ADP1829ACPZ-R7 datasheet(26/32 Pages) AD

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ADP1829

Rev. 0 | Page 26 of 32

LFCSP PACKAGE CONSIDERATIONS

The LFCSP package has an exposed die paddle on the bottom

that efficiently conducts heat to the PCB. To achieve the

optimum performance from the LFCSP package, give special

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

guidelines for the LFCSP package.

The pad pattern is given in Figure 36. The pad dimension

should be followed closely for reliable solder joints while

maintaining reasonable clearances to prevent solder bridging.

•

The thermal pad of the LFCSP package provides a low

thermal impedance path to the PCB. Therefore, the PCB

must be properly designed to effectively conduct the heat

away from the package. This is achieved by adding thermal

vias to the PCB, which provide a thermal path to the inner

or bottom layers. See Figure 36 for the recommended via

pattern. Note that the via diameter is small. This prevents

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 planes that the thermal pad is connected 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

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 coverage to effectively remove the heat

from the package. However, due to the presence of thermal

vias and the large size of the thermal pad, eliminating voids

may not be possible. In addition, if the solder paste

coverage is too large, solder joint defects may occur.

Therefore, it is recommended to use multiple small

openings instead of a single big opening in designing the

paste mask. The recommended paste mask pattern is given

in Figure 36. This pattern results in about 80% coverage,

which should not degrade the thermal performance of the

package significantly.

•

The recommended paste mask stencil thickness is 0.125 mm.

A laser cut stainless steel stencil with trapezoidal walls

should be used.

•

A no clean, Type 3 solder paste should be used for mounting

the LFCSP package. In addition, 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 liquid be

less than 75 seconds. The preheat ramp should be 3°C/sec

or lower. The actual temperature profile depends on the

board density; the assembly house must determine what

works best.

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