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METHODS TO INCREASE PWB DENSITY
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There are three basic ways to increase the connectivity or available conductor capacity of PWBs:8
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Reduce hole and pad diameters Increase the number of conductive channels between pads by reducing the widths of the conductors Increase the number of signal planes
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The effect of each approach on manufacturing yields, and thus on board costs, will be discussed in sequence. It should be noted that the last option is the simplest but the most costly solution, and thus should be used only after the methods suitable for resolving the first two conditions have been proven inadequate for achieving the desirable board density.
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ELECTRONIC PACKAGING AND HIGH-DENSITY INTERCONNECTIVITY
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Effect of Pads on Wiring Density The major obstacles preventing increase of conductor channel capacity are large pad diameters around the plated through-holes (PTHs), since, at the present state of technology, PWBs still require pads wider than the conductors at their location. These pads reduce the obtainable connectivity of PWB boards and must be accounted for in a proper analysis of interconnection density Id. For instance, in one design, the reduction of pad diameters from 55 to 25 mils (by 55 percent) doubled the interconnection density, while the reduction in conductor pitch Cp from 18 to 7 mils (by 61 percent) increased it only by 50 percent. It is obvious that the reduction of pad diameters, or their total elimination, could be a more efficient way to increase the wiring capacity of complex PWBs. The purpose of copper pads surrounding the drilled holes in PWBs is to accommodate any potential layer-to-layer or pattern-to-hole misregistrations and thus prevent any hole breakout outside the copper area of the pads. This misregistration is caused mainly by the instability and movement of the base laminate during its processing through the PWB or multilayer board (MLB) manufacturing steps. The base material standards specify that such movement be limited to a maximum of 300 ppm, but the actual base material excursions are closer to 500 ppm, producing 10 mils of layer movement within a 20-in distance. For many applications this tolerance is too wide, as it requires at least a 10-mil-wide annular ring around drilled holes, resulting in considerable conductor channel blockage. Another cause of material instability in MLBs is the excessive material movement that occurs if the laminating temperature exceeds the glass transition temperature Tg of the laminate resin. On the other hand, if the laminating temperature remains below the Tg of the resin, there is minimal dimensional variation of the base material, as the resin is still in its linear expansion phase. This explains the need for use of high-Tg resins in the PWB industry. The data obtained from the performance of new, more stable unidirectional laminates indicate that the base material movement is reduced, for instance to 200 ppm from 500 ppm, and the requirements for the annular ring width will be reduced to 4 mils from 10 mils. Table 2.4 illustrates the connectivity gains made possible when a more stable laminate material is used, permitting a reduction in the initial diameters of the pads (as given in the first column) spaced at 2.5 mm (0.100 in) while keeping the conductor pitches constant. The most effective use of the signal plane area is achieved when the pads are eliminated and the z-axis interconnects are confined within the width of the conductors forming the invisible vias. This derivation is based on actual data obtained from the performance of new, more stable, unidirectional laminates. While MLBs using these new, more dimensionally stable, unidirectional laminates with reduced pad diameters could be manufactured by conventional manufacturing methods, the production of MLBs with invisible vias requires the use of a sequential manufacturing process similar to the SLC process previously described. PWB manufacturers are reasonably comfortable with the production of boards with 4- or 5-mil-wide conductors, but they still require large pads around plated holes to ensure against hole breakout. This limits the currently available wiring density to about 40 to 60 in/in2 per plane, as seen from Table 2.4. A technology that will permit PWB manufacturers to fabricate invisible vias could increase the connectivity per PWB signal plane from this current range to the level of 100 to 140 in/in2. Conductor widths of 0.002 in will offer a PWB of 200 to 250 in/in2 per signal plane.
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TABLE 2.4 Effect of Pad Diameters on Interconnectivity Density Pad dia, in 0.055 0.036 0.025 0.025 Cond pitch, in 0.010 0.018 0.009 0.007 Id @ 500 ppm, in/in2 20 30 40 60 Id @ 200 ppm, in/in2 37 48 96 130 Id @ invisible via, in/in2 55 55 100 143
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