BGA packaging technology is steadily advancing, especially for wearable and IoT rigid-flex and flex circuitry. Today, both standard and micro BGA 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) designers.
A major portion of a BGA populated PCB design deals with the two basic methods the designer has to perform to correctly escape signal traces from a BGA. Those are dog-bone fan-out (Fig. 1) and via-in-pad (Fig. 2). Dog-bone fan-out is used for BGAs with 0.5 millimeter (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 center of one BGA ball to the center of the next one.
Check out our EE Times article on efficient use of BGA signal routing to get a broader understanding and more details.
In the meantime, here are some tips and good to know information go get you well on the way to understanding best BGA signal routing.
- High-density, multi-layer PCBs may have either blind or buried vias, also known as micro-vias. Blind vias are visible only on one surface, while buried vias are visible on neither surface.
- Via size is calculated using land diameter and ball pitch. The land diameter is the diameter of the pad of each BGA ball.
- Trace width and minimum space required between traces define a BGA’s escape routing. It’s good to know that reducing spacing between traces increases PCB fabrication cost.
- A number of traces can be routed through various channels. For instance, one versus two traces can be routed and sometimes three if BGA pitch isn’t very fine. With a one-millimeter pitch BGA multiple traces can be routed, for example.
- When using the via-in- pad technique for BGA signal escape and routing, vias are placed directly on the BGA pads and filled with conductive material, usually silver that provides a flat surface.
- Once trace and space width, the number of traces routed through one channel, and type of via to be used for the BGA layout are determined, the number of required layers can be estimated.
Manufacturing and functionality are two key aspects that need to be considered regardless whether dog-bone or via in pad is used. It’s critical to know the manufacturing limits of the fab shop that’ll be used. There are shops that can manufacture extremely tight designs. However, if the product is going to volume production it gets very costly. It is therefore extremely important to design in a way that average manufacturing facilities should be able to handle them.