at88rf1354 ATMEL Corporation, at88rf1354 Datasheet - Page 36

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

D.3.
36
13.56 MHz Type B RF Reader
PCB Design Guidelines
As shown in Figure D-1. the lands on the package bottom side are rectangular in shape with rounded edges on the
inside. Since the package does not have any solder balls, the electrical connection between the package and the
board is made by printing the solder paste on the board and reflowing it after the component placement. In order to
form reliable solder joints, special attention is needed in designing the board pad pattern and the solder paste printing.
D.3.1.
Typically the PCB pad pattern for a package is designed based on guidelines developed within a company or by
following industry standards such as IPC-SM-782. However, since the QFN is a new package and the industry
guidelines have not been developed yet for a PCB pad pattern design, the development of proper design
considerations may require some experimental trials.
IPC’s methodology is used here for designing the PCB pad pattern. However, because of the exposed die paddle
and the package lands on the bottom side of the package, certain constraints are added to IPC’s methodology. The
pad pattern developed here includes considerations for lead and package tolerances.
D.3.2.
The QFN package is designed to provide superior thermal performance. This is partly achieved by incorporating an
exposed die paddle on the bottom surface of the package. However, in order to take full advantage of this feature,
the PCB must have features to effectively conduct heat away from the package. This can be achieved by
incorporating a thermal pad and thermal vias on the PCB. While a thermal pad provides a solderable surface on the
top surface of the PCB (to solder the package die paddle on the board), the thermal vias are needed to provide a
thermal path to the inner and bottom layers of the PCB to remove the heat.
Normally, the size of the thermal pad should at least match the exposed die paddle size. However, depending upon
the die paddle size, this size needs to be modified in some cases to avoid solder bridging between the thermal pad
and the perimeter pads. The thermal pad design on the board should be based on the exposed paddle area,
excluding the ring area.
In order to effectively transfer heat from the top metal layer of the PCB to the inner and bottom layers, thermal vias
need to be incorporated into the thermal pad design. The number of thermal vias will depend on the application, the
power dissipation, and the electrical requirements. It is recommended that an array of thermal vias should be
incorporated at a 1.0 to 1.2 mm pitch with a via diameter of 0.3 to 0.33 mm. For optimum heat transfer it is
recommended that a minimum of nine vias be placed in the thermal pad, and a 1 ounce copper thickness be used on
all PCB layers on AT88RF1354 readers.
D.3.3.
The pads on the printed circuit board are either solder mask defined (SMD) or non solder mask defined (NSMD).
Since the copper etching process has tighter control than the solder masking process, NSMD pads are preferred over
SMD pads. Also, NSMD pads with the solder mask opening larger than the metal pad size improves the reliability of
the solder joints, as solder is allowed to wrap around the sides of the metal pads. For these reasons, the NSMD pad
is recommended for perimeter lands.
The solder mask opening should be 120 to 150 microns larger than the pad size resulting in 60 to 75 micron
clearance between the copper pad and the solder mask. This allows for solder mask registration tolerances, which
are typically between 50 to 65 microns, depending upon the board fabricators' capabilities. Typically each pad on the
PCB should have its own solder mask opening with a web of solder mask between the two adjacent pads. Since the
web has to be at least 75 microns in width for the solder mask to stick to the PCB surface, each pad can have its own
solder mask opening for a lead pitch of 0.5 mm or higher. However, for finer pitch parts, not enough space is
available for the solder mask web in between the pads. In such cases, it is recommended to use the “trench” type
solder mask opening where a big opening is designed around all the pads on each side of the package with no solder
mask in between the pads, as shown in Figure D-2. It should also be noted that the inner edge of the solder mask
should be rounded, especially for the corner leads to allow for enough solder mask web in the corner area.
Perimeter Pads Design
Thermal Pad and Via Design
Solder Masking Consideration
8547A−RFID−10/08