I need to design a miniature rigid pcb that will have a silcon die wirebonded to it and then take the signals out of this board using a flex cable to a 4in x 4in 4-layer rigid FR4 board (I have already designed this big board and laid out except for the signals coming in from the flex cable). at the bigger board end I intend to have a ZIF connector for the flex cable. There are 20 signals that the flex cable need to carry. My question is,
there are differential signals coming out from the smaller pcb, they must be routed together right ? i know they must be placed close to each other and maintain some distance from other signals. but the problem is i am using 3 mil traces and they are separated by 3 mils ( because of the restriction on the size of the board). so i can have other signals at 3 mils from the differential signals....how do i take care of this ?
i am thinking of using a flex cable to take out the signals. does this usually need to have all the signals on one layer ( I know there is rigid-flex board but i heard its very expensive )
what about the impedance mismatch, since i will be routing the signals at 10 mils on the bigger board...
i am having power and ground signals brought in through the flex cable. now the small board can that be made a 4-layer board with inner layers for power/ ground. but still the traces that carry the power to the pads i cant make them more than 5 mils...i am trying to check all these with the pcb manufacturer as well....but thought will get some ideas from people here...i am still learning these things....