Ground and power supply connections

AN3317

8/22

Doc ID 18249 Rev 2

4

Ground and power supply connections

For proper device operation, it is critical to provide a very low impedance, wide bandwidth

connection to ground and to the voltage supplies. To achieve this, minimize inductance

between the device power and ground balls, and the PCB ground plane and decoupling

network. This guideline also applies to other critical components:

●

termination resistors

●

decoupling capacitors

●

ICs

●

multiple ground or power pins from the same IC

Directly connect each ground ball to the PCB ground plane with its own via. Do not share

vias among multiple ground balls.

Exception: the center 10 x 10 ground ball grid should have a fully populated ground via

grid between the balls, and the surface layers may be filled.

Directly connect each power ball to the PCB decoupling network with its own via. Do not

share vias among multiple power balls.

Exception: when multiple power balls are adjacent to each other and are connected to

the same voltage plane, but use the maximum number of vias that space allows.

To avoid cross-contamination of ground or power supplies between different devices (for

example, an IC and a termination resistor), do not share ground connections among multiple

ground or power balls. Give each ball and pin its own via to the ground or power plane. Do

not simply connect power and ground connections to surface layer copper fill areas, which

are not good low impedance paths at high frequencies.

Ball-to-via trace: Connect each ground and power ball to its via with a short, wide trace. Do

not simply connect ground or power balls to surface fill areas; a close, direct via to the

ground or power plane is necessary.

Caution:

It is critical to minimize trace length and maximize trace width.

●

In the ball field, make trace length less than 1 mm.

●

Outside the ball field, make trace length less than 0.25 mm.

●

Make trace width wide.

Note:

When it is not possible to achieve a close direct trace, a relatively high impedance will result.

Make every effort to minimize trace length, and consider high impedance power connections

only for power connections that require lower bandwidth.