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RDL Routing, the Beginning of High Speed PCB Design

High resolution blue photo of bacteria, some of the smallest agents known


HG Wells classic story; “The War of the Worlds,” had an incredible impact on the sci-fi genre and storytelling in general. Not only has it been remade into several different movies and television shows, but the concept of alien invasion introduced to us in the book have been reimagined in countless stories since.

But for all of the excitement and heart-wrenching aspects of the original story, the theme that captures me the most is how the mighty Martians were brought down by the smallest of agents; the bacteria on the earth which they had no immunity to. As HG Wells penned; “…slain, after all man’s devices had failed, by the humblest things that God, in his wisdom, has put upon this earth.

There are often small and unseen devices and agents at work in our lives that we are completely unaware of. The smallest of water leaks can create a huge amount of dry rot damage, while a dash of salt can completely change the taste of your favorite meal. And in the components that we use on our printed circuit boards, there are many small details that we might not be aware of as well.

One of these is the redistribution layer (RDL) that conveys the signals from the die of the component out to the pins that solder the part to the board. This RDL routing is a whole new (and unseen) world that many PCB designers may not be aware of.

What is RDL Routing?

When flip chips were first introduced, the concept was pretty simple and ingenious. Normally the die in a component would have its bond pads on the top, and then wire bonds would internally connect those die pads down to the bottom of the component to the leads. As a PCB designer you know the rest, the leads of the component are then what would be soldered to the board.

The flip chip however flipped the chip, or die, so that its bond pads were on the bottom. This allowed them to connect directly to the pins that would be soldered to the board. The flip chip is smaller and has shorter connections which reduces inductance and improves signal integrity for higher-speed signals.

As the components increased their capabilities however, they become more complex and dense with a greater number of die pads. Soon the number of die pads became too great for direct connections to the solder pads on the circuit board, and a new solution was needed. The answer was to introduce a layer of metal circuitry between the die and the component substrate.

In the same way that a PCB designer would escape route out of a BGA to vias that connect to traces on other layers of the board, the routing on this layer connects the die pads to the pins that solder onto the PCB. This layer of metal connections in the component is known as redistribution layer (RDL) routing.


RDL routing will connect the die of a BGA like this to its pins

BGA’s like this will have very complex RDL routing within them


Enhancements and Growth of RDL Routing

As can be expected though, IC development has not sat still and components are becoming increasingly more complex with higher and higher pin counts. The difficulty that PCB designers experience routing traces with shrinking pin & pitch sizes and escalating pin counts, is equally difficult for the IC designer working with more complex components.

These designers are facing similar challenges in getting the signals out of the die and onto the component substrate for pin connections to the board. To do this the RDL lines and spaces need to shrink, and other answers are needed as well.

Component creation has evolved dramatically over the years to accommodate the greater connection requirements of newer devices. Fan-out wafer-level packaging is one new IC packaging technology that has allowed for more space around the die for connections.

Multiple layers of RDL are also used to route these connections, and 3D packaging techniques are also in use. Enhancements to the electroplating and etching processes for the RDL routing are another area of size reduction being worked on. All of these improvements will become increasingly important as IC development continues to drive up the number of connections needed in new components.


Screenshot of OrCAD layout showing routing out of a BGA part

Routing patterns like these will get denser as ICs become more and more complex


What this Means for PCB High Speed Routing

As more and more functionality is being put into BGAs and other high density parts that we use on our circuit boards, our jobs as PCB designers will run into increasingly more difficult challenges. To deal with these challenges will require stepping up our game when laying out PCB designs.

Careful floor planning of our component placement will become a requirement, and escape routing will need to be more precise. Setting up and using high speed design rules will become the standard mode of PCB design instead of it being simply an option.

To be ready for the challenges that the newer and more complex components with enhanced RDL routing are going to present, you need to have the best PCB design tools possible ready to go. Allegro PCB Designer is the premier PCB design system that contains the most advanced high speed design features and rules available to designers today.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.