Conductor width

Figure 1. Circuit pattern for hydrolytic stability testing. Circuit Tin/lead over copper. Conductor width 0.7 mm. Space between conductors 1.2 mm.

A series of standard rectangular conductors, each with a 3 1 aspect ratio, a twist pitch of 7 to 10 times the conductor width, and either 10 or 14 weight % (w/o) tin in the bronze phase, were reacted at 600 to 700°C from 6 to 108 hr. The number of filaments varied from 67,507 to 259,369, and the initial Nb filament diameter varied from 4.0 to 4.5 fim. 

The circuit configuration (microstrip, stripline, etc.) is the basic structure of the interconnect. The general relationship of the physical parameters to impedance is such that impedance is an inverse function of conductor width, dielectric constant, and conductor thickness, and a direct fxmction of the dielectric thickness or spacing between the signal conductor and the impedance reference planes. 

PWB manufacturers are reasonably comfortable with the production of boards with 4- or 5-nul-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/in per plane, as seen from Table 2.4. A technology that wiU 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/in. Conductor widths of 0.002 in will offer a PWB of 200 to 250 in/in per signal plane. 

An obvious method of increasing the connectivity of PWBs is to reduce the widths of conductors and spaces and thus increase the number of available wiring channels on each signal plane, as described previously. This is the direction that has been used in the IC and PWB industries for many years. However, it is impossible to decrease conductor widths or spaces indefinitely. The reduction of the conductor width is limited by the current-carrying capacity of thin, small conductors, especially when these conductors are long, as they frequently are on PWBs. There are processing limits to this conductor reduction, since manufacturing yields may plummet if the reduction stretches the process capabilities beyond their normal limits. 

Still, such conductor reductions, if achieved within the described Umits, can be an effective path for increasing the PWB density and the reduction of PWB mannfactnring costs. As seen from Table 2.6, constructed from cost data derived from the Columbus program of BPA, the reduction of conductor widths from 6 to 3 mils halves the number of signal layers necessary to... 

TABLE 2.6 Effect of Conductor Widths on Number of Layers and Board Cost for a 6-in x 8-in MLB, with I4 = 450 in/in, 65 to 68 Percent Yields... 

In a well controlled manufacturing operation, the process-dependent yields (such as plating) remain fairly constant for a given technology, permitting the yield function to be based solely on the changes in the conductor widths. 

FIGURE 2.12 Cost relationships between number of layers and conductor widths. 

The panel-plating method is ideal for bare copper board. However, it is a difficult way to make padless via holes, which are becoming more popular. Generally, the conductor width of 0.004 in is considered to be the minimnm realizable by this method for mass prodnction. 

The lower chart in Hg. 16.1 is used to determine the trace width for various copper thicknesses. For the same cross-sectional area, the width will be smaller for thick copper and wider for thin copper. Continuing with the example, follow the vertical fine down from 200 sq. mil. into the lower chart to the fine labeled (1 oz/fF ) 0.0014 see the discussion of copper thickness in Section 16.4.5. Following the Une across to the axis labeled CONDUCTOR WIDTH IN INCHES shows that the trace should be 0.15 in. wide for a copper trace that is 0.0014 in. thick. If 3 oz. copper were selected, the trace width wonld be approximately 0.05 in. wide rather than 0.15 in. wide for 1 oz. copper. 

For single conductor applications, a chart may be used directly for determining conductor widths, conductor thickness, cross-sectional area, and current carrying capacity for various temperature rises. 

Conductors Shape and arrangement of both conductors and nonconductor patterns, thickness, dimensions, and tolerances, including conductor width and spacing allowances. 

Conductor width (W) The finished width of the conductor can produce variance, which is likely to occur from lot to lot. Therefore, process control measures are necessary when producing controlled impedance circuits.The density of neighboring circuits will have an effect on the final etch. Often it is advantageous to modify the artwork line width at phototool generation for predicted variance. 

Conductor thickness (T) The conductor thickness or copper weight can also affect the final impedance value. Here, as with conductor width, manufacturing process variations can have an inverse effect on the precision of the impedance value. Some modem software simulation tools, such as those from Polar Instruments, Inc., allow values for conductor profile (area) to be included in the simulation. This is more significant when lines are of heavy copper. It is best to avoid routing of impedance lines on plated subcores due to the added variability. 

Condnctive pattern imperfections such as loss of adhesion, reduction of conductor width or thickness due to nicks, pinholes, scratches, surface plating, or coating imperfections... 

FIGURE 51.6 Overall conductor width (OCW) measurement W. (Courtesy ofIPC.)... 

Conductor Width. This attribnte is measured at the widest point of the circuit in the microsection. The procurement docnmentation should state the minimum allowable width. (See Fig. 51.6)... 

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