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BGA Layout Tutorial: What are the Guidelines for BGA Routing?

As of today, the standard when packaging different advanced multifunctional devices like microprocessors and FPGAs are BGAs (Ball Grid Arrays).

You can break BGA packages down into micro and standard BGAs. With the electronics technology of today the request for input/output availability poses some challenges, even for PCB designers that are experienced.

In this article, we will be giving you some guidelines on applying BGA routing, challenges of BGA routing and strategies to help overcome the challenges with BGA PCB design.

BGA Routing Guidelines

There is a continuous increase in the demand from consumers for the smaller electronic devices having great functionality, PCB designers continue to lean towards BGAs (Ball Grid Arrays). This is why we have decided to give some BGA routing guidelines to help you out.

Making use of BGAs will allow PCB designers the use of PCB boards having greater functionality in smaller products. Normally, BGAs should be used for multilayer printed circuit boards where vias are useful in the routing of signals between the copper layers. It seems difficult for BGA routing to happen in complex designs of PCB layout.

With vias, you are sure that the process of the PCB design will have some flexibility, and the PCB assembly and PCB fabrication process will have some complexity. Just the same way there are placement restrictions, tolerance considerations, as well as the best practices for traces and design, the same holds for BGA connections making use of vias.

When you apply the best practice for BGA routing, then you can be sure that the PCB boards will not be too susceptible to challenges, which could affect functionality and manufacturability.

What are the Challenges of BGA Routing?

BGA routing creates some challenges, which you must consider while designing the printed circuit board. Of all these, the most important has to do with ensuring the reliability, manufacturability, and to optimize signal integrity of the board.

Reliability

Your board’s reliability has to do with the quality of the board construction and how your design synchronizes with the PCB manufacturer’s capabilities by working with good design for manufacturability (DFM) practices.

Not pushing the limits or synchronizing the limits very far could result in a PCB board, which doesn’t function according to the design or at one point in time, may fail once it is deployed for use.

Thickness of the solder mask, structure of the copper grain, coverlay thickness, and copper weight are issues with the material, which causes the failure of the board in the field.

Furthermore, trace routing, which includes via choices, could also affect the operation, thereby leading to huge costs for replacement and recalls. Applying the DFM check at Rayming PCB, which integrates these considerations on any board, which includes Ball Grid Arrays, will align the design of the PCB layout with the capabilities of the board manufacturer.

Manufacturability

Asides from determining the available signal layers (reference and signal) to help in optimizing the signal integrity, then there is a need to consider the BGA scheme’s manufacturability. There are some specific guidelines and restrictions. This depends on the routing connections of the BGA externally, as well as with each other.

The via sizes, pitch angles, bga pitch spacing, hole drilling type, and available signal layers are limited by the PCB manufacturer’s capabilities.

Signal Integrity

 One major issue with signal integrity has to do with impedance control. This affects reflections and transmission. The control of the impedance is made possible by matching the trace width between the return and forward lines, making use of the appropriate dielectric thickness and also installing a plane of reference in-between the signal layers.

Furthermore, installing ground or reference planes between the adjacent signal traces is effective as well in reducing crosstalk. If the route signals remain on just one surface layer, then it is important to maintain the space between the traces.

Now let’s consider some Options that can Enhance PCB Manufacturability and Signal Integrity of the Board

BGA Via Options

When designing your scheme for BGA routing, the first step is to know the signal layers necessary based on the considerations of the signal integrity. Make sure to add the reference planes where required. With the design process of the quick-turn PCB, within a few days, you will have PCB layers with a thickness of 30 layers.

Furthermore, the next available step is determining the ways to route these BGA signals. There are some options available to you. However, before you settle on a particular strategy, make sure you take a look at how each of these will have an impact on the PCB design.

Now, let’s quickly consider what a through hole and blind vias is.

