Wise Words from a Touch Screen Engineer
By L. Gaulin – Rockstar Touchscreen Engineer
Projected Capacitive Touch Screen Technology and Borders
With the advent and ever increasing popularity of smart phones and tablet computers, projected capacitive (PCAP) touch sensors are becoming more prevalent every day. This prevalence is leading more and more product development teams to look at including touch features in their new designs. In turn these designs push the boundary of what is possible for PCAP sensor, in both capability and form factor.
The size of a PCAP sensor is directly related to the size of the display active area and the borders needed to have a linearly sensitive, reliable sensor that can be manufactured efficiently. Many different options are available for hosting the conductive traces that make up the bulk of that border, all with their own pros, cons and costs. Ideally, the sensor and the display would have the same active and outer areas, but as display borders get narrower, the touch sensor industry is striving to keep pace.
By far the most common type of projected capacitive touch screen traces is the printed metal trace, usually Ag (silver). There are three main methods for creating these traces: printing, laser ablation and sputter deposition. These are listed in increasing trace density and price.
The printing option is the cheapest and fastest method, but the traces are limited by the screen or ink deposition resolution. This typically results in traces in the above 100um width range. At ~200um pitch, these traces take up considerable amount of room that adds up quickly and can take up more space than the display. These traces are best used for a small number of I/O or instances where low trace resistance is paramount. This method is also well suited for dual sided conductive ITO coated glass (DITO) as it is independent of what is on the opposite side of the substrate.
The laser ablation technique is the Touch International preferred method when designing custom projected capacitive touch panels. This entails a large swath of Ag (silver) ink being printed where the traces need to be, followed by a laser ablation that isolates and shapes the traces. The traces that this process yields are usually in the 50um range, which allows twice as many traces as printing alone could accommodate in the same area. Unfortunately as the laser passes right through the glass, any material on the far side of the glass can be damaged. This means that DITO sensors cannot have overlapping traces or conductive areas if they hope to use the laser ablation technique. While the time needed to laser scribe each line is greater than printed traces, it is far less than the duration of a sputtering deposition.
The last common trace creation method is sputtered deposition. Capital expenditures needed to accomplish this process are in excess of all others, thus it is rare than any company has this capability in house, Touch International included. Due to this, the cost and lead time for the sputtered traces is by far the largest. Inversely, the traces are usually sub 50um, which makes them the smallest option and therefor the smallest borders. Only when all other options have been ruled out does sputtering become a viable option.
No matter what trace creation method is used, the bond between the flexible printed circuit (FPC) tails that is needed to move the sensor signal to the controlling electronics is, by comparison, a large feature. With this limitation, the tail exit edge is always the largest side and must be planned around accordingly. Balancing the border width, cost, time and external constraints is what Touch International does best. Our broad industry experience in tune with our eyes on the future of the industry keep our capabilities in pace with the ever evolving touch display and multi-touch panel marketplace.
Keep calm and multi-touch on.