One of the most expensive components in every mobile product in the world is the touchscreen. Product developers worldwide cringe when they must consider adding a touchscreen to their product. But, where demand or frustration lies, so does innovation and opportunity. Touchscreen manufacturers globally are inventing new and more aggressive industrial design solutions to make touchscreens better, cheaper, and more integrated. There are technologies that remove entire layers of material in touchscreen sensors, new processes and patterns that eliminate cost, and new technology integration that removes requirements for complete portions of the touchscreen system. There is an ongoing capacitive touchscreen evolution and it is imperative for any product development manager or engineer to understand recent innovations that will lead to the development of the world’s most high performance, inexpensive touchscreen products ever.
First, it is critical for designers to understand the underlying construction of a touchscreen system to be able to understand how technology changes are remaking this segment. The key components in a touch system include the coverlens, sensor, LCD, and PCB. The coverlens is the outward facing component of the product. This is where the consumer interacts with the screen. In some products, this coverlens could simply be a protective cover to prevent scratching and damage, or it can actually be part of the touch sensing system itself.
In most capacitive touch systems, a touchscreen “sensor” sits just below the coverlens. The sensor is a clear glass or acrylic panel with a touch responsive surface printed or deposited on it; this sensor is then often directly adhered to the coverlens itself. This sensor can be a very complex series of layers of conductive material, clear adhesives, and glass or clear plastics; this “stack up” of materials will be discussed further below. Next, the most common construction places the sensor over a graphic display so that the touch area of the panel covers the viewable area of the screen. The last key component to the hardware system is the touch controller itself. In today’s systems, the touch controller is a small microcontroller-based chip that is placed between the touch sensor and system’s host controller. This touch controller takes information from the touch sensor and translates it into information the system’s host controller can understand.
While the description above provides the basic breakdown of the system, let’s explore the technical details of the touch sensor “stack up” itself as it truly bears most of the cost in a touchscreen system. Figure 1 is an exploded view of the layering of multiple different conductive materials in a touchpanel. Depending on the pattern and materials chosen for the sensor, there are many different combinations of ITO layering, film or glass substrates, and interleaving adhesive (OCA: Optically Clear Adhesive) materials. These combinations are chosen to allow a manufacturer to tradeoff between thickness, cost, transparency, rigidity, bezel width, front window material, weight, and performance.
Most touch sensors today are built using a combination of layers of glass or acrylic, isolation layers, clear adhesives, and indium tin oxide (ITO). ITO is a ceramic-like material known for its high conductivity and excellent transparency. While ITO is broadly used and has been proven to be an excellent material for touchscreens, handling and manufacturing ITO has its disadvantages. The primary objection when manufacturing touchscreens today is that ITO is expensive, the materials are fragile and heavy, and the manufacturing process is labor intensive and expensive. ITO manufacturing is a step-intensive process described in Figure 2 below. Figure 2 shows a typical process flow for manufacturing ITO-based sensors. Steps include sputtering ITO powder over glass, thermal baking the ITO to its melting point thereby creating a conductive layer, and then etching the sensing circuit topology on the conductive layer by use of photo or laser lithography. Each step in the process adds additional cost as a result of materials cost, manufacturing time, and yield loss.
For both tablets and mobile phones, sensor thickness is also a major consideration. In fact, one of the key driving factors for reducing layers in a touchpanel is not only cost, but also thickness. A typical glass coverlens varies from 0.5-1.0mm think while a typical PMMA lenses, though lighter, are usually 1.0mm thick or greater because PMMA is a more flexible material and should not bend. A sensor’s secondary substrate of glass will generally have a thickness of 0.2mm-0.7mm while a similar structure of PET (polyethylene terephthalate) has a thickness of only .055mm. PET, however, cannot yet easily be printed with bridges or jumpers (similar to PCB vias in the ITO structure), so multiple layers are need as opposed to a glass substrate that better handles ITO etching and bridges. Adding multiple extra manufacturing steps for PET must then be considered against the higher materials cost, thickness, and weight of glass.
One of the newer trends in mobile products is the goal of directly laminating the touch sensor to the LCD. This gives the phone a more sleek profile as it eliminates gaps between layers and it puts the user’s finger just that much closer to the actual display. Unfortunately, directly laminating a sensor is electrically quite a challenging feat. Most developers don’t realize that LCDs emit a substantial amount of electrical noise.
Because capacitive touch sensors measure very minute changes in capacitive signature on a touchpanel, this capacitive field can be disrupted by LCD switching noise. In the case of a typical TFT LCD, the common electrode is driven by either a DC or an AC voltage. This DC or AC voltage can switch dramatically as the LCD changes patterns on the screen (it even changes noise profile as each picture changes onscreen). As a result, LCD switching noise can appear to be a false finger touch. To combat the false positives created by LCD noise, touch sensor vendors sometimes add an air gap between the sensor and LCD, which dissipates the electrical transmission of noise.
Vendors may also rely on more sophisticated touch controller capabilities for cancelling or filtering noise.