at88rf1354 ATMEL Corporation, at88rf1354 Datasheet - Page 38

• Manufacturer Part Number: at88rf1354
• Description: 13.56 Mhz Type B Rf Reader Specification
• Manufacturer: ATMEL Corporation

38
13.56 MHz Type B RF Reader
D.4.2.
In order to effectively remove the heat from the package and to enhance the electrical performance, the die paddle
needs to be soldered to the PCB thermal pad, 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 pad for larger size packages. Also,
out gassing occurs during the reflow process which may cause defects (splatter, solder balling) if the solder paste
coverage is too big. Therefore, it is recommended that smaller multiple openings in the stencil should be used instead
of one big opening for printing the solder paste on the thermal pad region. This will typically result in 50 to 80% solder
paste coverage. As shown in Figure D-3. some of the ways to achieve these levels of coverage.
Figure D-3. Thermal pad stencil design for 7x7 mm and 10x10 QFN packages
D.4.3.
Voids within the solder joints under the exposed pad can have an adverse effect on high speed and RF applications
as well as on the thermal performance. As the QFN package incorporates a large center pad, 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 the via pitch within the plane. This recommendation would assure
that any via would not be rendered ineffectual based on any void increasing the current path beyond the distance to
the next available via.
With regards to the voids in the thermal pad region, it should be emphasized that the presence of these voids is not
expected to result in degradation of the thermal and the electrical performance. No loss in thermal performance is
predicted from the thermal simulation of the smaller multiple voids covering up to 50% of the thermal pad area. It
should also be noted that voids in the thermal pad region do not impact the reliability of the perimeter solder joints.
Although the percentage of voids may not be a big concern, large voids in the thermal pad area should be avoided.
In order to control these voids, solder masking may be required for the thermal vias to prevent solder wicking inside
the via during reflow, thus displacing the solder away from the interface between the package die paddle and the
thermal pad on the PCB. There are different methods employed within the industry for this purpose, such as “via
tenting” (from the top or bottom side) using dry film solder mask, “via plugging” with liquid photoimagible (LPI) solder
mask from the bottom side, or “via encroaching”. These options are depicted in Figure D-4. In case of via tenting, the
solder mask diameter should be 100 microns larger than the via diameter.
Stencil Design for Thermal Pad
Via Types and Solder Voiding
1.5 mm Dia. Circles
@ 1.6 mm Pitch
Coverage: 37%
1.35 x 1.35 mm Squares
@ 1.65 mm Pitch
Coverage: 68%
1.0 mm Dia. Circles
@ 1.2 mm Pitch
@ 1.2 mm Pitch
Coverage: 50%
1.35 x 1.35mm Squares
@ 1.5 mm Pitch
Coverage: 81%
8547A−RFID−10/08