ADP2291

Rev. 0 | Page 16 of 20

PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS

Use the following general guidelines when designing printed

circuit boards:

•

Keep the output capacitor as close to the BAT and GND

pins as possible.

•

Keep the input capacitor as close to the IN and GND pins

as possible.

•

PC board traces with larger cross-sectional areas remove

more heat from the pass transistor. For optimum heat

transfer, specify thick copper and use wide traces.

•

Use additional copper layers or planes to reduce the

thermal resistance. When connecting to other layers, use

multiple vias if possible.

LFSCP LAYOUT CONSIDERATIONS

The CSP package has an exposed die paddle on the bottom that

efficiently conducts heat to the PCB. In order to achieve the

optimum performance from the CSP package, give special

consideration to the layout of the PCB. Use the following layout

guidelines for the CSP package:

•

The pad pattern is shown in Figure 27. Follow the pad

dimension closely for reliable solder joints while

maintaining reasonable clearances to prevent solder

bridging.

•

The thermal pad of the CSP package provides a low

thermal impedance path (approximately 20°C/W) to the

PCB. Therefore a properly designed PCB effectively con-

ducts the heat away from the package. This is achieved by

adding thermal vias to the PCB that provide a thermal path

to the inner or bottom layers. Note that the via diameter is

small to prevent the solder from flowing through the via

and leaving voids in the thermal pad solder joint.

Note that the thermal pad is attached to the die substrate,

so the thermal planes that the vias attach the package to

must be electrically isolated or connected to GND.

•

The solder mask opening should be about 120 microns

(4.7 mils) larger than the pad size, resulting in a minimum

of 60 microns (2.4 mils) clearance between the pad and the

solder mask.

•

The paste mask opening is typically designed to match the

pad size used on the peripheral pads of the LFCSP package.

This should provide a reliable solder joint as long as the

stencil thickness is about 0.125 mm. The paste mask for the

thermal pad needs to be designed for the maximum cover-

age to effectively remove the heat from the package. How-

ever, due to the presence of thermal vias and the size of the

thermal pad, eliminating voids may not be possible.

•

The recommended paste mask stencil thickness is

0.125 mm. Use a laser cut stainless steel stencil with

trapezoidal walls.

•

Use a no clean, Type 3 solder paste for mounting the

LFCSP package. A nitrogen purge during the reflow

process is recommended.

•

The package manufacturer recommends that the reflow

temperature not exceed 220°C and the time above liquidus

is less than 75 seconds. Make sure the preheat ramp is

3°C/second or lower. The actual temperature profile

depends on the board’s density and should be determined

by the assembly house.