I am a big fan of the World War II movie; “The Great Escape.” It is based on the true story of a group of allied prisoners of war that use their ingenuity to attempt a mass escape from the prison camp where they were being held. Using materials that they scrounged or stole, they recreated civilian clothes, passports, and official documentation to help them in their escape. They also dug a series of tunnels complete with carts to haul out the dirt, hand operated air compressors for fresh air, and electric lights. Their ability to accomplish what they did while confined to a prison camp is astounding.
I am often reminded of their tunneling when I am routing a printed circuit board with high density ball grid array (BGA) parts on it. Routing the signal traces out of the BGAs is a process that is generally referred to as “escape routing.” With the more dense parts however, this can be a real challenge. You typically have to plan out ahead of time where the routes must go, change the trace widths to a smaller size in order to fit, and tunnel through the board using a variety of vias. To get all the nets successfully routed out of a BGA can end up being “a great escape” in its own rights. Here are some techniques that can help you with your BGA escape routing.
Component Placement: The Essential First Step to Successful BGA Escape Routing
To successfully route out of large scale fine pitch parts requires a good component placement to begin with. As with any board, start with the fixed components like connectors first. Next, locate the major components such as your BGAs according to their connectivity. You certainly need to be aware of any thermal issues with the board as well as high speed needs so that you don’t place sensitive components in too hot or noisy areas of the board. Once those parts are placed, then you can focus on how best to place the rest of the board to help with your escape routing:
Keep the decoupling capacitors as close as possible to the pins on the BGA that they will connect to. This will help to keep their inductance low, and allow you to place them so that they won’t get in the way of your routing.
Place components according to their signal paths. Not only is this important for high speed signal performance, but again by placing those first you will give yourself more room for routing later.
Spread out your placement enough to fit in the escape routing that you are going to be doing. Here is where you will have to find a good compromise in your placement. You need to place your parts so that they are positioned optimally for their performance, while at the same time leaving yourself room for all of the routing that you will be doing.
Once you are placed, it’s time to work on the escape routing.
With the components placed, you can begin the escape routing for the BGA
Tunneling to Freedom: How Best to Work with Traces and Vias for BGA Escape Patterns
Let’s start with the trace routing of a BGA escape pattern first:
The first thing that you can do, which is the simplest, is to route your escapes out from the outer rows of the BGA. This is usually done diagonally to give yourself more routing channels.
For larger parts with enough space between the pins, escape routing is usually done in a dog bone pattern. This is a short trace that directly connects the BGA pad to the via immediately next to it. You may also route inward on a BGA package to vias in the center.
Once the pin pitch begins to shrink on the larger BGAs, the escape routing becomes more difficult. One tactic is to shrink the trace widths down, although you don’t want to get any smaller than 0.003 inches. Depending on the pitch of the part, this should allow for more routing between the BGA pads.
Along with your trace routing, you will also be placing the vias. Here are some via considerations to keep in mind:
On small BGAs with only a few rows, your regular routing vias will probably work fine. On larger BGAs though, you may need to shrink the vias in order to fit them within the BGA pattern.
In dense BGAs a very helpful option is to place your vias in the component pads. This will free up a lot of space on the board but may cause manufacturing difficulties. Make sure that your manufacturer is on-board with this plan first.
Another option is to use micro-vias. These are often used when the BGA pad pitches decrease to 0.5 millimeters. Micro-vias can be as small as .004 inch holes with a pad size of .008. Be warned though, using micro-vias will raise the cost of manufacturing your board.
Ultimately the escape routing strategy that you choose will depend on the BGA pad pitch, the size of the BGA part, the overall routing density of the board, and the size of traces and vias that you use. Large fine-pitch BGAs will typically require more board layers as you will usually have room to only escape route a couple of rows of pads to a single board layer. Additional board layers will mean additional cost however, and may impact the signal integrity of your design. Once again, make sure that you check into all of these factors before you commit.
Dog-bone escapes and narrow traces are part of this BGA escape routing pattern
Tips For a Great Escape
One of the best resources that you can turn to is the component vendor’s data sheets for the BGA parts that you are working with. Often you will find recommended escape routing patterns that can save you a lot of time. Another good resource is to work ahead with your PCB manufacturer. They can advise you on the best board layer configuration for your high speed design needs, as well as the size and type of traces and vias that you can safely use for routing your BGAs.
Another helpful tip is to maximize the use of your PCB design tools. Your design tools will have the capabilities to shrink traces and automatically change via sizes within a specified area that you can set up around your BGAs. They also will have features allowing you to set up the high-speed design rules and other requirements of your board.
The PCB design tools from Cadence give you these powerful capabilities. The different routing features in Allegro PCB Designer will give you the control you need to quickly and effectively route your BGA escape patterns. Allegro’s powerful design rules and constraint system will also give you the ability to set up your design for the exact PCB trace and space widths that you need.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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