I am working on a PCB layout on a very space constrained, 4-layer board. On the board is a MIPI-output image sensor (1 lane) which go to an FPGA.
- Layer 1: Image sensor
- Layer 2: Full Gnd plane
- Layer 3: Some signals, some 1.2v plane
- Layer 4: FPGA
Since the board is very small, the differential pairs are only about 10mm (40 mils) long. At this length, do I still need to be very careful about their layout?
Here's what I have now:
The FPGA is at the top of the image, on the blue (bottom) layer. The Image sensor is at the bottom of the image, shown in grey (top layer).
And here we can see Layer 3 under the differential pairs.
As you can see, this plane doesn't fully cover the differential pairs. And when the pairs change layers, and thus change reference planes, the nearest capacitor, to help move the return current between planes, isn't that close (red circles).
One last point, there is clearly a change in impedance as the tracks change from 0.075mm (3 mils) to 0.12mm (5 mils).
The question is, does all of this matter when the pairs are so short?