A Brief Description of the Technological Problem

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A very important electronic component is the electrical resistor. These components adjust most of the operating points of an electronic circuit. In the past so called variable resistors were used in such cases. A screwdriver was necessary to change its resistance.
In the frequent case of a single parameter adjustment those devices have a lot of disadvantages like:

  1. the adjustment is very difficult to automate
  2. they have a low reliability
  3. they show a drift when exposed to thermal and mechanical strain
  4. they are in contradiction to miniaturization issues
  5. they are expensive compared to film resistor solutions
SMD application sample

The Film Technology

A thick film resistor consists of a special burned-in resistor paste which is printed on ceramic. The conductors are made by the same procedure, but by using another sort of paste with a much lower resistivity. The resistor film thickness is about 25 µm.
A thin film resistor consists of materials like NiCr, SiCr or TaN. The film is typically sputtered on a glas substrate or direct on a silicon wafer in a high vacuum system. Thin film thickness ranges between 10 nm and 100 nm.
The actual resistance depends on the shape geometry and on the property of the film material. A LASER trim procedure uses the geometry dependence for the resistance change and thereby to adjust the operating points of an electronic circuit. LASER trimming is a material removing technique by vaporizing. A LASER cut into the resistor area lengthens the effective resistor length and in the same moment the effective resistor width will be shortened. This is the reason why the resistance can be only increased by this procedure. An effective cut can be placed vertical to the current flow direction, for example.
A LASER works here in a pulsed manner by power and thermal strain reasons. The LASER beam has a diameter of about 25 µm for thick film and 4...6 µm for thin film applications. The partial overlapping of the small burn spots generates a closed cut path or trim path. The following pictures show examples of cut paths on bar resistors, one of the most used resistor pattern:

sample cuts of flat rectangular resistors

Other resistor patterns are for instance:

other cut samples

The functional interdependence of the resistance and the trim path length is usualy non-linear and depends on cut path way and shape geometry. During the trim process the trim sensivity - that's the resistance change per LASER step - is neither constant nor linear. The correlation between the cut path way length and the resistance is called trim characteristic. (see the following diagramm)

trim characteristic of a hat-cut-resistor

For the purpose of an optimal resistor design in respect of trim behavior and of automation it is desirable to know the trim characteristic and the trim sensitivity (i.e. the reachable final precision) already at design stage. Because some configurations are not able to fulfill the necessary requirements, like final tolerance for example. A non trimmed film resistor has a typical tolerance of about up to 20% of the designed target resistance. To produce a reliable cut into a resistor film each single trim path should be carried out uninterrupted to avoid thermal material shocks and other harmful effects. That means especially no stops and starts, or velocity changes within a cut path way. To predict the accurate stop point of the trim path way the knowledge of each individual trim characteristic is necessary to guarantee a high and reproducible quality.
Modern trim devices monitoring the desired electronical parameters during the trim process and stop the trim process if the target values hit the specification. But measure, analyze, and stop execution needs a certain time what could cause that the accurate stop point can be missed in many cases. Because of the nonlinearity, trim functions like trim characteristic and sensitivity are needed for fast and precise stop point approximations as long as the trim process is taking place.

Now, algorithms are required to determine the trim behavior. Such algorithms can be employed as decision criterion for resistor designs and for evaluations of trim strategies in consideration of trim device and material parameters.


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