ADP1823 Datasheet, PDF (26/32 Pages) Analog Devices – Dual, Interleaved, Step-Down DC-to-DC Controller with Tracking

ADP1823
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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