Optimising BGA signal routing in PCB designs

Ball grid array (BGA) device packaging is today's standard for housing a range of highly advanced and complex semiconductor devices such as FPGAs and microprocessors. The technology of BGA packaging for embedded designs is steadily advancing to keep up with chipmaker technical advances, with this type of packaging splintering out into standard and micro BGAs. Today, both types deal with increasing numbers of I/Os, and this means signal escape routing is difficult and challenging, even for experienced printed circuit board (PCB) and embedded designers.

The top job for the embedded designer is to develop appropriate fan-out strategies that won't adversely affect board fabrication. There are several major considerations involved in selecting the correct fan-out/routing strategy: ball pitch, land diameter, number of I/O pins, via types, pad size, trace width and spacing, and the number of layers required to escape the BGA.

PCB and embedded designers will always be challenged to use the minimum number of board layers. The number of layers needs to be optimised to reduce cost. But sometimes a designer must rely on a certain number, for example, to suppress noise by sandwiching actual routing layers between ground plane layers.

Aside from those design factors inherent in particular BGA-based embedded designs, a major portion of the design involves two basic methods the embedded designer has to perform to correctly escape signal traces from a BGA: dog bone fan-out (figure 1) and via-in-pad (figure 2). Dog bone fan-out is used for BGAs with 0.5mm (mm) and above ball pitch, while via-in-pad is used for BGAs and micro BGAs with below 0.5 mm ball pitch, also known as ultra-fine pitch. Pitch is defined as the spacing between the centre of one BGA ball to the centre of the next one.

It's important to know some basic terminology associated with these BGA signal routing techniques. The term "via" is the most prominent. It refers to a pad with a plated hole connecting copper tracks from one PCB layer to other layers. High-density multi-layer boards may have either blind or buried vias, also known as micro-vias. Blind vias are visible only on one surface; buried vias are visible on neither surface.

Dog bone fan-out
Dog bone BGA fan-out provides partitioning into four quadrants with a wider channel in the middle of the BGA to run multiple traces from inside. Several key steps are involved to break out signals from the BGA and connect them to other circuitry.

The first step is to determine the via size needed for the BGA fan-out. Via size depends on a number of factors—device pitch, PCB thickness, and the number of traces to be routed from one area of the via or one perimeter to the next. Figure 3 shows three different perimeters associated with a BGA. A perimeter is the boundary of a polygon is defined in a shape of a rectangle or square surrounding the BGA balls.