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How is TI’s new MulTI-Touch Projected Capacitive Technology able to work with water spray?- Tyler Womack, Wapakoneta, Ohio

So Tyler,

I guess you know water can be a lot of trouble for touch screens. The most vulnerable is SAW which stops dead in its tracks, to the least vulnerable, resistive touch. Resistive can be sealed so that water does not get in between the PET and glass layers which allows for better performance with water.

The ultimate irony is that most touch screens work because humans are just big bags of salt water and the water in our bodies serve as the primary activating mechanism. The problem is that the touch screen must distinguish between you, the big salt water bag, and the salt water puddle. (There is one rare form of an acoustic touch system than operates in a fully submersible ocean environment, but Touch Guy will ignore that one for now because it is made by a competitor).

So Touch Guy, still the SUPER-CHAMPION of projected capacitive needs to come clean on water and multi-touch. And if you follow Touch Guy, you know his answer to most questions is, “it depends.” In this case, it depends a whole lot on which sensing IC (Integrated Circuit/Chip) you use. Most ICs will invariably yell, “I quit!”, when it gets a lot of water on it – check out your iPhone sometime. The best IC could be saying, “I love water!”, and works really well. The water-loving IC will even work covered in distilled water, but as the salinity increases (as would be the case on the ocean) a salt spray is as far as you can take it. So your projected capacitive fish finder will work on the local lake in a light rain, but bow-busting 40 foot ocean waves are probably a little much.

Still need to survive the salt water wave and keep on multi-touching? Then the Resistive MARS technology is the one for you. Silicone-sealed behind a water tight bezel, this touch screen can take that salt water puddle and keep on touching. Add sunlight readable filters to the MARS sensor and you are ready for a jaunt around the continent on the bridge of your new yacht!

Until next time,
Touch Guy

Is projected capacitive technology reliable when used outdoors in a marine environment where it will be exposed to rain, saltwater spray, bright sunlight, etc? What are the likely points of failure? – Charlie, Marblehead, Massachusetts

Hey Charlie:

This is a most appropriate question for national sock burning day because, as is the case with all good sailors, Touch Guy barbequed his socks and pledged to go shoe-to-skin all the way to the last blustery days of Fall.

Ah, but back to your question…Until recently, resistive touch was the only technology that was appropriate for marine applications. It could be fully sealed so that salt water was not able to corrode the protective hard coat, trigger a false touch or become impaired by a layer of salt.

The big problem with resistive touch is that the internal reflections make the display difficult to view in bright sunlight. Typically this problem is overcome by using a very bright backlight, a sun shroud or an expensive sunlight readable filter.

Projected capacitive technology is now the best outdoor solution because it does not have internal reflections and when optically bonded to the display, provides optimal readability. Some projected capacitive electronics are confused by salt water on the sensor but others are totally unaffected with by it – Touch Guy TouchGuy_Basiccan help with this critical ASIC (Application-specific Integrated Circuit) decision. And, if your marine vessel happens to be an aircraft carrier, it is easy to incorporate EMI and night vision filters into the touch sensor.

Other than lowering the boom on the sensor, there are no points of failure, although cold weather sailors will find that projected capacitive technology will not work with thick gloves. It is also important to note that you will need to seal and add a conformal coat to the circuit board to prevent corrosion.

So Charlie, let’s be sock-less ‘til November and get that projected capacitive touch screen on deck!

Until next time,
Touch Guy

What adhesives do you recommend for bonding the touch screen to the LCD? – Scott

Dear Scott:

Touch Guy could simply say to use any VHB adhesive (VHB stands for very high bond), most commonly 3M 4956 mm, but it is not his style to be anything less than verbose. Touch Guy is usually not negative either, but he needs to be this time. There are two big no-no’s… neoprene and common RTV Silicone. This is because both of them are acid producing (sulfuric acid from neoprene and glacial acetic acid [vinegar] from the RTV). Those acid molecules will jump out and eat away at your electronics and your touch screen thin films. You can use a water based RTV silicone, but the cure time is much longer. And, while you are at it, resist the temptation to make a watertight seal between your plastic enclosure and your resistive touch top screen. This is because plastic (usually PMMA) excursion under temperature is usually more than the touch surface, which may leave you with a puffy topsheet.

Where was I… oh, back to that VHB that you asked about. You should use 1.6 mil (mm) thick adhesive, because even the most noise resistant touch technology (resistive) benefits from being away from those busy-beaver transistors in the display and it will reduce “puddling” (as is the case with “puffy”, a very technical term) from heavy touchers. Most companies provide strips of VHB, but many of Touch International’s touch screens come with a continuous die-cut gasket which gives you an elegant dust and water resistant seal.

The VHB has a shelf-life and will begin to activate (grow stronger) over time and pressure, so don’t use old stuff. Once aligned to the LCD, you will have a few seconds to easily pull off the touch panel and reposition it if you make a mistake. Most important of all, you really need to do this in a clean-room because the smallest dust particle in the gap will be grounds for your customer to send product back.

Don’t forget, Touch Guy can optically bond your touch panel for better optics and more rugged performance.

So, Scott, stick away!!!

Until next time,
Touch Guy