BGA Breakouts and Routing Print E-mail
Written by Charles Pfeil   
Tuesday, 01 January 2008 00:00
Charles Pfeil

BGA miniaturization amplifies the design challenge of balancing high-performance signal integrity with fabrication cost reduction.

This is the first in a series of articles that will run in Printed Circuit Design & Fab about BGA routing methods. My focus will be on design techniques using common and advanced technologies for large BGAs to provide efficient fanouts, dense routing and layer reduction.

These articles are intended for PCB designers, exploring the impact of dense BGAs with high pin-count, and to provide solutions for these inherent design challenges on PCB design. There are volumes of information about BGA package technology that look at the device from all aspects – materials, thermal characteristics, assembly methods, reliability, and electrical performance – yet little guidance is available regarding effective PCB design methods.

BGA density and pin count continues to increase, and our ability to effectively design with these devices has not kept pace. Fortunately, significant advancements in PCB fabrication technology have enabled further miniaturization in the manufacturing process. These improvements, along with new software and methods specifically designed for BGAs, provide the means for successful design using these devices.

The increasing pin-count and decreasing pin-pitch of BGAs amplifies already difficult design problems. Maintaining signal integrity at high performance levels while reducing fabrication costs are the two most important, but conflicting, requirements. Reducing crosstalk is generally accomplished by increasing the space between conductors, which can increase layer count, plus routing through dense BGA pin and via arrays requires smaller design rules. Increasing layers while minimizing features contribute significantly to increased board cost. This is nothing new, yet further miniaturization of BGA packages will make it even more difficulty in maintaining performance and cost goals.

PCB designers who are working with large pin-count BGAs (over 1500 pins) often claim that the breakout of the device is the greatest contributor to increased number of PCB layers. An efficient breakout solution will provide a foundation for layer reduction. I use the term “BGA breakout” to describe a fanout design solution that routes escape traces from fanouts to the perimeter of the device prior to general routing of the PCB. Figure 1 illustrates the complexity of the BGA breakout problem.

figure 1

Why the need for breakouts? The answer is simple. If the BGA device has a large number of pins in a dense array, the only way to minimize board layers is to utilize available space inside the component area by incorporating patterns for fanouts and breakout traces. Routing such a device without effective patterns will certainly waste space and require more layers.

Not all BGAs present a routing challenge. Medium and low pin-count devices (less than 800 pins), even with a pin-pitch of less than 0.8 mm, do not present a significant breakout problem and are usually routed without a breakout method. This means the pins are generally accessible and can be routed without using an excessive number of layers.

The high pin-count devices with a pin-pitch of 1 mm or less require a strategy for getting traces out of the array. Without a breakout strategy, layer count will be excessive, thus affecting the fabrication cost and reliability of the PCB.

Additional factors complicate the breakout process. To attain performance and cost goals, there are many variables that must be defined and managed properly in concert with each other.

Layer stackup. Early in the design process, the layer stackup will be defined. If the board has large, dense BGAs, a High Density Interconnect (HDI) with a laminated core and buildup layers may be required. There are many different material and process options. Cost and reliability are usually the primary factors in determining the stackup, and there may be a tradeoff between layer count and the fabrication process in order to reach your goals.

Via models. Within the context of any given layer stackup, you have many options regarding via models. The decision on which type of via to use (through-hole, laminated blind and buried, or HDI micro vias) will likely be driven by the density of the board and the BGA packages. There are many options regarding stacking and vias inside pads that effect cost. In addition to this, board fabricators tend to focus on a limited set of manufacturing processes, making the choice of vendor dependent upon the technology you desire. From the design point of view, choosing the appropriate via models directly impacts the routeability of the board.

Design rules. PCB fabricators continue to find methods that allow further miniaturization and increased reliability. Design rules must balance the tradeoffs of cost, signal integrity and routeability.

Signal integrity. Although the fabricators continue to improve their processes and produce reliable boards with increasingly smaller features and clearances, maintaining signal integrity at high performance levels usually requires greater spacing between critical nets, especially when attempting to eliminate crosstalk effects at higher speeds. This conflict is exacerbated with high pin-count and dense BGAs. Choosing appropriate layer stackups and via models will not only improve routeability, but signal integrity as well.

Power Integrity. Managing power distribution effectively for large pincount BGAs is a challenge and is significantly impacted by the layer stackup. There are methods that can minimize the number decoupling capacitors required, thereby increasing the space available for signal routing. Because of the high number of variables with any PCB design, it is not possible to have a single BGA breakout solution for all. It is possible however, to develop solutions within a reasonable set of variables. PCD&F

Charles Pfeil is a product marketing director for Mentor Graphics, Systems Design Division. He was the original product architect for Expedition PCB and an inventor of XtremePCB; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Thursday, 03 January 2008 15:33
 

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