Motherboard Design Process - Page 4

..:: Design Simulations - Tuning ::..

Yet another important topic that must be addressed by designers is the tuning of the high speed interconnect traces, such as the front side bus connections running from the CPU socket pins to the MCH pins. If you take a look at either the top or bottom layer of a motherboard, you’ll notice that many of the traces do not follow a straight line path, rather they seem to snake around. Some traces do go in a straight line, while a trace next to it might twist and turn. This is referred to as “serpentining” for obvious reasons.

Why is serpentining used? This process is used in order to meet the target lengths for traces that are determined during the simulation process. This is also used in order to make sure that when the signals travel down the traces, they will meet up at the device in unison. Each signal takes a finite amount of time to travel between the source and destination, called the propagation delay, and therefore, varying trace lengths are needed if one signal needs to arrive at the destination at the same time as another.

Since they deal with modern high speed busses that are ever increasing in transfer rates, the designers use something called source synchronous signaling for the clock. This means that the "driver" of the trace also sends clocks with the data signals; these clocks are typically called "strobes". This is another process used in order to properly time the signals so that they reach their end destination when needed so as to not force further delays. This does not mean, however, that all signals must arrive at the destination at the same time. There may actually be a need for a signal to be offset. The designer may also have to deal with other propagation delays of something called differential strobes, and non-differential strobes. Once again, these topics are well beyond the scope of this article.

Once all of the necessary simulations have been run, and all of the manufacturing parameters have been given to them, the designers then run numerous routing studies for the various components that need to be placed on the PCB using all of the tricks that we have just covered. This task it undertaken by the CAD designers who create the layout and make sure that the motherboard can have the needed devices without experiencing any routing problems. The major goal of this part of the process is to see how closely the various components on the motherboard can be placed together. The tuning requirements that are given by the simulations dictate the placement of the traces, as well as attempting to make several areas of the board as compact as possible to leave room for additional onboard features such as audio.

When circumstances arise that a trace may need to transverse through multiple layers of the PCB, we come across something called a “via.” These are used in order to pass the trace from one layer to another when needed. When you’re talking about single or double layer PCB’s, these vias will actually penetrate the entire board. If you have a sound card, or something or the sort lying around, simply hold it up to a light and you might be able to spot a few of these vias that pass through the entire board. When you’re working with multiple layer PCB’s however, you may not want the vias to completely pass through the PCB, rather only go a certain amount of layers in. These vias are termed “buried” or “blind” vias. You can easily see several blind vias on the surfaces of the motherboard as these vias pass from the surface layer inward. A buried via cannot be seen as it is used to connect the inner layers of the PCB.