As market demand for smaller electronic devices with greater functionality continues to increase, PCB designers are turning more and more to ball grid arrays (BGAs)1. Using BGAs allows PCBs with greater functionality to be used in progressively smaller products. BGAs are intended for multilayer PCBs where vias are used to route signals between layers.
Vias add flexibility to the PCB design process and complexity to the PCB fabrication and assembly process. Just as there are tolerance considerations, placement restrictions and best design practices for components and traces, so too for multilayer routing of BGA connections and using vias. By applying BGA routing best practices, you can ensure that your boards are less susceptible to common challenges that can impact manufacturability and functionality.
BGA Routing Challenges
BGA routing presents unique challenges that must be considered as you design your PCB. The most significant of these involve ensuring the signal integrity, manufacturability and reliability of your board.
• Signal Integrity. A key signal integrity issue is impedance control, which affects transmission and reflections. The impedance can be controlled by matching trace widths between forward and return lines, using the appropriate dielectric thickness and installing a reference plane between signal layers. Installing reference or ground planes between adjacent signal traces is also effective in minimizing crosstalk. When the signals are on the same layer, maintaining the spacing between traces is key.
• Manufacturability. In addition to determining the number of layers (signal and reference) to optimize signal integrity, you need to consider the manufacturability of the BGA routing scheme. There are specific restrictions and guidelines depending on how the BGA connections are routed with each other and externally. The number of layers, pitch angles, via sizes, type of hole drilling and other concerns are limited by the capabilities of the manufacturer.
• Reliability. The reliability of your PCB depends on board construction quality and synchronization of your design with the capabilities of your manufacturer by employing good DFM practices. Not synchronizing or pushing the limits too far can result in boards that don’t function as designed or may fail at some point once deployed in the field. Copper weight, solder mask thickness, copper grain structure and ionic contamination are material issues that cause failure in the field. Trace routing, including via choices, may also affect operation, leading to significant costs for recalls and replacements.
BGA Via Options
The first step when designing your BGA routing scheme is to determine the number of signal layers required based upon your signal integrity considerations. A good rule of thumb is to use the following equation:
Remember to include reference planes when needed. With the fast turnkey process of companies like Tempo Automation, you can have your PCB up to 30 layers thick in a matter of days.
The next step is to determine how to route your BGA signals. A number of options are available to you, but before settling on a strategy, you should consider how each may impact your PCB design:
Through Hole Via
The most conventional method of connecting through vias is through hole routing, which extends through the entire board. Through-hole vias provide good signal integrity and are easy to implement, but they require the most space.
For routing directly from the BGA, blind vias are the alternative to through-hole. They offer the advantage of being able to incorporate stacked vias, staggered vias or a combination of these. Stacked vias allow for a denser design; however, staggered vias do not require the precise alignment that stacked vias do. From a fabrication standpoint, the relaxed tolerances make it simpler to implement staggered vias.
Finally, you need to determine whether to use via-in-pad routing or dogbone routing. There are tradeoffs in complexity and space that must be considered:
The space requirements of through-hole vias can be minimized by using via-in-pad routing. For through-hole vias, the footprint is small and routing is easy, but the design is complex. Blind vias, which improve signal integrity, present the tradeoff of being difficult to route. The steps necessary to implement standard via-in-pad add significant time to the PCB fabrication and assembly process. This can double or even triple the manufacturing turnaround time.
When using via-in-pad routing, minimum via and pad sizes are used to save space. Due to the precision needed, this may cause breakout. Breakout is when the via extends outside of the pad. This may be a significant issue if the breakout occurs in the direction of an incoming trace to the pad. In those cases, signal integrity loss may occur. An alternative to avoid this is dogbone routing.
In this style of routing, the via is offset from the pad and connected by a short trace, thus making a “dogbone” like shape. Dogbone routing does not need the fill and cap process steps needed for via-in-pad routing so it is less complex and faster to manufacture as opposed to via-in-pad. If the BGA pitch spacing allows it then this style of routing may be a better option than via-in-pad. However, designs can be complex and it can sometimes be unclear which style of via is appropriate. This is where our team can help.
Your choice for BGA routing may significantly affect the signal integrity and manufacturability of your PCB design. Additionally, your selection may add days to your board’s turnaround time. By implementing BGA routing guidelines and best practices, you can ensure the manufacturability of your board and reduce build time for your PCB.
We can help you incorporate these BGA routing best practices into your design. To get started, send us your PCB information using our quote tool. Or if you would like to speak with one of our experts, please contact us for more information on BGA routing or other topics of interest.