Where touch technology is concerned, “PCAP has started its triumphal march, and accounts for about two thirds with us — and upwards.” Much improved drivers mean high functionality. And there is a demand for multitouch, with at least two fingers, plus frequently an additional cover glass, for reasons of design or IP protection category, for which PCAP is “optimal”. Accidental operation can be excluded by Kyocera’s Haptivity technology (haptic feedback of keys). Resistive touch sensors were simpler to implement, and still to be found in low-cost applications. As regards the ratio of displays with and without touch, Kyocera usually delivers standard displays without touch, “but 95 percent of the final applications work with a touch sensor”, says Schill.
“Nearly all inquiries are for displays with touch”, says Matthias Keller, head of sales & marketing components at Distec. “The good old resistive touch still has a lot of life in it, but the trend is obviously moving to modern full-glass fronts with PCAP technology.” Here the bonding technology is exclusively dry VacuBond, guaranteeing the customer zero error quality in the bonding layer.
Overview of bonding technologies
LOCA, OCA and gel bonding
LOCA bonding (liquid optical clear adhesive). This is very flexible in application. The liquid adhesive is suitable for hard-to-hard connections, and is cured by UV light. Primarily LOCA serves for connecting cover glasses with SITO sensor technology. It is also used for direct bonding, to adhere TFT displays with a cover glass (with or without touch). The major advantage of this technology is that the single components are stressed neither by high temperature nor by pressure when curing. Nor are follow-up processes necessary. Production-related unevenness of the individual components (like in the cover glass), and differences in the height of the metal bezels on TFTs are easily compensated by the LOCA adhesive. The adhesive is silicone-free and does not diffuse, making it resistant to aging, and it does not yellow. LOCA is not fully suitable for frameless displays because in liquid adhesion — even taking the greatest care — penetration of the adhesive into the display, depending on how it is designed, cannot be excluded.
OCA bonding (optical clear adhesive). Dry adhesion or OCA roll lamination with ITO film/film and mesh film/film PCAP sensors is used to deposit the touch sensor on the cover glass (hard to soft). Here the OCA of the touch sensor is used so that no extra bonding adhesive is necessary. The join is subsequently cured by means of an autoclave. Within the possible diagonals the advantage of OCA bonding is fast implementation of projects for optimized cost. The variability of the cover glass as regards printing and curing is limited because the adhesive layer on the touch sensor is limited in its ability to compensate differences in height. TFTs cannot be bonded or laminated by this process.
Gel bonding: This process is especially suitable for the further working of frameless displays without a metal bezel. Dry adhesion with OCA is not applicable on frameless displays because neither sensor/glass nor display can be deposited by roll lamination. Nor can the adhesive band of the air gap bonding be attached without difficulty — or not at all — because the necessary metal frame on the TFT of frameless displays is inadequate. Adhesion by gel bonding can produce temperature stability of –30 to +80°C — depending on the materials used and the individual components. Instead of the widely used and less expensive silicone, Data Modul uses a high-grade polyacrylate exhibiting significantly higher adhesiveness and UV resistance than silicone. Curing is analogous to OCA lamination in an autoclave.