AD8155ACPZ-R7 datasheet(31/35 Pages) AD

31 / 35 page

Data Sheet

AD8155

Rev. A | Page 31 of 35

PRINTED CIRCUIT BOARD (PCB) LAYOUT

GUIDELINES

The high speed differential inputs and outputs should be routed

with 100 Ω controlled impedance differential transmission

lines. The transmission lines, either microstrip or stripline,

should be referenced to a solid low impedance reference plane.

An example of a PCB cross-section is shown in Figure 48. The

trace width (W), differential spacing (S), height above reference

plane (H), and dielectric constant of the PCB material determine

the characteristic impedance. Adjacent channels should be kept

apart by a distance greater than 3 W to minimize crosstalk.

PCB DIELECTRIC

SIGNAL (MICROSTRIP)

SOLDERMASK

PCB DIELECTRIC

REFERENCE PLANE

SIGNAL (STRIPLINE)

Figure 48. Example of a PCB Cross-Section

Thermal Paddle Design

The LFCSP is designed with an exposed thermal paddle to

conduct heat away from the package and into the PCB. By

incorporating thermal vias into the PCB thermal paddle, heat

is dissipated more effectively into the inner metal layers of the

PCB. To ensure device performance at elevated temperatures,

it is important to have a sufficient number of thermal vias

incorporated into the design. An insufficient number of thermal

vias results in a θJA value larger than specified in Table 1.

Additional PCB footprint and assembly guidelines are

described in the AN-772 Application Note, A Design and

Manufacturing Guide for the Lead Frame Chip Scale Package

(LFCSP).

It is recommended that a via array of 4 × 4 or 5 × 5 with a

diameter of 0.3 mm to 0.33 mm be used to set a pitch between

1.0 mm and 1.2 mm. A representative of these arrays is shown in

Figure 49.

THERMAL

VIA

THERMAL

PADDLE

Figure 49. PCB Thermal Paddle and Via

Stencil Design for the Thermal Paddle

To effectively remove heat from the package and to enhance

electrical performance, the thermal paddle must be soldered

(bonded) to the PCB thermal paddle, preferably with minimum

voids. However, eliminating voids may not be possible because

of the presence of thermal vias and the large size of the thermal

paddle for larger size packages. Also, outgassing during the

reflow process may cause defects (splatter, solder balling) if the

solder paste coverage is too big. It is recommended that smaller

multiple openings in the stencil be used instead of one big

opening for printing solder paste on the thermal paddle region.

This typically results in 50% to 80% solder paste coverage.

Figure 50 shows how to achieve these levels of coverage.

Voids within solder joints under the exposed paddle can have

an adverse effect on high speed and RF applications, as well

as on thermal performance. Because the LFCSP incorporates a

large center paddle, controlling solder voiding within this

region can be difficult. Voids within this ground plane can

increase the current path of the circuit. The maximum size for a

void should be less than via pitch within the plane. This assures

that any one via is not rendered ineffectual when any void

increases the current path beyond the distance to the next

available via.

1.35mm × 1.35mm SQUARES

AT 1.65mm PITCH

COVERAGE: 68%

Figure 50.Typical Thermal Paddle Stencil Design

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