Tips for BGA Escape Routing

BGAs are used more commonly now than ever before and are increasing design density and complexity. At the same time, the BGAs, themselves, are shrinking in size and adding greater numbers of pins. This calls for more precise design and manufacturing techniques.

However, size of the traces is shrinking. Currently, trace routing between BGA pads is 2 – 3 mils. This poses difficulties at fabrication and assembly. These thin traces demand tight control of compensation factors to highly precise levels and at different manufacturing stages.

The escape routing technique takes into account such factors as ball pitch, land diameter, number of I/O pins, via type, pad size, decoupling capacitor placement, trace width/spacing, and the number of layers required to escape the BGA.

Here are some helpful tips for routing BGAs:

1. Placement of decoupling capacitors is very critical. A decoupling capacitor must be placed as close as possible to the BGA pad. Otherwise, a higher inductance path than necessary is created.
2. Modern device complexity requires the use of fine pitch BGAs. This calls for diagonal placement of via capture pads compared to in-line with surface pads. Flared dog bone fan-out is the most popular method used today.
3. Via pad size also affects the number of traces that can be routed within each channel.
4. In a BGA pad design using silver-filled vias, the need for dog-bone fan-out is eliminated, and a small increase in density is realized by being able to route a few more signals on the outer surface.
5. Factor in extra time. Silver-filled vias are a specialized process and extend PCB fabrication time by three to five days to incur extra costs.
6. On 0.5mm pitch BGAs, micro-via on the BGA land is the most common approach, as pads are not typically large enough for through vias. In this case, the outer perimeters can be fanned out using blind vias.
7. A more productive technique could be to route on a 45º angle from the corners of each quadrant. This method aids in utilizing all possible escapes and in turn could reduce the number of layers required.

As device packages are able to handle more I/Os with minimum increase in size, signal escape routing will continue to be more difficult. The number of PCB layers required to escape all the pins depend on a number of factors. Some of these factors, like the ball pitch, and land size are device specific and need to be factored in prior to starting board layout. Others have to be figured out by the designer. Board designers will always be pushed towards using the minimum number of routing layers to reduce cost, especially when using a strip-line structure. Hence successful and effective routing will always be a challenge for the PCB designer.

It is therefore important for a contract manufacturer (CM) or EMS provider to have a capable, experienced design team to correctly design trace routing between BGAs. A high level of experience is also vital at the manufacturing level to run proper DFM checks. This assures a BGA-populated PCB is easily and effectively manufactured in small quantities, as well as mass produced overseas.