Reducing capacitive touchscreen cost in mobile phones

Trevor Davis, Cypress Semiconductor

February 25, 2013

Trevor Davis, Cypress SemiconductorFebruary 25, 2013

With consumers clamoring for the next generation of mobile phones, manufacturers must figure out how to reduce the cost and complexity of today’s touchscreen technology. For mobile phones to evolve, so must their core technologies, including touchscreens. This article explains key touchscreen performance parameters, critical touchscreen manufacturing processes and their impact, significant physical design tradeoffs, and new discoveries in advanced capacitive touchscreen technologies including single-layer sensors, and in-cell and on-cell technology.

Despite the fact that millions of users enjoy the benefits of capacitive touchscreen enabled phones, few users understand the underlying technology. A capacitive touchscreen, like those used in iPhone and Samsung Galaxy products, is commonly constructed of several layers of materials. This layering construction is called the 'stackup' (Figure 1).

Click on image to enlarge.

Figure 1: Typical stack sensor with multiple layer technology

In mobile products, the top layer is a protective layer composed of glass with an anti-scratch coating, or PMMA (polymethyl methacrylate), commonly called plexiglas or acrylic. This top layer is often shaped, back-printed with the company logo or decorative graphics and button indicators, and is the outward-most facing material of the touch product. Directly underneath the surface layer is a layer of thin adhesive and then the electrically conductive layers for touch sensing.

Most touch sensors 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 is that ITO is expensive, the materials are fragile and heavy, and the manufacturing process is labor intensive and expensive.

Basics of ITO stack sensor fabrication
The manufacturing of an ITO sensor is similar to printing a complicated multi-color poster or graphic where every individual color must be printed in a separate pass. Much like a sophisticated printing process, ITO is deposited on the sensor substrate in multiple layers to attain the desired electrical pattern.

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. Every manufacturing step adds cost as a result of materials cost, manufacturing time, and yield loss (every step risks further defects).

Click on image to enlarge.

Figure 2: Typical touchscreen manufacturing process flow

Traditional capacitive touchscreens use 1, 2, or even 3 layers of ITO depending on the specific product design considerations and the touch panel supplier technical capabilities. Unfortunately, every required layer of material adds costs, thickness, and weight.

In the case of mobile phone touchscreens, the thickness can depend heavily on the materials used (Figure 3). For example, a typical glass coverlens varies from 0.5-1.0 mm. Typical PMMA lenses, though lighter, are usually 1.0 mm thick or greater.

Click on image to enlarge.

Figure 3: Example of layering and thickness differences in sensor designs

The primary reason is that PMMA is a more flexible material than glass. If the PMMA is not thick enough, it can actually flex from finger pressure, causing mechanical and electrical problems as the ITO materials in the stackup get closer or even touch the LCD. A sensor’s secondary substrate of glass will generally have a thickness of 0.2 -0.7 mm while a similar structure of PET (polyethylene terephthalate) has a thickness of only .055 mm.

However, PET cannot yet easily be printed with bridges or jumpers, so multiple layers are needed 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.

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