Simplify BGA Routing With Safe Neck Down Trace Width

Steve Tran
|  Created: February 21, 2017  |  Updated: September 25, 2020

BGA Routing neck down example header

How many times have you attempted to route into your BGA (ball grid array) only to be prevented by clearance and trace width constraints? It's possible to math out thinner traces, but computers are particularly good at this kind of work while maintaining a safe current carrying capacity. 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®.

Not being able to trace a BGA because of clearance and trace width constraints really slows the design process down, forcing you to stop routing and adjust your rules or the size of PCB trace widths. The tight space within a BGA routing is fairly difficult to deal with by itself. Combine that with having to change the width of the PCB trace and it becomes even more time consuming. Since a large portion of your time is spent routing your board, it is especially important to have a continuous process that doesn’t stopping to make adjustments to trace width. But how can you control that?

How to Leverage Neck Down for Enhanced BGA Routing

Neck down is the process of shrinking the PCB trace 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 PCB trace rules in place to constrain the design within safe 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 system to automatically modify the track width as it enters and leaves 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 at a width still able to maintain the current carrying capacity. This can be done in the background of Altium Designer® by setting the PCB trace width rules specifically for an area generated around the BGA, as shown below.

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

Route Uninterrupted in Altium Designer

You can configure Altium Designer to automatically neck down the PCB trace width as it enters a predefined printed circuit board area. As the routing enters the area of the BGA, the PCB trace 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 PCB trace width neck down in Altium Designer? Download our free white paper Using Neck-Down Simplifies BGA Routing today.

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About Author

About 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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