AT02876: Atmel REB212BSMA Hardware User Manual [APPLICATION NOTE]
Atmel-42097B-WIRELESS-AT02876-REB212BSMA-Hardware-User-Manual_ApplicationNote_062014
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Figure 5-3.
Board Layout – Transceiver GND Routing
The soldering technology used allows the placement of small vias (0.15mm drill) within the ground paddle
underneath the chip. During reflow soldering, the vias get filled with solder, having a positive effect on the
connection cross section. The small drill size keeps solder losses within an acceptable limit. During the soldering
process vias should be open on the bottom side to allow enclosed air to expand.
5.4 Digital GND Routing and Shielding
With the Atmel AT86RF212B, consider pins 12, 16, 18, and 21 as digital ground pins.
Digital ground pins are not directly connected to the center paddle. They may carry digital noise from I/O pad
cells or other digital processing units within the chip.
In case of a direct paddle connection, impedances of the paddle ground vias could cause a small voltage drop for
this noise and may result in an increased noise level transferred to the analog domain.
There is a number of pro’s and con’s when it comes to the shielding topic. The major con’s are:
•
Cost of the shield
•
Manufacturing effort
•
Inaccessibility for test and repair
The number of pro’s might be longer but the cost argument is often very strong. However, the reasons to add the
shield for this reference design are:
•
Shield is required for a certification in Japan
•
Shield is recommended for FCC certification in North America
•
Increased performance
Besides the function to provide supply ground to the individual parts, the ground plane has to be considered as a
counterpart for the antenna. Such an antenna base plate is required to achieve full antenna performance. It has
to be a continuous, sustained metal plate for that purpose. The shield, covering the electronic section will help to
form this antenna base plate.
For that reason, any unused surface should be filled with a copper plane and connected to the other ground side
using sufficient through-hole contacts. Larger copper areas should also be connected to the other side layer with
a grid of vias. This will form kind of a RF sealing for the PCB material. Any wave propagation in between the
copper layers across the PCB will become impossible. This way, for an external electromagnetic field, the board
will behave like a coherent piece of metal.
