Modelling the Effect of Laser-Trimming on the Electrical Properties of Thick-film Resistors

In this paper, various thick-film resistor trimming geometries and their effects on resistor properties, including resistance value, current density distribution and perceived inhomogeneity of power density within the functional layer. Experimental EDAG 6017 SS polymer paste resistors were stencil printed on an FR4 substrate using immersion silver plated ISO 162525-2 compliant measuring pattern and cured at datasheet recommended parameters. Resistance values were measured with a DC 4-wire method, and the trims were cut using a frequency tripled Nd:YAG UVA laser; they were measured again at each step. The topology of the experimental resistor samples was recorded using an Olympus DSX100 motorised stage measuring microscope using the focus stacking method. This resulted in height (thickness) information at a fine resolution. The thickness data was used to build a 3D model geometry for finite element method (FEM) simulation of untrimmed and trimmed resistors. The prepared geometry was procedurally trimmed using the same length and spot size parameters as the laser micromachining. The various geometries' stationary current behaviour was FEM simulated using MATLAB PDEtool, and properties were determined by post-processing of nodal results. A comparison of measured and numerically approximated resistance values proves sufficient predictive power at the current abstraction level.