Projected Capacitive Borders

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.

Capacitive Touch Borders

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.

 

LCD Display Product Lifecycle

TECH TALK:  DISPLAY (LCD) PRODUCT LIFE

by Shaun Detmer, Director of Marketing

I have a project coming up that needs a touch display.  We know it will use a custom capacitive sensor, and we know the project will need to be in production for 5-7 years, with 3-5 year replacements and repairs.  What’s troubling for us though is how do we know our display will be around for that long?  Since you guys design touch display solutions, how do you deal with this?  – Inquisitive Engineer

I believe the phrase goes something like “the only thing you have to fear, is fear itself.”  Display end-of-life (EOL) is a concern for every display project out there, whether people are conscious about it or not.  The good thing is, you’re thinking about this now as opposed to 3 years into production, or even worse, when you find out you can no longer order the displays.    Now, some manufacturers are great about notifying you about the projected life of a product, and even send a nice EOL letter up to a year ahead of time.  This usually only applies to those customers that buy direct from the manufacturer.  If you’re going through distribution, the chances that EOL notification is going to filter its way to you decrease greatly.

It may sound overly simple, but the key to ensuring long display life is really just finding the right display from the right manufacturer.  I know, not much help right?  Let me elaborate.  I’ll be careful not to name any names, as I don’t want to ruffle feathers, but let’s just say some manufacturers have a better at display lifespan than others.  For example, if you’re picking a display from a manufacturer that produces mostly consumer grade products – you’re taking a risk.  Consumer grade displays have their own issues in the industrial space, and we’ll get to that, but the manufacturers of these displays tend to play by a different set of rules.  EOL can come within 6 months of a product released, for no other reason than a left field higher up decision.  When you play in the consumer products game, you are at the mercy of consumer product manufacturers.  Because of immense volumes, consumer device manufacturers have the buying power to heavily influence display manufacturers product lines.  It’s a two way street as well, the display manufacturer will look at trends and customer information, and might just decide that 7-inch display you just ordered in the exact resolution you wanted, isn’t the resolution their giant consumer products customer wants, so they cull the herd and replace it accordingly.  If you have the volumes to influence this kind of behavior, more power to you.  For most industrial and specialty applications, this is simply not the case.

Now as far as the display itself, avoid consumer grade products at all cost.  Consumer LCD’s are developed to be thinner, lighter, and cheaper.  Thinner and lighter can sound great, but don’t take the bait so fast.  Sometimes these displays can lead to engineering nightmares, quickly diminishing any profits you could have gained from choosing the lower cost display.  Some displays have thin plastic bezels, others are taped together.  This is not always indicated on drawings, and is often missed by misguided customers seeking clarity in all things LCD.

Industrial LCD Displays

 

Bottom line, unless you’re knee deep in the consumer products market, avoid consumer grade displays.  Industrial applications require industrial components.  In some cases, we are able to find a better performing industrial LCD for a lower cost than the consumer grade LCD the customer originally planned on.  Here at Touch International, we can source and enhance almost any size or brand panel.  Our experience has given us preferred brands, competitive pricing, and we certainly have our sweet spots like anyone else.  We only source A-Grade panels manufacturer direct or from qualified distributors.   If you want to design your product right the first time, you must define the right display.

What are the differences (if any) in anti-reflective, anti-fingerprint, anti-smudge, and anti-glare?? Every piece of information I find ends up as a sales pitch. Can you clarify things for me?

Touch guy thought that the new fad of taking and sharing selfies meant everyone wanted to see themselves (a lot), so he wonders why there are so many requests for anti-reflective/anti-fingerprint films (AR/AF) on his products. He is so out-of-touch (haha). Ok, enough cracking myself up.

What customers want when they ask for AR coatings is to reduce the ability to see oneself when looking at the display, especially when you are outside. Ultimately, AR coatings make the display easier to see.

In legacy touch products, glare reduction was done using anti-glare (AG) coatings. AG treatments are made by either coating the touch surface with silica “bumps” or lightly etching the glass. The result is that some of the light hitting the touch surface is diffused or scattered, and not reflected back at the user. This was good enough for 25 years of touch technology.

Then along comes Apple and their beautiful high resolution displays to upset the cart. To preserve that super display image, polished glass was used on the iPhone instead of the traditional AG treatment. It is kind of OK for mobile devices because you can tilt the surface to reduce reflections, but less easy to “fix” on bigger devices such as pads.

So the market is asking for something that reduces reflections but preserves that pretty little picture on the display. This has traditionally been done using anti-reflection treatments. Reflection reduction can be achieved in a number of ways, including moth-eye treatments, circular polarizers, eliminating the reflective surface, or that which I will talk about, multi-layer thin film interference coatings.

AR thin film treatments work by “trapping” the light. OK, English majors, stay up with me here….if you took a physics class you know that most energy can be represented by a wave of peaks and valleys (sine pattern). In your physics class you probably also played with a 3 by 5 card with slits, or (as touch guy did) a wave generator made of Popsicle sticks which showed that light can “cancel” itself. Angstroms wide thin films, made up of multiple layers of transparent materials, are put down (on the touch surface) to a precise thickness relative to a visible wave length (550nm) so that light (from your smiling face) passes into the thin film, reflects on the touch surface, and then is reflected back on itself by the thin film, and, as if by magic, the reflection is gone (canceled). Another benefit of AR coatings is that because the light is not reflected, much more light from the display passes through the AR coating so the display is actually brighter. (OK, physics majors, I know this is not a compete explanation, but I might have lost most readers by now)

Touch Guy finds that customers want less than 1% of the light (your pretty face) to be reflected, and that is pretty commonly (though still not cheaply) achieved. If you have enough money (and don’t mind parting with a chunk of it), you can get an AR stack to have less than 0.1% reflection. It used to be that only a few companies could make production quantities of AR coated material but the demand for this stuff has dramatically reduced the cost and increased the number of suppliers, both on plastic and glass.

There are two problems with AR coatings on touch screens. The finger print problem has been fixed with anti-finger print (AF) coatings. Recalling the description above, you know that the thin film coating must be the precise thickness of a light wave (say ¼ of the width) to work. Along comes your dirty, oily finger tip, laden with hand cream, to touch the screen. Boom, the oil you left on the AR coating has changed the thickness of the coating which reduces the ability to trap the light and most often shows up as a finger print. AF coatings work by resisting the ability of the oil on your finger to “stick” to the AR coating; these are called hydrophobic (fear of water) coatings. Of course, for the AR coating to work, the AF coating must be built into the precise thickness of the thin-film stack. Touch Guy is not impressed with the after market spray-on AF coatings over AR stacks. Not impressed at all.

AR problem number two is that it wears off…think about it, angstroms thick material in an abrasive and chemically active environment, and you have limited time before its anti-reflection properties go away, especially on frequently touched areas. There seem to be two solutions…the first is that the AF coating on the AR stack is made of (secret) tough material that will wear-off your finger tip (just kidding) before it gives up. The other is to use thicker, but less effective (2% reflection) organic AR coatings, that seem to have better resistance to finger wear.

Another way around the AR wear problem is to eliminate the reflective surfaces in the first place. Touch International is a big supplier of optically bonded (touch panel to display) products which increased the ruggedness of the display while eliminating the internal reflections. An AG coating and an optically bonded p-cap touch panel eliminates the wear, and anti-finger print issues, with only a minor loss in display sharpness. And the new p-cap+ product has a secret method of AR reduction.

Until next time,
Touch Guy