A look back at the most important ideas for fanout and routing of BGAs.
For over a year, I have presented various methods and ideas for fanout and routing of BGAs. This article marks the last of the BGA Bulletin series, and I will be passing the back-page baton to others who will cover additional solutions related to PCB design. I felt it would be appropriate to review the most important ideas from previous articles– hence the Top 10 Tips (in no particular order). The PCD&F issue in which the tip is detailed is included after the title. I hope to write more articles of interest to you in the future; until then, I wish you the best of success with your PCB design challenges!
1. Align Vias (May 2008)
Of all the methods, aligning blind vias (mechanically or laser drilled) in columns and rows is the most effective way of increasing the route density. Depending on the size of the via, the route density can increase from 24% to 36% per layer over vias in a standard matrix using the same pitch as the pins. This method is most affective with high-pin-count BGAs and when escaping the device is the primary contributor to the layer count.
2. Push Perimeter Vias (December 2008)
When using BGAs that have a pin-pitch less than 0.8 mm, aligning the vias cannot be done because there isn’t enough room between the ball pads for via patterns; in fact most of the time, there is only room to put the vias in the pads. However, if you push the fanout vias for the first two rows of pins around the perimeter and away from the BGA far enough, you can then route the next two rows of pins on the same layer, thereby eliminating one to two layers for the escapes.
3. Adopt HDI (June 2008)
The smaller feature sizes available when using HDI enable great improvements in route density and will reduce your layer count significantly. A new book, “HDI Handbook,” is completed and is available at http://www.hdihandbook.com. The authors include: Happy Holden, John Andresakis, Eric Bogatin, Michael Carano, Karen Carpenter, Karl H. Dietz, Mark Laing, Christophe Vaucher, Per Viklund and Mat Wuensch. This book provides incredible depth and detail about HDI.
4. Spread Vias-In-Pad (December 2008)
When using via-in-pad methods, you can spread the vias within the pads and potentially open up a channel to route differential pairs without splitting them.
5. Use Ground Fanouts (April 2008)
It is tempting to minimize the number of ground fanout vias in order to increase route density; however, there is a trade-off with this method. It can affect power and signal integrity by reducing the ground return paths. Ground ball pads are almost always distributed in some kind of pattern around the BGA, and they are not in nice columns or rows. Therefore, not using fanout vias will not open up route channels. I recommend using a through-via or a combination of blind and buried vias for each ground pin to ensure the ground planes are highly connected, providing excellent return paths.
6. Adjust Patterns Locally (May 2008)
When using via patterns to increase route density, the vias for the differential pairs sometimes are spread apart. Each set of fanouts for the differential pairs should be reviewed and moved locally in order to fulfill your coupling and phase matching requirements.
7. Layer-Biased Via Patterns (August 2008)
If you have only two to four layers to route a BGA, then escaping in all directions on each layer makes sense because of the extreme route density. If you have more than four layers to route, then applying a layer-biased concept, where the escape routes obey the layer bias, enables more efficient routing– especially if an autorouter is used.
8. Use Regions (July 2008)
Applying different via patterns in regions is an effective way to increase route density when you have a variety of via spans to work with. For example, you could use a 1:2 via span on the outer four rows of ball pad fanouts (Region 1), a 1:3 via span for the next four rows in from the perimeter (Region 2) and through-vias for the fanout in the center of the BGA which is usually for power and ground distribution.
9. Investigate Any-Layer Vias (November 2008)
Of all the HDI via types, the Any-Layer-Vias hold the greatest promise for effective fanout and routing of fine-pitch BGAs. These vias may start and end on any layer, thereby giving you the greatest flexibility and route density. I have heard of a 12-layer board using any-layer vias, and over time, I expect this technology to be applied to boards up to 20 layers.
10. Prepare for Further Minituration (January 2009)
Although the highest pin-count BGAs (over 1500 pins) are still at 1 mm pin pitch, there are many 0.6-mm pitch BGAs with hundreds of pins. The minituration will continue, and new methods for the packages and fanouts will be required. PCD&F
Charles Pfeil is an engineering director for Mentor Graphics, Systems Design Division. Email: This email address is being protected from spambots. You need JavaScript enabled to view it..
