Signal Integrity TUTORIAL





Need of Multilayer Board


The first basic thing a Multilayer board does is that it provides more routing space. Experiences PCB designers know how easy is it to drop down a via for power and ground nets. It would not have been easy without defining Power and Ground planes.

If a board is not very dense and does not has BGA ICs, you can probably do with a 4 Layer PCB. If it has a BGA of pitch greater than or equal to 0.8 mm pitch you can probably do it with 6 layers, assuming that the power nets are not many and BGA is not very big. For BGAs of pitch 0.65 mm or lower and many power nets 8 layers or more may be required.

This is just a general guidance. We would like to minimize the number of layers to keep the cost of PCB down. However, the multilayer PCB fulfills some important High Speed PCB design requirements. We must ensure that these requirements are fulfilled.

Power Plane for Impedance Requirements


SOme high Speed Signals need their characteristic impedance to have a certain value, like 50 ohms. For the characteristic impedance to have a fixed value like 50 Ohms , it needs to refer to a metallic place ( power or ground ) at a certain distance from it. The value of the characteristic impedance depends upon the distance of the power plane from the trace ( besides trace width and relative permittivity). We must place the power plane beneath the signal requiring their characteristic impedance to be have a fixed value.

Power Plane for Power Supply Requirement


The second important function of a power plane is that it provides a return path for the power signals. Without Power planes the power signals would have to travel to the thin traces. The traces have inductance, because of which fast returning current may induce noise in the power system.

Power Plane and Stitching Capacitors


It is often happens that we have many power islands in a single power plane. In this scenario we must be careful while routing the controlled impedance traces which pass accross the power islands junctions. We must place the stiching capacitors at the juncions of the power islands.The stitching capacitors provide the AC return path and keeps the impedence constant during the flight of the controlled impedance nets.

A typical 4 Layer stack up


A typical 4 Layer Stackup looks as follows

Signal 1
Ground
Power
Signal 2

Notice that the Ground and Power are interchangeable. If it this PCB has many power islands AND there are controlled impedance traces, then it may be a better idea to keep all the controlled impedance traces on Signal 1 and Ground on Layer 2. If controlled impedance traces refer the power planes it must not either crtoss the split power plane junctions. If it is unavoidable to pass the split power plane junctions, we should place stitching capacitors.

A typical 6 Layer stack up


A 4 layer stackup is pretty much straight forward. A 6 layer stack up can have a number of variations. We should know the reasons behind choice of these variations and select the one that suites our requirements.

A typical 6 Layer Stackup looks as follows

Signal 1
Ground
Signal 2
Signal 3
Power
Signal 4

In this stack up the separation between signal1 and signal2 will be as large as possible to avoid any potentila cross talk. Care must be taken not to route traces in parallel along signal 2 and signal 3.

All 4 signalling layers can refer to at least on power plane. So all the four signalling layers can have controlled impedance traces.

One practical problem with this stack up is the lack of the power planes. A typical microprocessor based system will need many small power islands, for which a single power plane is not sufficient. In this scenario some of the power islands will have to be on signal2 and/or Power plane. If you find the things hard you may use 8 Layer stack up.

A typical 8 Layer stack up


A 4 layer stackup is pretty much straight forward. A 6 layer stack up can have a number of variations. We should know the reasons behind choice of these variations and select the one that suites our requirements.

A typical 8 Layer Stackup looks as follows

Signal 1
Ground
Signal 2
Power
Ground
Signal 3
Power
Signal 4

The 8 Layer stack up provides ample routing space for multiple power islands. In the above typical stack up all signalling layers have at least one referencing power plane. The only issue with is power plane is that the space for routing signalling routes may be too small.There are two ground layers while only one may be sufficient. If the routing space is enough this stack up is good, otherwise you may explore alternative stack ups.