Attachment Method for a Highly Reliable, Low IR Drop Bolted Bus Bar Interface to a Printed Circuit Board
Original Publication Date: 2001-Nov-09
Included in the Prior Art Database: 2003-Jun-20
The power requirements in high-end processor backplanes created a distribution problem. These backplanes required efficient distribution of 1700 Amps at 1.2V to four 3.4 inch X 3.4 inch multi-chip module sites through six 1/2 oz. copper planes with 30 mV or less voltage drop from the common sense point. The distribution system was also required to have low inductance to minimize the decoupling capacitance required on the backplane to absorb high frequency switching noise in the power system. The magnitude of the design point was complicated further by wiring constraints and physical obstructions from logic card connectors and L3 memory modules. These restrictions resulted in a bus bar pad design that was 0.38 inches X 3.0 inches. An illustration of the pad is shown in Fig. 1. Note: Fig. 1 only shows the general long rectangular shape. In reality, the pads come in pairs since the bus bar is a two layer laminated design. The close proximity of ground to the voltage on the bar helped to reduce the inductance and overall impedance of the power system. Due to wiring constraints in the bus bar area as well as the number of vias required to support the current from the pad to the inner planes, only three holes could be tolerated to hold the bus bar to the pad on the backplane as can be seen in Fig. 1. This invention, however, could be used over longer bus bars that would utilize more attachment points. Lab test results indicated that direct bolting alone was not sufficient to preserve the integrity of the interface over time. The bus bar contact interface integrity was preserved by incorporating a spreader plate on the backside of the printed circuit board (PCB). The illustration of the concept is shown in Fig. 2. The invention solves the interface integrity problem by distributing the load created by the bolts along the length of the bus bar pad. This allowed the interface to have more points of contact between the bus bar and the PCB which reduced the overall contact resistance of the interface. The spreader plate also helped to get closer to the ideal "gas tight seal" than could be achieved without the plate. The constant distributed pressure along the entire length of the bus bar pad in conjunction with the greater number of contact points created less opportunity for atmospheric gases to penetrate the interface and create insulative oxides. In addition, an identical bus bar pad was created on the opposite side of the PCB to make contact with the spreader plate. This sandwich effect created redundant points of contact for current to spread into the PCB. More specifically, this allowed current to travel through the screw shaft, spread along the spreader plate, and enter the PCB through the pad on the backside.