Attached is a guidelines presentation that is appropriate for 2 layer boards. It relies on transmission line techniques to prevent noise problems – emissions and susceptibility. The transmission line techniques are meant for emissions, but the concepts aid susceptibility as well since noise voltages and currents are generally kept under control.
The length of traces versus the wavelength of the signal transitions is not an easy subject to grasp. A ¼ wavelength antenna is an efficient antenna for propagation and reception. Traces of any length greater than about 1/10 of a wavelength that do not have controlled impedances will present discontinuities that can cause system supply noise and crosstalk. One basic element for high speed design is to not split ground planes, or more properly, to not cross a split in a return plane. Two layer boards do not have effective ground planes, and the return paths are difficult to find. It’s the return paths that cause most of the problems in EMC. The guidelines document shows how we have built 2 layer designs that excelled in harsh noise environments. One of the basic points was to keep each signal adjacent to a return path all along its length. This meant stitching top and bottom grounds to direct the energy where we wanted it to go.
Your 2 layer board may work fine in normal operation. My concern is the transient cases, whether ESD, line transients, or self-generated noise.
Also see: Daniel Beeker has a great training “Effective Printed Circuit Board Design” here.
