BGA/PCB interconnect design guidelines: a successful BGA design is the right combination of pad diameter, drill diameter, anti-pad and aspect ratio that maximizes the routing density.

Stacked microvia technology can go down to layer five by stacking microvias and using a solid copper plate to eliminate via-in-pad issues at assembly. Additionally, stacked microvias can be utilized to route out of a 0.4-mm pitch device by routing the outside row on layer one and laser drill remaining I/Os to layer two. Now layer two is the outside row, which can be routed using a 0.004" trace. This inverted pyramid approach can be used to fan-out all rows. Laser drills can be offset within the 0.4-mm pitch device to create every other channel available for routing with a 0.0033" trace and space.

Stacked microvias can also be utilized for 0.25-mm flip-chip technology. This is routed with a 0.0068" pad diameter around a 0.004" laser-drilled hole and uses a 0.003" space, routing outside rows with a 0.003" or 0.004" trace using an inverted pyramid scheme to escape the BGA. This technology is only available at leading-edge PCB fabricators (FIGURE 8).

Inches vs. Millimeters

We discussed BGA routing techniques in this article using inches. However, we can promote the same message in millimeters as it relates to drill-to-copper, pad diameter, drill diameter and aspect ratio (TABLE 2). For example, a 1-mm pitch BGA consists of a 0.5-mm pad diameter and a 0.25-mm drill, with a 0.1 mm trace and space.

Conclusion

The key element to success with BGA/ PCB interconnects or BGA escape/ routing techniques is to determine the right combination of trace width and space, pad diameter, drill diameter, anti-pad diameter and aspect ratio. This allows the PCB to be fabricated with the highest yields for the lowest cost while maximizing routing density, thus reducing layer count and improving electrical characteristics.

It is also important to utilize DFM guidelines and IPC specifications for high PCB yields, low cost and long-term reliability. The combination of DFM guidelines (PCB fabrication tolerances) and IPC specifications equals the BGA/ PCB interconnect design guidelines.

Additionally, the use of an 0.8-mm pitch BGA is the time to consider microvia technology, and a high I/O count 0.5-mm pitch device pushes the consideration for stacked microvia technology.

We suggest contacting your fabricator when starting the design process or after a netlist is extracted and a layer assignment is complete. Discuss your plan for drill sizes, pad sizes, anti-pads and thermal reliefs, as well as the construction and PCB material selection. The fabricator can provide a PCB construction with material recommendations, impedance calculations and confirm your choice of pad, drill, anti-pad, thermal reliefs, and trace and space. Incorporating these design guidelines will allow the PCB to maintain efficient routing, reduce the number of layers required, enhance electrical characteristics, provide a robust, reliable design and allow the PCB to be manufactured with the highest yields in a timely manner at the lowest cost.

GIL WHITE is a technical marketing manager at DDI. He can be reached at pgwhite@ddiglobal.com. TABLE 1. Standard BGA design guidelines, IPC 6012B Class 2.

DRILL PAD ANTI-PAD .006" (.152 mm) .016" (.406 mm) .026" (.660 mm) .008" (.203 mm) .018" (.457 mm) .028" (.711 mm) .010" (.254 mm) .020" (.508 mm) .030" (.762 mm) .012" (.304 mm) .022" (.558 mm) .032" (.812 mm) .0135" (.342 mm) .024" (.609 mm) .034" (.863 mm)

DRILL THICKNESS ASPECT RATIO .006" (.152 mm) up to .039" (.990 mm) 6.5:1 .008" (.203 mm) up to .062" (1.575 mm) 7.75:1 .010" (.254 mm) up to .100" (2.54 mm) 10:1 .012" (.304 mm) up to .120" (3.04 mm) 10:1 .0135" (.342 mm) up to .135" (3.429 mm) 10:1 TABLE 2.

DRILL PAD ANTI-PAD .15 mm (.0059") .4 mm (.0157") .65 mm (.02559")

.2 mm (.0078") .45 mm (.0177") .7 mm (.02755") .25 mm (.0098") .5 mm (.01968") .75 mm (.0295")

.3 mm (.0118") .55 mm (.02165") .8 mm (.03149") .345 mm (.0135") .6 mm (.0236") 85 mm (.0334")

DRILL THICKNESS ASPECT RATIO .15 mm (.0059") up to 1 mm (.03937") 6.5:1

.2 mm (.0078") up to 1.575 mm (.062") 7.75:1 .25 mm (.0098") up to 2.54 mm (.100") 10:1

.3 mm (.0118") up to 3.048 mm (.120") 10:1 .345 mm (.0135") up to 3.4 mm (.1338") 10:1