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Circuit board PCIe BGA fan-out and escape routing

Circuit board PCIe BGA fan-out and escape routing

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  • Time of issue:2020-06-24 16:39
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(Summary description)Since all chips are present on PCI or PCIe cards, the placement 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.

Circuit board PCIe BGA fan-out and escape routing

(Summary description)Since all chips are present on PCI or PCIe cards, the placement 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.

  • Categories:Company News
  • Author:
  • Origin:
  • Time of issue:2020-06-24 16:39
  • Views:
Information

Since all chips are present on PCI or PCIe cards, the placement 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 tips for fan-out and escape routing.
PCIe BGA fan-out
Like most components with BGA, there is no golden rule about 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 the fan-out strategy.
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 common choices. Regardless of the number of layers, the total thickness of the card is limited to 1.57 mm. For four-layer circuit boards, since there are two internal copper layers, the wiring space will be limited to two layers .
With a very wide pitch BGA, you may be able to lead it directly out of the package without placing vias on the signal line. PCIe routing guidelines specify symmetric routing even under 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 bending of the two traces on the differential pair as closely as possible. It is best to route differential pairs between pads, rather than placing pads between a pair of traces.
The dog-bone fan-out strategy is suitable for coarse to medium pitch BGAs, but the trick is to keep the trace under the package. Considering the limitation of the board thickness, this may be difficult because it limits the number of available layers. 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, without vias) compared to a typical dog bone 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 wiring through the copper layer, make sure to include the appropriate pad diameter.
For a very high pitch BGA, the number of pins is very high, you may have no choice but to choose a higher number of layers through HDI routing. Due to the number of cuts required for the connection, the fine BGA pitch may not support the typical fan-out strategy. You will use VIPPO through holes to access the inner layer of the circuit board, because the plating in VIPPO prevents the solder core from sucking to the back of the circuit board.
BGA circuit board
BGA's typical dog bones fan out
Escaped route
Once your trace escapes from the BGA, what happens next depends on the device to be installed on the BGA. Although formal PCIe layout and routing specifications define the maximum allowable trace lengths, differential impedance values, 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 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 using different components. These requirements are often more stringent than the limits provided in the official PCIe standard, regardless of age. Therefore, always check the data sheet of the component before you start designing the layout and wiring.
BGA circuit board
To keep the impedance differential traces consistent and within the required tolerances, PCB design software with controlled impedance design and routing capabilities can be used. This allows automatic routing or interactive routing to automatically set trace spacing and geometry when you route. Make sure to observe the "5W" rule for the spacing between differential pairs, and mirror any deviation of one of the adjacent traces to ensure symmetry. In addition, make sure to define a tolerance for length mismatch according to the selected signal transmission standard.
Today's device speeds require designers to define differential trace geometries with consistent characteristic impedance as design rules that conform to the PCIe standard. Working with rule-driven PCB design software can greatly simplify layout and routing, making it easier to design circuit boards based on PCIe specifications.

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