ADP3408 查看數據表（PDF） - Analog Devices

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ADP3408

GSM Power Management System

Analog Devices 

ADP3408

LFCSP Layout Consideration

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, special consideration must

be given to the layout of the PCB. Use the following layout

guidelines for the CSP package:

1. The pad pattern is given in Figure 7. The pad dimension

should be followed closely for reliable solder joints while

maintaining reasonable clearances to prevent solder bridging.

0.08

5.36 3.80

3.96 3.56

0.70

0.30

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. Also, 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 9. This pattern will result in about

80% coverage, which should not degrade the thermal perfor-

mance of the package significantly.

CREATE SOLDER PASTE WEB FOR APPROX. 80% COVERAGE

125 MICRONS WIDE TO SEPARATE SOLDER PASTE AREAS

THERMAL PAD AREA

0.50

0.20

Figure 7. LFCSP Pad Pattern (Dimensions Shown in

Millimeters)

2. 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 8 for the recommended via pattern. Note

that the via diameter is small. This is 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 VBAT. Do not con-

nect the thermal pad to ground.

Figure 9. LFCSP Paste Mask Pattern

5. The recommended paste mask stencil thickness is

0.125 mm. A laser cut stainless steel stencil with trapezoi-

dal walls should be used.

A “No Clean,” Type 3 solder paste should be used for

mounting the LFCSP package. Also, a nitrogen purge during

the reflow process is recommended.

6. The package manufacturer recommends that the reflow tem-

perature should not exceed 220ЊC and the time above liquidus

is less than 75 seconds. The preheat ramp should be

3ЊC/second or lower. The actual temperature profile depends

on the board’s density and must be determined by the as-

sembly house as to what works best.

ARRAY OF 9 VIAS

0.25mm DIAMETER 35␮m PLATING

0.60

1.18

THERMAL PAD AREA

1.18

0.60

Figure 8. LFCSP via Pattern (Dimensions Shown in

Millimeters)

3. The solder mask opening should be about 120 microns

(4.7 mils) larger than the pad size resulting in minimum 60

microns (2.4 mils) clearance between the copper pad and

solder mask.

4. 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.

–18–

REV. A

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