Question from Tim in Ann Arbor, MI: You’ve mentioned through-hole vias and stubs, and their effects on high-speed serial links, but I don’t recall your mentioning blind vias. Are these a better choice than through-holes for controlling signal effects? Are there any downsides to using blind vias in place of through-hole vias?
Answer: Blind vias that start on one signal layer and terminate on the final signal layer can be designed with minimal stubs. If the signal transition is from a layer near the top of the board, a back-drilled via, with the bottom stub drilled out, could have just as good performance as a buried via.
Some backplane vendors quote a 10 to 15% premium to backdrill a board. For a board with many backdrilled vias, or with signal layer transitions deep within the board that would require backdrilling both sides of the board, buried vias may be a lower cost solution.
Another alternative is using HDI or microvias. Each via layer is built sequentially, and all stubs can be eliminated in the design. With microvias, it’s not a question of stubs, but of excess capacitance from the pads. Unlike through-hole vias that do not require capture pads on intermediate layers, microvias do require capture pads on each intermediate layer. The pads must be kept small, with optimized anti-pads or clearance holes; otherwise the capacitive load of the microvia would contribute its own rise time degradation.
Question from Frank in Santa Cruz, CA: I just got my hands on a 2D field solver and it’s giving me negative capacitance values for the coupling capacitance. Is this an error or numerical noise? If not, what does a negative capacitance mean?
Answer: The negative capacitance in the 2D field solver is perfectly correct due to the definition of capacitance used in the field solver. In circuit models, capacitance is defined from a circuit element whose current through it is proportional to the dV/dt across it.
Increase the voltage at a steady rate and there is a steady current though the capacitor. In this definition, the direction of current is from the side with the increasing voltage to the side with the decreasing voltage.
In the world of circuit elements, capacitance is always positive. Whether we use a capacitor element to define the capacitance between a signal line to its return path, or the capacitive coupling to an adjacent signal line, capacitance is always positive. Current will only flow one way through a capacitor.
The field solver capacitance, and the capacitance matrix, have a different definition. To distinguish these two different types we refer to the capacitance used in a circuit model as the SPICE capacitance and the capacitance used in a field solver as the Maxwell capacitance.
In a field solver, which is used to calculate the electric and magnetic fields around a collection of conductors, the capacitance between two conductors is defined in a very special and subtle way so as to make it easy to calculate the fields.
When calculating the capacitance between any two conductors with a field solver all the conductors except one are tied to the ground reference point. A voltage is placed on the one floating conductor.
This is illustrated in Figure 1. In this environment all the voltages on all the conductor surfaces are precisely known.
|
FIGURE 1. The field solver will apply 1V to one conductor and, keeping all other conductors grounded, calculate the charge induced on all conductors. The induced charge will always be negative.
|
Using conventional numerical simulation techniques, the electric field distribution on the surface of the conductors can be easily found. Using Gauss’s law, the charge on each conductor can be found from the fields around each conductor. The capacitance between the floating conductor and any other conductor is the ratio of the charge on the conductor to the 1V applied.
You might think that since each conductor is grounded, the charge on it should be zero. However, the presence of the adjacent conductor, charged to 1V, will suck negative charges from the ground reservoir. This means that every other grounded conductor will have negative charge on it, induced by the positive 1V applied.
Since the charge on all other conductors is negative, the Maxwell capacitance between any two signal conductors will always be negative. To convert it to the SPICE capacitance, just change the sign. PCD&M
Send questions for the Signal Doctor to This email address is being protected from spambots. You need JavaScript enabled to view it..
Dr. Eric Bogatin is president of Bogatin Enterprises.
