Signal layer

A variety of transmission line structures can be fabricated in planar layers of conductor and dielectric (Figure 9). The stripline and offset stripline are best suited for multilayer structures. The offset stripline, with two orthogonal signal layers between a pair of reference voltage planes, eliminates one intermediate plane and achieves higher characteristic impedance for a given dielectric thickness than do two stripline layers but increases the possibility for crosstalk between layers. 

Polyimide Thin-film Signal layers with Ground Planes... 

Signal Layer Signal Layer Ground Mesh... 

The capability to layer interconnection levels is critical for many complex circuit designs. One particular application called for assembling devices on a silicon wafer with solder connections. The necessary interconnection density was obtained by using two signal layers above the power plane separated by a dielectric Iayer.(j3... 

Four signal layer dual stripHne construction with shared reference plane. 

SIGNAL LAYER REFERENCE PLANE REFERENCE PLANE... 

Printed wiring board density the amount of wiring a PWB has as measured by the average length of traces per square inch or the area of that board, including all signal layers. The metric is inches per square inch. 

T = tracks per channel L = nnmber of signal layers G = channel width... 

Table 2.5 illustrates the most important result of increased connectivity per layer a reduction in the number of signal layers needed to provide the same wiring density W. Table 2.5 was constructed by applying connectivity data from Table 2.4 to a 50-in MLB with total wiring length of 10,000 in. Note also that the layer count in Table 2.4 has been brought up to the next higher full-layer value, i.e., the calculated 1.4 layers have been recorded as 2 layers. 

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... 

Line-space Total no. of layers No. of signal layers Board cost, %... 

Table 2.6 also shows that any increase in the number of signal layers in boards operating at frequencies reqniring transmission line characteristics will double the total number of layers, due to the need to interleave gronnd or DC power planes between signal planes. 

Placement tools range from completely manual to fully automatic. All have some form of graphical feedback to the designer that assesses the quality of the placement in terms of its ability to be routed or connected in the desired number of signal layers. Most have spacing rules that ensure that the components have enough room between them for successful assembly, rework, and testing. 

The PCB trace thickness is another important parameter that interacts with the trace length and width. If the traces are thick, they offer less thermal resistance to heat transfer. If the traces are thin, their thermal resistance is increased and the heat will not spread as far. For best thermal performance, use the thickest possible Cu foil material with the thickest possible electroplating. Unfortunately, the trace thickness is often specified to achieve the best possible etch performance for ti t pitch routing, which in tnrn gives the smallest PCB with the fewest signal layers and lowest cost. Within these limits, ensnre that Cn traces are as thick as possible. Use them to provide direct thermal condnction paths to thermal featnres snch as thermal vias, thermal side rails, or thermal condnction screw holes. 

Top Cu. Layer is a positive signal layer named sigf... 

Structure This factor determines the number of signal layers and the combination of through and buried vias that permit interconnection between layers and the complex blind, stacked, and variable depth vias available in high-density interconnection (HDI) technologies. 

To calculate a potential set of design rules and signal layers, first the wiring demand (Wj) should be calculated. Wiring models help accomplish this. 

Figurel9.13 advises two tracks on two signal layers (nsing Eq. 19.9).

The wiring demand indicates that a 0.007 in trace and a 0.008 in spacing(two track) for a 0.100 in grid (channel) represent more density than is required (see Sec. 19.4.5). A one-track wiring on two signal layers could achieve the required wiring density or a 0.012 in trace... 

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