Meshed Interconnect Approach Reduces Crosstalk
John Baliga, Associate Editor -- Semiconductor International, 3/1/2001
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A typical substrate has conductor planes dedicated for power and ground. Other layers contain signal traces, which basically make them stripline waveguides. The interconnected mesh power system (IMPS) has the power and ground contained in two metal layers, but not on separate planes.
Figure 1 illustrates the IMPS construction. One metal layer has conductors running in one direction, and a second layer has conductors running in the orthogonal direction. Vias connect the layers at each crossing point (1a). Two such constructions are built in an interpenetrating, or interdigitated fashion (1b). The blue mesh is the ground mesh, and the red one is the power mesh.
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Test vehicles were constructed using co-fired ceramic substrates. The metal traces for both vehicles were built on a 10 mil (250 µm) pitch. Passive tests of the power and ground traces showed that the IMPS traces had much higher capacitance, with higher resistance and comparable inductance compared with conventional traces. The higher capacitance occurs because of the closer spacing of power and ground traces, which helps to condition the power better. Measurements of power distribution noise illustrate the reduced noise (Fig. 2), and pulsed signal-line measurements showed crosstalk to be two to three times less for the IMPS package.
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The improved performance and reduction of material each represent an improvement. As interconnect densities continue to grow, it is not sufficient to rely on the properties of new materials to enable the increased density. Design methods must be used to do more than simply route the signals from one point to another. The IMPS design approach is one example of using design to make better use of the available material to enable increased density and performance.
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