[SOLVED] A question on capacitive touchscreen and LCD assemblies in consumer electronics

[ernpao]

Hi, I've been looking at some of the product teardowns of smartphones at ifixit and noticed that the touch screen components (LCD/Digitizer/Protective glass) are usually a single assembly in products, and are connected to the logic boards/pcbs via flex circuit connectors.

My first question is, how are the these LCD assemblies designed? If I wanted to design a product with a capacitive touch screen, LCD, and protective glass, how should I go about it? (The touchscreen assembly is what baffles me the most so far).

It would be great to hear from someone who has professional experience with LCD screens/consumer electronics. Thanks.

 

[ArticCynda]

I worked on a home automation project a few years back that required custom LCD displays. As designer you can lay out the segment topology and then send it in to a specialized manufacturer. Flex interfaces are default these days, but glass ribbons and other options are also available on demand. Of course, the more custom you want, the more expensive it gets.

If I'd redo the project, I'd opt for an off-the-shelf assembly which would have comparable results at a much lower cost.

 

[ernpao]

Thanks for the response ArticCynda. If you don't mind I'd like to ask you a couple of questions :grin:
Let's say I found some TFT screens I'd want to use and on the site, the manufacturer says they can customize the FPC connector depending on our requirements. Is it simply a matter of telling the manufacturer where your FPC connector is going to be on your main PCB and the position of the screen with respect to the PCB? What CAD or design files do manufacturers usually ask for if you want something custom?

What about capacitive touchscreens? I've seen a lot of off-the-shelf TFT modules with resistive touch screens, but I'd rather use a custom capacitive touch screen. When approaching a manufacturer of custom touch screens, are there any specific CAD files they look for? Or just mechanical drawings of your requirements?

Heck, all I have are rough sketches and doodles of what I want and maybe my main problem is I don't know how to translate those sketches into technical documents the manufacturers can use :lol:

 

[ArticCynda]

Everything depends on the manufacturer you're working with. Keep in mind that, whenever you're going towards custom designs, you'll immediately facing an initial set up cost ranging from several thousands to tens of thousands of dollars. So don't think of it as sending in Gerbers and NC Drill files to a PCB fab to get your boards manufactured, for this kind of work you'll typically meet up with the engineering team of the manufacturer and discuss your project requirements face to face with them.

What they typically require is a box model of the assembly, such as STEP or STL, which constrains the size of the assembly. There are no limits to the form factor and length of an FPC connector, but the larger the flex part is the more expensive the manufacturing process will be. Therefor it's usually better to optimize the PCB to keep the flex FPC connection as short as possible (added advantage: mechanical robustness) and supply the manufacturer with a complete 3D model of your product so they can design and fit the assembly as good as possible. Of course, you'll always have to provide a complete set of mechanical drawings (autocad and DXF are de facto industry standard) as reference.

Once again, however, keep in mind that you'll be facing a considerable NRE cost when choosing custom display technology, so it might be a good idea to spend some more time on re-engineering your product so that a standard assembly can be used.

 

[ravindragudi]

TFT LCD module with capacitive touch screens are available in market. These ready modules will speed up the development and also will be cost effective for lower number units production or development.

example - https://au.mouser.com/new/newhavendisplay/newhavencaptoTFT/

 

[ernpao]

Thanks for the info guys. And yeah, I thought it was like how you had to send Gerber files to a PCB fab :-D Okay that clears things up a bit. Using an existing assembly will reduce costs, but unfortunately I need a capacitive touch display that measures about 2" diagonally. Most capacitive touch displays I've found are for smartphones and tablets usually (like in the link ravindragudi posted).

 

[ArticCynda]

You don't necessarily have to use the entire surface of an off the shelf part, i.e. to make a circular display most designers pick a rectangular one and hide the edges behind a circular cut out in the front panel.

Perhaps the recent "smart" watch hype will flood the market with 2" touch displays?

 

[ernpao]

Yup, I understand. But I do want to use the whole display area. Just saying that most of what i have found so far use resistive touch and not capacitive. I agree with the smart watch statement and I've seen a lot of android powered watches on alibaba while looking for a display to use for prototyping :grin: