Printed Circuit Design & Fab - April 2008 - (Page 48)

BGA Fanout Patterns, Part 2 Through-vias provide the lowest fabrication cost but can limit routing density. THIS IS THE fourth in a series of articles on BGA routing methods. My objective is to reveal the routing problems often associated with large pin-count CHARLES BGAs, and provide PFEIL the PCB designer with effective techniques that enable higher route density and reduction of layer count. Through-Vias There are not many fanout options, due to the large via pad relative to the 1-mm ball pitch. Either a “Quadrant Dog-Bone” or “Via-in-Pad” method is appropriate. When using through-vias for fanouts, rather than trying to gain a few route channels by eliminating connections to the planes, it is best to gain the power integrity benefits by adding fanouts for all power and ground pins. In FIGURE 1, it is clear that the vias for power and ground are scattered such that even if they were removed, little would be gained. The outer perimeter of through-vias dictates the space available for routing, and at least in the case of the Virtex-4, few of them are assigned to power and ground. If an ASIC can be packaged to provide adequate power integrity and have the power and ground pins aligned in such a manner so as to increase route channels (by not using fanout vias) that would be a good design, but this is likely to be a rare occurrence. Quadrant Dog-Bone This fanout pattern has the fanout vias in the center spaced between the ball pads, and angled in one of four directions. Advantages over via-in-pad. ■ It opens up additional routing channels in the center row and column, giving room for two or three more routes per layer, which could contribute layer reduction, seen in FIGURE 2. ■ On the side of the board opposite the BGA mount, the column and row channel gives a convenient place to add capacitors and pull-up resistors. ■ A lower cost and less risk of soldering problems related to the via-in-pad. Disadvantages over via-in-pad. ■ When using a ground or power plane on the BGA mount side, the fanout via pads prevent a continuous plane fill under the BGA. Theory: Removing Vias One method proposed to increase route channels on inner layers is not to use fanout vias where possible, for power, ground and unused pins. When using through-vias, there is very little benefit in not adding fanout vias for the 30% to 50% of the BGA pins that will likely be assigned to power, ground or unused. The power and ground pins will be assigned to the center of the device and distributed among the other pins. It is highly unlikely that they, or any unused pins will be distributed in columns and rows. Shifting Through-Vias Choosing to place the fanout via Continued on p. 39 FIGURE 2. Quadrant dog-bone pattern detail. FIGURE 4. Shifted vias additional route space. FIGURE 1. Xilinx Virtex-4 with power (orange) and ground (green) vias. 48 FIGURE 3. Space between vias (mm). FIGURE 5. Via-in-pad pattern with ground plane on mount layer. APRIL 2008 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - April 2008 Contents Our Line Market Watch Around the World Happenings ROI Tip Jar Interconnect Strategies Positive Plating Improving Fabrication Yields by Design Solve Design Problems with Signal Integrity Optimization Laser Direct Imaging Made Easy Troubleshooting the Innerlayer Process SIPS Give More to Moore Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - April 2008

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