AN2797

Power integrity

Doc ID 14841 Rev 1

5/23

1.4

Ground and power supply connections

For proper operation of devices, it is critical provide a very low impedance wide bandwidth

connection to ground and the voltage supplies. To achieve this it is necessary to minimize

inductance between the device power and ground balls and the PCB ground plane and

decoupling network.

The above guideline also applies to other critical components: termination resistors,

decoupling capacitors, ICs, multiple ground or power pins from the same IC.

Each ground ball should be directly connected to the PCB ground plane with its own via. Do

not share vias between multiple ground balls. An exception to this rule is the center 6x6

ground ball grid, which should have a fully populated ground via grid between the balls and

the surface layers can be filled.

Each power ball should be directly connected to the PCB decoupling network with its own

via. Do not share vias between multiple power balls except when multiple power balls are

adjacent to each other and are connected to the same voltage plane. In any case, 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), multiple ground or power balls should not share

ground connections. Each ball/pin should have their own via to the ground or power plane,

not a shared via. Power and ground connections should not simply be connected to surface

layer copper fill areas - these are not good low impedance paths at high-frequencies.

Ball to via trace: Each ground and power ball should be connected to it's via with a short,

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

should be less than 1 mm in the ball field, 0.25 mm outside a ball field, Trace width should

be wide. Do not simply connect ground or power balls to surface fill areas. A close, direct via

to the ground or power plane is necessary. In cases where it is not possible to achieve this

length, there is a relatively high impedance. Every effort should be made to minimize the

length, and to consider only power connections requiring lower bandwidth for high

impedance power connections.

Ferrite beads and RC filters: The perceived need for ferrite beads and RC filters is very

often the result of poor grounding and power supply decoupling, resulting in excess noise on

power rails. A well-designed power delivery network eliminates the need for these remedies.