This white paper provides an overview of the touchscreen supply chain. It describes the key components of the supply chain and the touchscreen modules available in the market.
A detailed understanding of the capacitive touchscreen supply chain is vital for customers considering touchscreen end products. The engagement model chosen should match the in-house design capabilities, component supplier value added services, and available options for module design and supply.
Controller IC Supplier
The capacitive touchscreen controller is a semiconductor device which senses finger position on a touchscreen sensor. Different silicon architectures and sensing methods are used by various suppliers, which is beyond the scope of this document. Capacitive controllers can be grouped into two broad categories.
This method senses finger position on a sensor from the electric field projected through a substrate. It is typically implemented with one or more patterned ITO layers, applied to one or more transparent substrates. Projected capacitive is very durable, because users touch the top of a glass or plastic substrate, while the ITO layers are protected below. Coordinates are calculated from measuring signals off patterned ITO with a deterministic position. This makes projected sensing more accurate and repeatable than other capacitive sensing methods. Projected capacitive controllers generally require more sensing input pins for larger screens. In practical terms, they are limited to approximately a 10″ diagonal screen for single chip solutions. Alternative solutions using multiple controller ICs or unique ITO patterning are also possible.
This method employs a single sheet transparent conductor applied to the sensor substrate, typically ITO on glass, film, or plastic. The method is cost effective and suitable for high volume manufacture with many well known techniques. Because finger position is determined by four sensors on the corners or edges of the panel, touchscreens may be scaled to thirty or more inches without increasing the controller IC pin count. There are also significant drawbacks, especially for smaller screens. First, sensing is accomplished via ITO on top of the touchscreen substrate. Even though the ITO may be protected by a thin hard coat, it is susceptible to scratches from fingers, rings, keys, and so on. In addition, inherent non-linearity of the sensing field for surface capacitive must be corrected though edge patterning, software, or hardware methods. This linearization effect becomes more pronounced with smaller screens. Other implementation details such as proximity effect and finger shadow must also be addressed.
Flexible printed circuit assemblies (flex) are used to connect the touchscreen sensor to a system controller board. The flex sometimes has a touchscreen controller mounted for supply as a “flex module”. Alternatively, a simple flex circuit without controller may be supplied to the sensor manufacturer, in which case a controller IC is built into the customers’ system controller board.
Typically referred to as the “glass” or “film” supplier, this member of the supply chain has in-house capabilities to create conductive patterning on a transparent substrate material. The most common conductor is indium-tin oxide (ITO) used for its relatively low resistance and high optical clarity. Other transparent conductors are in use or in development, but are not yet main stream. Typical sensor substrates are glass, film, or polycarbonate plastic derivatives. Some sensor suppliers have additional vertical integration, such as touchscreen controller IC ASIC products, electrical engineering for controller boards, LCD supply, flex connector supply, or complete module integration.
Touchscreens are typically used in conjunction with video displays, so it is logical for LCD manufacturers to be essential in the supply chain. Their core competencies already involve depositing patterned ITO on glass and flex circuit design for LCD manufacture. It is therefore much easier for them to vertically integrate these key elements of the touchscreen supply chain. LCD manufacturers have chosen to simply supply displays to a third party for integration into touchscreen modules, or integrate the controller, sensor glass, or flex circuit into their assembly process.
Supply Chain Logistics
Supply chain logistics for touchscreen manufacture has several permutations, depending on the supplier-customer sales engagement, customer preferred business model, and vertical integration of each supplier. Customers may initially consider purchasing touchscreen controllers, sensors, flex, and LCD directly from each supplier, and performing their own design and integration. Although this may have perceived cost advantages, it places all design, performance, and quality issues directly on the customer. Each touchscreen design is unique, so the controller must be configured to work properly with the sensor characteristics, LCD noise, and environmental and industrial design factors. This is often outside the customers’ capabilities for a first design. The more often used approach is for customers to purchase a custom designed and tested known good “touchscreen module”, from one of the supply chain members.
Touchscreen modules are sold in three main varieties depending on the specific end customer:
This is a combination of controller IC and flexible printed circuit. Flex modules are mainly purchased by ITO sensor suppliers to reduce touchscreen design cycle time and improve touchscreen yield. The flex module consists of a flexible circuit with an attached touchscreen controller IC, tested as a known good unit. Flex modules are typically supplied to ITO manufacturers or design end customers via controller IC distribution channels. For the Asian market, these are typically “Stocking Reps”, who function as a combination of manufacturer representatives and value added supply chain managers.
This product combines a flex module and ITO sensor. Using the flex module improves time to market and product reliability, while maintaining flexibility for sensor design. Touchscreen modules are usually offered by suppliers of the ITO sensor glass or film to large OEM or ODM design end customers. Some controller IC manufacturers also offer complete touchscreen modules for ease of engineering design, although this offering is usually limited to large direct customers. Most large customers have very specific marketing requirements for industrial design and product “look and feel”. Subsequently, very few companies use an off-the-shelf touchscreen module. The majority are custom designed directly with component suppliers.
Touchscreen LCD Module
This is the highest level of integration. It combines a touchscreen module with an LCD to create a full touch sensitive display device. Design end customers see this device as a black box, capable of displaying images and sending touch coordinates to the main system. It is highly preferable in terms of manufacturing reliability, because each unit guarantees a known good display and touch interface. Some LCD manufacturers offer this type of module as a product to the general market. For high volume OEM customers, touch sensor manufacturers also take LCDs on consignment and produce a touchscreen LCD module from glass, flex, and controller IC components.
Understanding the details of how touchscreens are produced helps you make the right choice of technology and suppliers for your touchscreen enabled device.