How to repair my benq p50

First we need to know china phone keypad connected with PCB how way
(A) Direct on PCB
(B) Flex System (flex cable direct soldering pcb to keypad panel)
© Flex and socket system include (flex one side soldering with key panel other side connect to pcb with socket)
(D) only Socket System (here have 2 socket, 1 with key panel others with pcb)
Keypad problem have various type
1. All key not work.
2. some key work and some other not work.
3. sometimes work and sometimes not work.
1. All key not work
china phone all key not work for this type of problem
(A) though its touch screen phone, first try to disconnect touch panel line from phone pcb or display flex (4 line)
(B) though not solve and keypad direct on pcb first you try to good clean full pcb with cleaner.
© though not solve now you try to format/flash your phone because some time key problem come from software problem.
(D) though still not solve now you check must be CPU. 95% china phone keypad directly going in cpu. and other 5% first going to resistor or capacitor near the cpu or keypad than going to cpu. so now first check them than you can try to re-hot, re-soldering or changing cpu.
(E) though its flex or socket system, first check GND connection continuslay pcb to key panel. next check full flex or socket all line going continuously pcb to key panel or not. also sometimes flex or socket shorted with other line or gnd so check also them.
(F) Also you can check all key anyone is short 2 line.
(G) sometimes keypad line broken from cpu pin on pcb to key panel, so check them.
2. Some key work and some other not work
Some key work and some other is not work for this type of problem
(A) maximum time this problem we face for flex or socket system keypad. so check flex cable and socket all line any line is broken or not and though need change them. and check all key connection on key panel.
(B) though its direact on pcb, some of key line broken internaly. and maximum china phone have alternative test point near every key or with other key on keypad. so check them with multimeter and though need connect to not work key with near testpoint or other working key.
© common problem is keypad metal ball (which is short 2 line on key when we press) sometimes its damage for phone water damage or over key voltage, so clean them or change.
(D) also try to check line continuously from cpu to not working key.

3. sometimes work and sometimes not work
this problem come from this defect
(A) flex or socket system keypad any line is bad soldering or do lose contact. check them and
re-soldering or keep there line join with extra ware.
(B) for cpu bad soldering or cpu pin is broken. so re-hot or re-ball cpu.