differential transmission

Consider the cross section of the differential line in Figure 1. In addition, the distance between lines is represented by G (Gap) for single transmission, and S (Space) for differential signals, but please note that this is not particularly generalized and is just my personal preference. .

For higher density

Since it is necessary to pass wiring between mounted parts on the surface layer, and especially between multi-pin ICs such as BGA, there is a demand to make the pattern width as thin as possible.

On the other hand, the pattern width considering the cost is usually about 0.1 mm, as mentioned in "How to determine the cross-sectional dimensions of the​ ​board" and "Cross-sectional dimensions of the surface layer of the board - Part 1". If it is thinner than usual, the pattern may be cut off during etching. When forming a pattern, as shown in Figure 2, the parts not covered by the resist are etched away, leaving the pattern under the resist. The result of the etch is therefore a generally trapezoidal cross-section, as shown. If the copper foil is extremely thin, the etching will be completed in a short period of time, and the degree of trapezoidal shape will be less, and a narrower pattern can be achieved.

Plating is formed on the surface copper foil in order to apply through-hole plating to the surface layer. For example, an ultra-thin surface layer can be achieved by performing "selective plating" in which plating is applied only to the holes in the through-hole plating process, or by scraping off the plating by polishing or etching. Figure 3 is an image diagram of it.

If we can make it as thin as W = 75 um, we can make the distance from GND 50 um and achieve Z0 = 50 Ω (Fig. 3, upper left diagram).

If G is also chosen to be 75 um, which is the same as W, the crosstalk coefficient can be reduced from 0.18 to 0.12 compared to W = G = 130 um (W = 75 um and W = 130um). It may be possible to reduce the number of layers by significantly improving the mounting density.

I would like to consider it and make it a reality, and I have been proposing it whenever I have the opportunity, but so far I have not received a report that it has been realized. My first reply when I made the proposal was to explain the reason why "selective plating", polishing, and etching are expensive, but I would appreciate it if someone could make it a reality.