High-voltage thick-film resistors can fail when long-term electric field stress creates local heating, partial discharge, tracking, or insulation breakdown.
This article outlines layout factors that raise field stress, such as narrow sections, sharp corners, and proximity to edges, and explains how trimming can amplify these risks. It then compares air-abrasion trimming with laser trimming, focusing on edge geometry, debris, and the importance of trench filling and post-trim cleaning.
Resistor Trimming
High-voltage resistor manufacturers try to avoid trimming wherever possible, or at least minimise it. They do this by using a layout where the main HV stress path is defined by the printed resistor geometry. When trimming is used, it is only to fine-tune the resistance to the specified value and tolerance.
There are two methods of thick-film resistor trimming: laser cutting and air abrasion (discussed below). Trimming can be performed by cutting notches into the printed resistor pattern, but more often, a block of resistor material is printed as part of the resistor pattern specifically to facilitate trimming. The positioning of that block on the resistor substrate is critical.
During trimming, the manufacturer removes parts of the trim block to achieve the required resistance value and tolerance. When creating these trim cuts or patterns, they aim to avoid very narrow sections, sharp corners and tight meanders in the resistor path.
As trimming can impact resistor performance and reliability (see below), the trim area is often perceived as a potential weak spot. Hence, in laser trimming, it is often moved towards the zero-voltage (grounded) end of the resistor where the voltage is lower. This is a potential problem for single resistors if the resistor is installed incorrectly. With Air abrasion trimming the trim areas are usually located in the middle of the resistor substrate.
Laser High Voltage Resistor Trimming
Laser trimming is a closed-loop process where a laser cuts (burns away) the resistive film on a resistor while its target electrical value is measured in real time,
In laser trimming the cut is often through the resistor coating material and the resistor element in a single step.
Thick Film Resistor Air Abrasion Trimming
Air abrasion resistor trimming uses a small, high-velocity jet of abrasive particles to physically remove part of the resistive film. The rate of film removal is determined by the compressed gas pressure, particle size and flow rate. The nozzle size and distance from the substrate are critical. Unlike laser trimming, the process is usually carried out pre-coating.
A fixture holds the substrate in a precise, repeatable, X-Y position via a micrometre or motorised controller. A precision ohmmeter continuously monitors the resistor value during trimming.
The Impact Of Resistor Trimming
The following relates to flat substrate resistors. In Axial leaded HV rod resistors, helical laser trims are the standard because they naturally equalize the voltage gradient around the cylinder.
Resistor trimming issues to consider include:
- Sharp resistor track edges.
- Debris and contamination.
Sharp resistor track edges
Sharp or abrupt edges generate high localised electric fields. Once partial discharges start, they can form conductive tracks and cause progressive insulation failure over time. Repeated stress at sharp edges accelerates aging and causes early resistor failure.
Laser cuts tend to leave steep walls and sharp corners in both the resistive film (at the edge of the trimmed slot) and the overglaze where the laser broke through. With careful process control (power, pulse width and focus), edges can be rounded, but it is difficult to eliminate the problem entirely.
If the main goal is to avoid sharp resistor track edges, on high-voltage thick-film resistors, Air-abrasion trimming is generally better than laser trimming. Air abrasion thick film resistor trimming tends to deliver smooth, shallow, distributed material removal. The process tends to produce more rounded, tapered edges (both laterally and in thickness) and a slightly “feathered” transition zone between full film and no film.
Debris
Laser trim always creates debris, including ejected molten/solid particles from the resistive film, the glaze / protective coating and (if the process is poorly controlled) elements of substrate material. If that debris deposits as a semi-conductive splatter or bridges across regions of different potential, it creates parasitic resistive paths or whiskers that can cause partial discharge in high-voltage applications and increase electrical noise.
Narrow laser cuts increase the risk of debris or carbon residues bridging the gap and creating a leakage path. Wider cuts improve electrical isolation and voltage standoff between the trimmed sections, but produce more debris. Organic binders or partially decomposed compounds can turn into carbonaceous residue.
Air Abrasion trimming of HV thick film resistors can also cause debris, resulting in bridging and resistive paths as outlined above. Fine particles can cling stubbornly to ceramics and glassy films. Abrasive media and nozzle tool wear material can embed into the surface or stick electrostatically. Re-deposited dust can collect on the substrate surface, especially around cut edges.
As in laser trimming, excellent process control minimises debris issues. In Air abrasion and laser trimming, vacuum removal of material during the trimming process can help, but cleaning after the trim is critical.
Filling
Failure to fill trim trenches can result in increased leakage current, noisy behaviour, and resistance drift. An insulating filler is applied to the trench, targeting no voids and strong interface adhesion.
Any air pockets have a lower dielectric strength than a solid fill. Under high-voltage stress, these can result in micro-discharges inside the void and gradual fill degradation. Part-filled trenches leave corners, lips, and “knife edges” exposed that can concentrate the field (see above) and accelerate surface damage. Poor wetting/adhesion leads to fill separation during thermal cycling.
The laser fill process is similar, but as the laser cut is often made through the final resistor coating after laser trim, the final coating process may be localised. The final overglaze/conformal coat must wet, round, and seal the abraded zone with no pinholes/voids.
Air abrasive trimming is superior for high voltage applications due to wider cuts giving superior voltage gaps across the trim cuts as it leaves a clean areas, whilst laser trimming leaves debris and possible high voltage stress areas.
Conclusion
The choice of the trimming process largely depends on the application. In high-volume manufacturing, where speed and repeatability are critical, laser trimming is often the preferred option, as higher production volumes can offset the cost of specialist equipment and the initial set-up.
For low- to medium-volume applications that demand exceptionally high-voltage performance with minimal electric-field hot spots, air abrasion is often preferred. This assumes post-trim cleaning and inspection can be tightly controlled.