ADN8102ACPZ-R7 datasheet(30/36 Pages) AD

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ADN8102

Rev. C | Page 30 of 36

Stencil Design for the Thermal Paddle

To effectively remove heat from the package and to enhance

electrical performance, the thermal paddle must be soldered

(bonded) to the PCB thermal paddle, preferably with minimum

voids. However, eliminating voids may not be possible because

of the presence of thermal vias and the large size of the thermal

paddle for larger size packages. Also, outgassing during the

reflow process may cause defects (splatter, solder balling) if the

solder paste coverage is too big. It is recommended that smaller

multiple openings in the stencil be used instead of one big

opening for printing solder paste on the thermal paddle region.

This typically results in 50% to 80% solder paste coverage.

Figure 47 shows how to achieve these levels of coverage.

Voids within solder joints under the exposed paddle can have

an adverse effect on high speed and RF applications, as well as

on thermal performance. Because the LFCSP package incorpo-

rates a large center paddle, controlling solder voiding within

this region can be difficult. Voids within this ground plane can

increase the current path of the circuit. The maximum size for a

void should be less than via pitch within the plane. This assures

that any one via is not rendered ineffectual when any void

increases the current path beyond the distance to the next

available via.

1.35mm × 1.35mm SQUARES

AT 1.65mm PITCH

COVERAGE: 68%

Figure 47. Typical Thermal Paddle Stencil Design

Large voids in the thermal paddle area should be avoided. To

control voids in the thermal paddle area, solder masking may be

required for thermal vias to prevent solder wicking inside the

via during reflow, thus displacing the solder away from the

interface between the package thermal paddle and thermal

paddle land on the PCB. There are several methods employed

for this purpose, such as via tenting (top or bottom side), using

dry film solder mask; via plugging with liquid photo-imagible

(LPI) solder mask from the bottom side; or via encroaching.

These options are depicted in Figure 48. In case of via tenting,

the solder mask diameter should be 100 microns larger than the

via diameter.

(A)

(B)

(D)

(C)

VIA

SOLDER

MASK

COPPER

PLATING

Figure 48. Solder Mask Options for Thermal Vias: (a) Via Tenting from the

Top; (b) Via Tenting from the Bottom; (c) Via Plugging, Bottom; and (d) Via

Encroaching, Bottom

A stencil thickness of 0.125 mm is recommended for 0.4 mm and

0.5 mm pitch parts. The stencil thickness can be increased to

0.15 mm to 0.2 mm for coarser pitch parts. A laser-cut, stainless

steel stencil is recommended with electropolished trapezoidal

walls to improve the paste release. Because not enough space is

available underneath the part after reflow, it is recommended

that no clean Type 3 paste be used for mounting the LFCSP.

Inert atmosphere is also recommended during reflow.

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