CircuiTree - March 2009 - (Page 39) Happy Thoughts Figure 4 The Four-Region Via Patterns2 Figure 5 The BGA Diagonal Region on the Corners for the Diagram in Figure 42 regions have been defined. Usually, the center of the BGA has power and ground pins, and thus putting the through-vias in a standard dogbone pattern as shown in Figure 4d makes sense. Note that the vias are not located in the exact center of the ball pad matrix—this allows for a greater ground plane fill on Layer 1.” 2 “The Diagonal Region (1): The pins along the diagonals could have conflicting patterns when the Region 1 and Region 2 fanouts We don’t see too many significant changes in the design of circuit boards, but the blind-via rotation and four-region approach to BGA fanout is superior to the normal north-eastwest-south via fanout method. • 24 percent increased route density per layer over through-vias and unshifted blind vias; • More room for a ground plane on the mount layer (but not as much room as with via-in-pad); • If you route the high-speed single-ended nets on the layers using blind vias, via stubs are eliminated and via-to-via crosstalk is minimized; • Any signal routed on the blind via layers will not need to have a buried via, thus opening up route space on the buried via layers as well; and • The disadvantage is that blind and buried via stackup is slightly more expensive than a through-via stackup. Summary We don’t see too many significant changes in the design of circuit boards, but the blind-via rotation and four-region approach to BGA fanout is superior to the normal north-east-west-south via fanout method. By moving the fanout vias closer together and even occupying the same channel, and stagger them as shown, more traces can be routed for the breakout and fewer signal layers are required. ■ merge. The example in Figure 5 shows a method that not only merges the patterns, but also spreads the vias away from the centerline, providing greater route density along the diagonal. Dividing up the BGA into regions enables maximum route density, and can thereby reduce the number of layers needed. When the BGA has over 1,500 pins, simply routing out of the BGA tends to be the primary contributor to increased layer count. By varying the number of rows used in each region, based on the stackup and via spans available, you can obtain the most optimal fanouts and routing in the context of your own specific design.” 2 This aligned-via pattern of shifted blind vias increases breakout from big BGAs significantly. But it also has other advantages: References 1. Pfeil, C., “BGA Breakouts & Routing, Version 1.5,” Mentor Graphics, 2008, www.mentor.com 2. Pfeil, C., “BGA Breakout Challenges,” On Board Technology Magazine, Oct. 2007. Happy Holden is the senior PCB technologist at Mentor Graphics-System Design Div., Longmont, Colo. Email: Happy_Holden@mentor.com circuitree.com • March 2009 39 http://www.mentor.com http://www.circuitree.com
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