Programmable Substrate Inductor
Original Publication Date: 1989-Nov-01
Included in the Prior Art Database: 2005-Jan-29
Described in the following is a programmable substrate inductor (PSI). The PSI is a low-cost replacement for chip inductors on substrates and boards. Because the inductor is programmable by means of a laser trimmer or equivalent, the inductor can be fine tuned to a precise circuit inductance value. (Image Omitted)
Programmable Substrate Inductor
Described in the following is a programmable substrate inductor (PSI). The PSI is a low-cost replacement for chip inductors on substrates and boards. Because the inductor is programmable by means of a laser trimmer or equivalent, the inductor can be fine tuned to a precise circuit inductance value.
The inductance calculations for thin and thick film inductor designs can be found in a number of reference sources, e.g., 1. This design of a thin film PSI on a substrate is unique in that the material which makes up the inductor is the same material which makes up the normal signal and bus pattern lines. In most thin and thick film designs which are used on substrates or boards, the inductor coil lines are made from magnetic material. Since the magnetic material is a different material from the signal lines themselves, multiple processes must be used to make the inductors. After the completion of the inductor, the inductor may still be a +/- 20 percent tolerance component due to the process variations involved. By making the programmable inductor out of the same material as the signal or bus lines, the inductor can be constructed at the same time the standard circuit line patterns are developed on the substrate or board. The same processes are used to apply both the inductor and the signal/bus lines.
A typical substrate inductor is shown in Fig. 1. The inductor starts at the I/O pin 1 point A and wraps around the I/O pin 1 until it reaches the signal line at point B. The signal line 2 from the inductor point B adds some additional self- inductance to the total inductance seen by the remainder of the circuit at point C. As shown in Fig. 2, (depending on the number of inductor turns) from 12 to 74 nanohenries to the total inductance value. If fewer turns are used, the mutual inductance contribution between signal lines decreases and there is a slight increase in the self-inductance of the external signal line's contribution due to a longer line length between points B and C.
The basic substrate inductor can be programmed or fine tuned by two methods or the combination of the two methods. The first fine- tuning method can be understood by examining the equation for the self- inductance of a flat conductor with a rectangular cross-section over a ground plane 2. The equation is: 6
L = 5 length LN over + m
T + 0.8 W where
L is the self-inductance in nanohenries
length is the conductor length in inches
T is the conductor thickness in inches
W is the conductor width in inches
H is the height of the conductor above the
is the relative magnetic permeability of the
& is the skin depth factor
If the partial derivative is taken of the equation, the sensitivity or change in inductance due to the various parameters can be evaluated. When the partial derivative equation is evaluated with normal substrate parameter values, it can clea...