ADP1823ACPZ-R7 datasheet(26/32 Pages) AD

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ADP1823

Rev. A | Page 26 of 32

LFCSP PACKAGE CONSIDERATIONS

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

efficiently conducts heat to the PCB. 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 LFCSP package.

•

The pad pattern is given in Figure 35. The pad dimension

should be followed 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 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 35 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 over a single

big opening in designing the paste mask. The recommended

paste mask pattern is given in Figure 35. 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 should not exceed 220°C and the time above

liquid is less than 75 seconds. The preheat ramp should be

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

depends on the board density; the assembly house must

determine what works best.

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