Through Hole Via

 Through hole routing is a conventional method to use when connecting the through vias. This extends through the whole board. For through hole vias, you will get a great signal integrity and implementing it is easy. However, one drawback of this is that they need much space compared to the others.

Blind Vias

When routing from the Ball Grid Array directly, blind vias offer you an alternative in contrast to through-hole. Blind vias offer the benefit of incorporating staggered vias, stacked vias, or combining these two.

For stacked vias, this permits a design that is denser. However, for staggered vias, there is no need for precise alignment, which stacked vias require. Considering the PCB fabrication, the tolerances ensure that the staggered vias are implemented in a simple way.

In the end, you have to determine if you should make use of the dogbone routing or via-in-pad routing. Here are the tradeoffs in space and complexity that you must consider.

Dogbone Routing

For this routing type, the via is neutralized from its pad and then connected via a short trace. This creates a shape that is similar to that of a dog bone. Furthermore, dogbone routing doesn’t have to be capped and filled as a process necessary for the via in pad routing, therefore pcb manufacturing it is faster and less complex in contrast to the via in pad.

Furthermore, there could be complex designs, and at times it may become unclear, which via style is appropriate. Now, this is an area where Rayming PCB’s design team can assist. The BGA routing choice you make affects the integrity of the signal significantly, as well as the turnaround time for board manufacturing.

When you implement the guidelines for BGA routing, you will make sure of the manufacturability of the PCB board, as well as reduce the time for manufacturing the printed circuit board.

Via in pad Routing

You can reduce the space requirements of the through-hole vias by making use of the via in pad routing. The footprint for through hole vias is small. It also features an easy routing; however, it has a complex PCB design.

Blind vias help to better or improve signal integrity but routing it is difficult. The steps that are important for implementing the standard via in pad adds a huge time to the assembly process and manufacturing of printed circuit boards. The turnaround time for manufacturing can even double or triple.

With via in pad routing, the minimum pad sizes and via hole are utilized in saving space. As a result of the required precision, this could result in a breakout. Breakout occurs when a via extends outside the pad. This issue may be a significant one if the breakout happens where a trace is coming from. For cases like this, there may be loss of signal integrity. To avoid this, an alternative is the dogbone routing, which we have discussed earlier.

Strategies to Overcome Challenges Related to BGA PCB Design

Make sure you start the PCB Layout Using a BGA. BGAs are usually a device’s main processor. Therefore, they may have to interface with different components present on the board. The common practice is placing the largest component of the BGA first and then utilize it in floorplanning your PCB layout.

Though, placing this component before any other thing isn’t a must, neither must the location be locked when placed. However, the largest BGA partially determines the fanout strategy and layer count that will be useful when routing in the component.

Define Appropriate Exit Routes

BGAs having a greater layer count will ensure that the planning of an exit route has to do with routing traces via many rows of pins. Furthermore, some traces may have high-speed signals, thereby requiring the appropriate spacing of the traces to prevent any crosstalk.

Also, other signals may be signals with slower configuration, which can be brought close together without excessive noise or reduced risk of crosstalk.

Power and Ground

A very easy and simple way of managing power connections into the BGA is using power rails. This can either be on two or a single plane layer. Placing power & ground on the adjacent layers with a thin dielectric separation also helps in maintaining the power integrity by offering a high capacitance for the interplane

Determine the PCB Layer Stack

You can use the BGA I/O count and pinout on a BGA in determining the layers required for your PCB stackup. Immediately the designer determines the trace width, then it needs to route the lines of the controlled impedance into the BGA. With this, you will be able to know the layer thickness and required layer count required to maintain the impedance.

Conclusion

Whatever BGA routing you choose will have a significant impact on the manufacturability of the PCB design and the signal integrity. Also, note that your selection will most likely delay the turnaround time of your board. Implementing BGA routing guidelines as well as the best BGA routing scheme and practices, you can be sure of the board’s manufacturability, as well as reducing the PCB’s build time.