Issue link: https://iconnect007.uberflip.com/i/884055
54 The PCB Magazine • October 2017 low as possible, the inductance of wire bonds can swamp any gain. With a partial self-induc- tance of a wire bond of roughly 1 nH/mm, and typical wire bonds on the order of 1 mm long, the in- ductance of one bond can be 1 nH. To reach the 10-pH range, a minimum of 200 wire bonds would be needed just for power and ground. Microvias on an 8-mil (200 micron) pitch can provide the connections for most high pin count flip chips [or BGAs]. With a solder ball length of only 5 mils (0.15 mm), the chip attach induc- tance can be reduced over an order of magnitude from wire bonding" [2] . The HDI structure in Fig- ure 11 has ground on the outside with power as lay- er two. Microvias connect the power land to the pow- er plane. These vias are only 40 pico-henrys, while the ground connections have no inductance or capaci- tance. Figures 13a and 13b are the microvia-decoupling capacitance simulation. No- tice that the response in Fig- ures 13c and 13d are almost flat. They are almost invisi- ble to the circuit and do not affect the energy storage ca- pability. Radiation When resonances are present in a PDN, the ener- gy usually escapes out of the end of the boards, or some components can even act as antennas and cause electro- magnetic interference (EMI). As frequencies go up, this ra- diation becomes a bigger is- sue. It has been demonstrat- ed that less noise in the PDN results in less EMI. Taking the 14-layer PCB as an example (from Ch5-HDI Handbook), we measured the radia- HDI'S BENEFICIAL INFLUENCE ON HIGH-FREQUENCY SIGNAL INTEGRITY Figure 12: Simulations of TH vias on a bypass capacitor taking into account pads, traces, and vias (a, b). Results of the simulation show- ing (c) phase and (d) impedance. (Courtesy: C. Grasso, ANSOFT) Figure 13: Simulations of a microvia bypass capacitor taking into account pads, traces, and vias (a, b). Results of the simulation show- ing (c) phase and (d) impedance. (Courtesy: C. Grasso, ANSOFT)