understand the laptop keyboard for repair purposes

[m4rtin]

My Lenovo T60 laptop keyboard(FRU 39T7142) does not detect "a", "s", "d", "f", "j" and "k" keys any more. At first simply not every key press was detected, but gradually it became worse up to point where those six keys do not work at all. All the other keys work fine. It is definitely a hardware issue(tested with Linux Live CD) and it does not change anything if I push the switch directly. How are such keyboards built and what might cause such behavior? I mean do they have a controller inside the keyboard? I would say yes because while I could not find it, the amount of traces going into the ribbon cable(connected to laptop motherboard) is far less than amount of keys on keyboard. Maybe this keyboard controller is faulty?

[Shock]

http://en.wikipedia.org/wiki/Computer_keyboard

[monz]

On a laptop keyboard the controller is usually on the motherboard - no active circuitry on the keyboard itself. The reason there are less connections than keys is that the key switches are arranged in a matrix. Google keyboard matrix and also look at image results for a better understanding. The controller on the motherboard scans the matrix to see what key was pressed.

https://en.wikipedia.org/wiki/Keyboard_matrix_circuit

On a desktop keyboard it's the same, except the controller is inside the keyboard, providing ps/2, usb or other interface to the comouter.

Now, the reason some of your keys is not working is you have a broken connection in the keyboard.  A broken trace on it will affect several keys. If you break a wire in the matrix, you'll knock out several keys.  This often happens if a liquid was spilled inside and corrodes the trace.  These traces are often on flexible plastic, it's very difficult to try and fix these. Conductive ink pen would be best bet.

Luckily, you can usually find new replacement keyboards on eBay realitively inexpensively.

[electr_peter]

You have almost zero chance of repairing keyboard - all traces are out of reach printed on flexible plastic. Just buy a keyboard replacement if you care/can do that.
See this video for a reference:


By the way, I found that the best way of cleaning any PC/laptop keyboard is replacing it with a new one 

[German_EE]

Most of the early Lenovo machines have the same keyboard and keyboard connector, this makes replacement and test easier. It.s possible that reseating the keyboard connector will cure your problem so...........

1) Unplug the power lead and remove the battery

2) Turn the laptop over and look on the underside of the machine, you will see a series of screws that have small icons next to them, this is to tell you which screws to undo. Note that some screws are the same diameter but a different length to the others so it's best not to mix them up.

3) Remove the screws for the wrist rest / mousepad / fingerprint reader (if installed) and CAREFULLY unclip the wrist rest and lift it up. The mouse pad is attached with a ribbon cable that is easy to tear so pull off the connector carefully.

4) Remove the screws for the keyboard and, again, CAREFULLY lift it up until you can pull off the connector by the tab

5) Try reseating the connector and see if it cures your problem. If it doesn't then order a replacement keyboard and fit it in the reverse order to the steps above.

[m4rtin]

On a laptop keyboard the controller is usually on the motherboard - no active circuitry on the keyboard itself. The reason there are less connections than keys is that the key switches are arranged in a matrix.

Thanks for explaining this!

Now, the reason some of your keys is not working is you have a broken connection in the keyboard.  A broken trace on it will affect several keys. If you break a wire in the matrix, you'll knock out several keys.  This often happens if a liquid was spilled inside and corrodes the trace.  These traces are often on flexible plastic, it's very difficult to try and fix these. Conductive ink pen would be best bet.

In case of this keyboard, there is a single sheet of flexible plastic which holds the matrix. I was not able to find any corrosion or physical damage to traces..

You have almost zero chance of repairing keyboard - all traces are out of reach printed on flexible plastic.

It is also the case for this T60 keyboard. The plastic is basically impossible to reach without breaking the keyboard.