated, thereby improving technological autonomy and production capacity, and significantly reducing manufacturing costs.

　　SCT technology is one of the capacitive touch technologies, so it also retains many advantages of capacitive: "better visual enjoyment", "easier and more flexible control" and "longer service life", etc. In addition, SCT's requirements for touch screen characteristics are relatively loose compared to PCT, and its mass production technology is more mature, which makes it have the advantages of high yield and low cost. Moreover, in actual operation, users' demand for single-point selection, single-point gesture and handwriting functions is more urgent than multi-touch, and these functional requirements happen to be what SCT can currently provide stably. Therefore, before the PCT solution matures (functional stability, patent issues, high production yield and low cost), the SCT solution is a touch solution that is very in line with market expectations.