Alps Electric Touchpad Internals

Another day, another salvage. Someone smashed apart a laptop and left the touchpad sitting out in the open – why not take a peek at it and see what it’s made of?

Well, the first thing to realize is that it’s pretty much a thin PCB with a piece of durable plastic glued to it. After removing this opaque plastic (which is what the user drags his/her finger over), and then rubbing off all of the adhesive – this is what you get:

A very interesting pattern indeed. Despite rubbing off all of the adhesive, there is an overcoat of a thin, almost sticky-tape layer, which might be a dielectric layer for ensuring no charge accumulation and particular performance characteristics.

Unfortunately, I’m no expert, but the golden fingers are likely to be connected to the ground plane and isolates each column from each other. The thin wire in the middle is probably a sense wire for each column which is connected to the vias at the bottom which are connected individually to the chip on the rear. The interesting thing is the “alternating” isolated pads that are in pairs, closer to sense wire, further from sense wire. This might have to do with the horizontal row-co-ordinate sense.

So how does this touchpad sense what row your finger is at? Well, a good inspection with bright light reveals that this is a multi-layer board, with the layer just underneath appearing to be responsible for the row-sensing. Now, with a bit of Photoshop curves abuse, I’ve tried to emphasize this in the image below:

You see those ends which almost look like a serpentine arrangement crossing horizontally behind the vertical column sense areas? Well those look like a horizontal arrangement with the vias on the right being responsible for connecting the sense wires for the rows to the chip on the rear.

Pretty cool …

A scan of the rear is a bit fuzzy, owing to the tiny depth-of-field on Canon LiDE scanners. Anyhow, it’s clear, there’s a chip which is individually connected to each row and column sense line. There’s a crystal, and a flat-flex connector for hooking the ribbon to the motherboard.

So there you go … that’s what the insides of a touchpad look like.