Table of Contents:

Definition and Overview

1. Introduction

2. Broadside versus Edge-Coupled Differential Pairs

3. Point of Diminishing Returns: Line Width Increases

4. Mechanical Materials Properties

5. Launch Issues

6. System Measurements

7. Conclusion

8. Appendix

Self-Test

Glossary

PDF of this tutorial

After transmission through the PCB, launches were isolated as another effect on data transmission. In addition to stubbing effects, the parasitic parameters of the PTH cause electrical degradation. The goal was to optimize the inductance and capacitance of the PTH so that the launch behaved as an ideal transmission line. As backplane systems have large barrels and long PTHs as result of board thickness, lumped capacitance was used as a good first cut.

Capacitive launches act as a low-pass filter. The effect of this filter is to prohibit the transmission of high frequencies.

Using a different type of plated hole, such as micro-vias, can minimize many of these effects. Traditionally these have induced a cost penalty. Blind vias takes care of the electrical capacitance and stubbing problems; however, launching into the board onto any layer becomes a problem. To correct this, buried vias could be used to distribute the signal into deeper layers. Implementing blind and buried vias requires multiple lamination and processing steps yielding higher costs. Four different launches were explored for electrical gain (Figure 7).

Figure 7. Signal Launch Concepts

The standard PTH shown in the picture is a 0.022-inch finished hole. This is the specified hole size for Teradyne's press-fit VHDM family of connectors used in the system simulations and in the connector individual section of the report. The second type of PTH shown is a pad-type via. This type of hole would be used with a pressure-mount or a surface mount solder-attach-type of connector. The third PTH is a standard PTH that has been sent through drilling again, adding a secondary drilling operation after plating. This removes the unused portion of the via below the layer where the signals are routed out. The fourth type of hole is a semi-intrusive surface-mount technology (SISMT); a press-fit version of this hole (PRESMT) is also on the drawing board.

The standard PTH is a baseline measurement. The pad-type via shows electrical promise for reducing capacitance theoretically. The board thickness will determine the minimum drilled-through-via diameter due to plating aspect ratios. For example, a 0.250-inch board and a 12:1 aspect ratio yields a 0.021-inch drill and a 0.017-inch finished hole size. The pad itself has additional capacitance, so much of what is picked up in the smaller PTH is lost again on the pad. Based on the simple capacitance comparison, the only way to realize an electrical advantage from the padded system is to use it in conjunction with a blind and buried via or micro-via. Because of line widths needed in backplane systems and thick dielectric layers, the 1:1 or 1:2 aspect ratio needed for micro-vias is not always possible. As stated before, blind, buried, and micro-vias limit the depth to which signals can be launched into the board.

The counter-bored PTH has several advantages. Because part of the plating is drilled out, the capacitance and the stub are both reduced. Routing is not as efficient as micro or blind vias; the intent of this method, however, is to provide an electrically equivalent blind via at a lower cost. A press-fit-type connection requires approximately 100 mils of plated barrel length. On a very thick board, this type of launch could be used with press-fit pins. Table 2 shows measured data for different amounts of plating removed from a standard VHDM 0.022-inch plated hole. From the data, it is apparent that this technique can drastically reduce the capacitance of the launch.

Finally, the SISMT/PRESMT–type of PTH allows routing to all layers and reduces capacitance but does not take care of stubbing effects. This graduated hole allows for smaller drills in the lower section of the hole, and if the top section is long enough, a press-fit-type connection can be used (PRESMT in Figure 8). Otherwise a stubby pin solder termination to the large diameter barrel can be used (SISMT in Figure 8).

Figure 8. PRESMT/SISMT