Fanout

 As circuits get more complex, designers try to make better use of the available space. The introduction of SMD components certainly helped on this matter.

Through-hole components had the advantage of presenting accessible pins from both sides of the PCB. Some SMD components, though versatile in functionality, do not offer this feature, which means that accessing the pins requires wiring and/or the use of vias. These types of vias are called escape vias, and the tracks leading to them (also called clines in Allegro PCB Editor) are known as escape wires. The pattern generated, using escape wires and vias, is referred to as a fanout pattern. Fanout is the process of creating dispersion vias for SMD devices on Printed Circuit Boards.

There are several types of fanout. The first one to mention is called Quadrant Dog Bone style, and it gets its name because of how it looks when implemented:

Here, the vias are placed diagonally to the pads, allowing the device to be partitioned into four quadrants. It is a very cost-effective technique, especially when paired with through-hole vias. Its main disadvantage is the disruption of the ground plane due to the vias.

The second type of fanout is the Pushing Perimeter one. It is similar to the Quadrant Dog Bone fanout, except that the vias for the two external rows/columns are extended away from the component (around its perimeter). The result is the creation of more channels for routing, which facilitates the placement of other components (like decoupling capacitors or termination resistors) near the IC. It looks as follows:

The next type of fanout is the Via in Pad. Here a through-hole via can be placed at the center of the pad. This fanout style effectively makes the SMD component a through-hole-alike. However, the soldering technique required to implement this type of fanout is different and more complicated, increasing its cost as compared to the previous two fanouts.

A microvia (or a blind via) can also be used in place of a through-hole via, but it will further increase the cost of the PCB and will narrow down the available manufacturer options. However, it will allow for the placement of decoupling capacitors and termination resistors on the other side of the PCB.

In PCB Editor, you can create a fanout using Route > Create Fanout. The command creates clines and vias and connects these to the chosen pins or symbols. To ensure the vias extend to the appropriate routing layer, use this command after placing the component but prior to routing.

In the Options panel, you will find settings to customize the style of the fanout:

In the previous image, the style is set to Quadrant Dog Bone, but if you click on it, you will see all available choices:

Nowadays, the use of BGA devices has been popularized. Most ICs will have a packaging option matching this type. The consequence is favorable for designers since it means it does not have a significant impact on the cost of the PCB. However, as the number of pins increases and their pitch decreases, it becomes more difficult to route the escape traces more effectively. An effective breakout can minimize the number of layers required. An appropriate stackup and via model pattern can make a significant impact on routing the board.

You may refer to our Knowledge Website for additional information and a step-by-step guide on fanout-related commands offered by the PCB Editor.

Team PCBTech

Cadence Design Systems