November_EDFA_Digital

edfas.org 15 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 4 base plate at the bottomside and the charged antenna at the front side, where the RFID chip is going to be placed in the next assembly step. In the worst case, both types of antennas are printed onto the foil. With the latest RFID generation, devices now include both near and far field operation capabilities. For low frequency (near field), the coil is used while the dipole operates the device for UHF operation. In this case, the device is connected to four pins: two for the coil antenna with a backside contact through the foil and two for the dipole. Without useful ESDprecautions inmanufacturing, such a setup may include a very high risk of ESD-related destruction (Fig. 3): While the coil antenna is grounded through the backside contact and bridge, the dipole antenna is charged to the electrostatic potential of the nearby plastic. When placed onto the foil, the chip is faced with two pins at ground level and two additional pins at high electrostatic charge with the energy of a charged foil capacitor behind. Therefore, the discharge path starts entering the chip at the dipole pads and leaves it through the coil pads directly to ground. This event can neither be described by the ESDmachinemodel (MM) nor by the charged device model (CDM). The power of such a discharge is orders of magnitude higher than when the discharge is basedon the surface capacitanceof thedevice itself, as in a CDM discharge. As a result, CDM-specified, chip-internal ESD protection structures cannot copewith the scenario described here. Some assembly lines that produce smart cards have reported a few periods with as much as two-digit percentiles of unclarified yield loss, most of it associated with nonspecific ESD. However, thorough investigations were never conducted. Most tool manufacturers are suf- ficiently experienced with the mechanical challenges of high-throughput smart card production. Unfortunately, only basic knowledge exists regarding electrostatic effects and the related risks of high-speed processing of plastic foils—assuming they don’t interfere with mechanical issues such as electrostatic adhesion in stacking and separation. After themarriage of chip and antenna(s) (the secondproduction stepafter unrolling foilswithprintedor glued antennas), the antenna/chip foil is cut (third step), and stacked (fourth step) with some 10-20 further blank foils in order to achieve the standardized card thickness. These foil stacks are then pressed between metal plates and thepackage is exposed toaplasticmelting andbaking process within a hot furnace (Fig. 4). The metal plates also become slightly baked onto the plastic stack, which gives them strong adhesion when they are removed after furnace baking. Themetal plates, with the plastic stack in Fig. 2 Printed RFID coil on foil. In order to close the coil loop electrically, a backside connection is required, including two contacting vias through the foil. Fig. 3 Vertical setup during the assembly, also showing potential ESD paths. While the coil antenna is grounded, the dipole antenna is charged. The discharge takes place through the chip. If the nozzle is dissipative, some of the charge will pass through the substrate. Fig. 4 Foil stacks betweenmetal plates (top); stacks baked in a furnace (bottom).