PCB Tech - Circuit board PCIe BGA fan-out and escape routing

Since all the chips appear on the circuit board PCI or PCIe card, the layout and routing of these circuit boards seems very complicated. However, PCIe's standardized architecture provides designers with considerable flexibility.

A somewhat complicated issue is the PCIe BGA fan-out of the components on these cards. The trick to implementing fan-out and escape routing strategies is to ensure that you comply with PCIe layout and routing specifications. With this in mind, let's delve into some techniques for fan-out and escape routing.

PCIe BGA fan out

Like most components with BGA, there is no golden rule for BGA fan-out. The correct choice depends on the spacing between the balls in the BGA. Component manufacturers may recommend different fan-out strategies for specific components, so it is best to check their data sheets before implementing fan-out strategies.

The actual escape routing strategy will depend in part on the layer stack. PCIe devices are mainly built on 4-layer circuit boards, although 6-layer circuit boards are also a common choice. Regardless of the number of layers, the total thickness of the card is limited to 1.57 mm. For a four-layer circuit board, since there are two internal copper layers, the wiring space will be limited to two layers.

Using a BGA with a very thick pitch, you may be able to directly lead it out of the package without placing vias on the signal line. The PCIe routing guidelines specify symmetric routing even under the BGA. When you route under the package between adjacent balls, you may need to place an elbow on the signal line to make the required connection. Try to mirror any bends in the two traces on the differential pair as closely as possible. It is better to route differential pairs between pads instead of placing pads between a pair of traces.

The dog bone fan-out strategy is suitable for BGAs with coarse to medium pitch, but the trick is to keep the traces under the package. Considering the board thickness limitation, this may be difficult because it limits the number of layers available. In fact, the requirement to route differential pairs between balls actually makes it easier to reach the first two rows in the BGA directly on the top signal layer (ie, no vias) compared to a typical dogbone fan-out strategy. Then, on the inner row, a dog bone fan-out structure with through holes can be used to reach another signal layer. When routing through the copper layer, be sure to include the appropriate pad diameter.

For the extremely high pitch BGA, the pin count is very high, you may have no choice but to select a higher number of layers through HDI routing. Due to the number of shears required to connect, the fine BGA pitch may not support the typical fan-out strategy. You will want to use VIPPO vias to access the inner layers of the circuit board, because the plating in VIPPO prevents solder wicking to the back of the circuit board.

BGA circuit board

BGA's typical dog bone fanout

Route after escape

Once your trace escapes from the BGA, what happens next depends on the device to be installed on the BGA. Although the formal PCIe layout and routing specifications define the maximum allowable trace length, the differential impedance value, and the maximum number of vias that may appear on the interconnect, your components may have different requirements. The routing specifications outside the BGA depend more on the components and signaling standards used, rather than just looking at the maximum allowable amount in the PCIe standard.

Due to the sensitive tolerances of the components themselves, the minimum, typical and maximum wiring requirements for these changes caused by the use of different components. These requirements are often stricter than the limits provided in the formal PCIe standard, regardless of their age. Therefore, you should always check the data sheet of the component before you start designing the layout and routing.

BGA circuit board

To keep the impedance differential traces consistent and within the required tolerance range, PCB design software with controlled impedance design and routing functions can be used. This allows automatic routing or interactive routing functions to automatically set trace spacing and geometry as you route. Make sure to follow the "5W" rule for the spacing between differential pairs and mirror any deviations in one of the adjacent traces to ensure symmetry. Also, make sure to define the tolerance for length mismatch according to the selected signal transmission standard.

Today's device speed requires designers to define differential trace geometry with consistent characteristic impedance as a design rule that conforms to the PCIe standard. Used together with rule-driven PCB design software can greatly simplify layout and routing, making it easier to design circuit boards according to PCIe specifications.