Ground pad

Walkenhorst, A., Hagemeyer, J., 1997. Distribution of Cd, Pb, and Zn in above ground pads of a 55-year-old spruce tree (Picea ahies [L.] KARST.). Angew. Bot. 71, 164-167. 

For packages with solder joint pitches less than 0.8mm, a mix of NSMD and SMD pads is sometimes inevitable. At small pitches, it is difficult to route the traces from the pads in the typical dog bone fashion.Thus, to allow routability, the common power and ground pads are bussed together to form one large pad.The connected pads are SMD, whereas the individual signal pads are kept NSMD. 

Due to the specific tissue resistance and the energy dispersion between the needle-like electrode and the dispersive ground pad there is a rapid decay of energy and consequently of heat around the electrode, where the temperature is inversely proportional to the distance from the electrode (r l/r T = temperature, r = radius around the electrode). This means that destructive thermal energy can be deployed sufficiently only within a volume of a maximum diameter of 2.2-2.4 cm around a single, needle-like electrode (Fig. 2.1.1). As a consequence, the energy is delivered quite precisely and focused... 

Complications related to the thermal ablation itself include unintentional thermal harm to non-targeted areas including burns at the grounding pads and interference with metallic implants such as pacemaker wires and cardioverter defibrillators. 

Fig. 5.1.3. Grounding pads properly attached to the patient s thighs, equidistant to the RF site...

First-, second- and third-degree burns at the grounding pad site during and after RFA do occasionally occur. We have experienced two severe skin burns after open liver RFA and one after percutaneous liver RFA (Fig. 5.1.10). All dispersive pads are believed to have been placed correctly none of the pads have been noticed to be improperly attached at the end of the ablation procedures. The use of higher currents for a longer period of time in RFA of tumors has led to an increased incidence of pad burns in our experience as well as that of others. 

Fig. 5.1.10. Skin burns at grounding pad site on both thighs...

The current density at the dispersive pad site is intended to be small due to the large surface area of the grounding pad. Mitchell et al. determined the minimum area of the pad for the diathermy units (Pearce et al. 1997). They suggest a maximum safe power density of approximately 1.5 W/cm. Therefore, for a maximum power output of 200 W the minimum required surface area of the dispersive pad is 133 cm. ... 

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