suggestions on my pcb design..

[TD]

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.
i can and should provide a solid ground plane. but the traces that
carry the power to the pads i cant make them more than 5 mils. how do i
overcome this coz there will be a voltage drop. the supply current
drawn is aroun 30 mA. please provide any suggestions..thanks !

 

[Robert Baer]

TD wrote:

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.
i can and should provide a solid ground plane. but the traces that
carry the power to the pads i cant make them more than 5 mils. how do i
overcome this coz there will be a voltage drop. the supply current
drawn is aroun 30 mA. please provide any suggestions..thanks !

1) The chip can be directly placed on the flex; an area around the chip

would be restrained from bending by some mounting method.
Better yet, buy the IC in standard package as the total cost may be
less than 100K minimum and up quantity requirements for the chip; the
package does the strain relief.
2) One could use 2-sided flex where ground and power "planes" are used
to help decrease inter-trace coupling and lower the impedance *and*
decrease the power IR drops.
I say "planes" in that wide strips would be used on the back side;
obviously the whole side would not be at one voltage level (ground,+5V,
etc).
They can be bypassed with chip capacitors mounted on the flex: +5 to
gnd; gnd to -5 (reading left to center to right).
I have seen this trick used before.
You mentioned IR drop; the above trick would help that and help
matching if there are signal reflection concerns.

Another way is to run the +sig on the top and the -sig on the bottom,
which keeps the differential E-field close to the 3-mil stripes and
severely reduces coupling to nearby stripes.
In addition, the nearby stripes could be (bypassed) power or ground
stripes; killing any coupling elsewhere.
That means a given power or ground is carried by multiple stripes,
thereby reducing the IR drop by a large fator.

You did not say how many hundred of feet long the flex would be.

 

[TD]

thanks for your reply. i also thought about going for double sided flex
printed circuit with a solid ground plane or like you said wide strips
for power/ground. but the traces leading into the pads on the top layer
are going to be run at 5 mils( i do not have an option here !)

1) The chip can be directly placed on the flex; an area around the chip

would be restrained from bending by some mounting method. Better yet,
buy the IC in standard package as the total cost may be less than 100K
minimum and up quantity requirements for the chip; the package does the
strain relief.

We have to use the silicon die because the board with die has to be
placed inside a box and hence its dimension has to be max 3mm x 10mm.
couple of companies i talked with were willing to do this as flex
printed circuit. now is the wirebonding to FPC and the stiffness of it
an issue or is it generally followed practice.

You did not say how many hundred of feet long the flex would be.

the signals are differential signals and will run max at 15-20MHz. And
I am trying to use a flex cable or a flex circuit that will be
1-2inches long !

 

[TD]

we decided it is going to be a double sided Flex printed circuit and
chip-on-a-flex with FR4 stiffner at chip location seems to good. and i
should have a ground/power plane at the bottom layer.