Simplify Your BGA Routing Using Neck-Down

January 10, 2017 Steve Tran

BGA Routing neck down example header

How many times have you attempted to route into your BGA only to be prevented by clearance and track width constraints? Learn how to effectively route into your BGA without interrupting the routing process to adjust rules or width size with the neck down features in Altium Designer®.

How many times in your routing process have you attempted to route into a BGA only to be prevented by clearance and track width constraints, forcing you to stop routing and adjust your rules or the size of track widths? Due to the tight space within a BGA routing into it becomes fairly difficult just by itself. Combine that with having to change the width of the track and it becomes even more time consuming. Since a large portion of your time is spent routing your board, it is especially important to be able to have a continuous process that doesn’t require you to stop what you’re doing to make adjustments. But how can you control that?'

How to Leverage Neck Down for Enhanced BGA Routing

Neck down is the process of shrinking the track width to a smaller width within the rule constraints to allow for trace routing in tight clearances. Typically, neck-down is the percentage width change of pad size to a track width size. However, it is also possible to set a size reduction from track-to-track as it enters or leaves an area.

With any PCB design, you have a set of rules in place to constrain the design rules within defined parameters. One of those rules defines the track width of your routes. This rule, along with a clearance constraint rule, normally restricts the track from entering a BGA. To resolve this, you exit routing mode and change the rules to fit within that BGA. Obviously, this isn’t ideal when you’re trace routing and you want a way for the system to modify the track width as it enters the BGA and then reestablish its track width as it exits the BGA. This is where having the tracks neck down comes into play.

The width for the PCB design rules should not be changed just to meet the space constraints of the BGA as this would affect the rest of your design. Instead, width reduction rules should be restricted to just the BGA area. This can be done in Altium ® by setting the width rules specifically for a room generated around the BGA, as shown below.

Placing a room around the BGA allows rules to be changed only for that room

Route Uninterrupted in Altium

You can configure Altium to automatically neck down as it enters a predefined area. As the routing enters the area of the BGA, the width automatically shrinks to match the track width rule for that area. This is especially important when you are routing between several pads of a BGA to connect with an inner pad but the width of the track is too large to reach the pad.

This time-saving feature lets you continuously route your board without stopping and changing the PCB design rules to meet the constraints of the BGA.

Want to learn more about how to utilize neck down in Altium ? Download our free white paper Using Neck-Down Simplifies BGA Routing today.

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About the Author

Steve currently serves as an Application Engineer for Altium and is responsible for post sales support, customer training, and technical writing. He holds a B.S. in Engineering with an emphasis in Computer Engineering from Wentworth Institute of Technology.

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