I have found that there are two equally intense emotional moments when putting together a jigsaw puzzle. The first one is the absolute feeling of joy and accomplishment that envelops you when you finally finish the puzzle. The second however is an overwhelming sense of doom and gloom when you open the box and dump out all the pieces on the table in front of you.
I would tell you what my typical reaction is while staring glassy-eyed at all of those pieces, but that kind of language is usually frowned upon in print. The reason that many of us become agitated when opening a new puzzle is that the idea of recreating the beautiful picture on the cover of the box from the mess of pieces in front of us seems impossible.
In that same way when you first look at the jumbled mess of unrouted nets that are coming from the ball grid array (BGA) device in your PCB layout, it can also seem impossible. But experience has shown me that if I just take one piece at a time and start working the puzzle, I will eventually put it all together. It’s the same thing with PCB design, by taking it one step at a time you too will be able to finish routing BGA packages. Here are some ideas that should help.
Be Prepared Before You Start Routing BGA Packages
One thing is for sure in PCB design, as circuit board technology keeps improving there will be a continual need for more fine pitch devices. This includes the use of BGA devices, which will continue to grow in pin numbers while their pad pitch and size steadily decreases.
As these devices become more complex, it will get harder and harder to route out of them as you may have already found out. To stay on top of this, you need to keep informed by doing as much research into designing with these parts as you can.
You can find a lot of information from doing online searches for video tutorials, white papers, technical articles, and blogs that will help you to design using these devices. Another excellent source of information is to go to the vendor web sites and look for recommended land patterns, placement suggestions, and routing examples.
This information will save you a lot of time trying to resolve some of these complex design issues on your own. Lastly, don’t be afraid to ask others that you work with for help too. Tribal knowledge can be a great tool to leverage, so go ahead and ask the question. And in that vein, here are a few ideas for you to consider while routing BGAs that should prove to be helpful.
Some examples of BGA escape routing patterns on a PCB design system
Some Practical Tips for BGA Routing that Can Help
Before jumping into how to rout a BGA fanout technically, it is best to be sure you know exactly what you’re dealing with. What is a BGA fanout, and how to decide number of layers in PCB depending upon BGA pins? How many traces do you have to get out of the BGA and how many nets can be routed? What is your required impedance and acceptable minimum line width?
For the best results in routing your BGA devices, you need to start with the component placement of your design:
Make sure to give yourself room to route. Although critical components need to be placed for the best signal path, you also need to keep in mind that you have a lot of routing and vias that need to go in.
Keep the signal paths correct. This is the counterpoint of what we just stated above, but that doesn’t negate its importance. Your circuit board won’t have the signal performance it needs if your critical components and their routes are wandering all over the board.
Keep decoupling capacitors as close as possible to their BGA pins to reduce inductance.
Before you can route out of the BGA, you need to route your via escapes, or fan-outs. Here are some ideas to keep in mind:
Larger BGAs will work well with diagonally routed escape patterns. This short trace from pad to via is usually referred to as a dog bone pattern.
Another tactic that works well on the outer rows of BGAs is to diagonally route the escape traces from the corners.
BGA escapes will also be routed with vias in their pads. This makes life a lot easier for the PCB designer, although you must confirm first that your PCB manufacturer will support this.
As pad pitch decreases to 0.5 millimeters, micro-vias are often used in the BGA pads.
To accomplish the high density routing within the BGA pattern, smaller trace width and spaces are used along with smaller vias:
Trace and space sizes can get down to 3 mils (0.003 inch). Although this will probably not be used throughout the entire board, manufacturers will often support this within the BGA area.
Vias sizes can also be smaller with their size being dependent on the pitch of the BGA, the amount of routing in the area, and the overall thickness of the PCB.
Micro-via sizes can get down to 8 mils with a 4 mil hole depending on the capabilities of the PCB manufacturer.
It is very important to confirm first whether or not your manufacturer can handle the smaller trace, space, and via sizes before you use them. Another consideration to work on with your manufacturer is the board layer stackup and how many layers you will need in your design. This can be driven in large part to the BGAs due to routing requirements. You may need an extra routing layer for each two rows of BGA pins in order to completely fan-out the BGA.
Careful planning goes into routing out of a BGA package like this
Let Your Design Tools do the Heavy Lifting
Fortunately, your PCB design tools can do a lot to help you with your routing of BGA packages. You can set up specific high speed design rules for the sensitive nets that you will be routing out of the BGA. Additionally, you can work directly with many manufacturers on your board layer configurations, and by using the IPC-2581 format you can import their recommended board layer stackup into your design. Your design tools will also give you the following abilities:
Set net types for traces that have to be a certain width. This will become very useful for routing narrow BGA escape traces.
Create zones around BGAs for unique routing requirements. This works together with the smaller trace widths for BGA escape routing as you can designate the zone where these smaller widths need to be used.
Designate keepout zones for areas of critical routing. This way you can restrict the placement of parts in high density routing areas giving you more room for your BGA routing.
To get the help you need with this heavy lifting, you will need to use PCB design tools that have the capabilities that we’ve talked about here. Allegro PCB Designer has the constraint driven design functionality that you are looking for to quickly and easily set up the unique routing requirements that you need to successfully route your BGA devices.
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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