Ordinary thick-film resistors can display varying qualities depending on how the manufacturer elected to trim the device.
It pays to know what to look out for.

Kory Schroeder, Stackpole Electronics Inc.
Surface-mount thick-film chip resistors are the predominant type of resistor in electronic circuits today. This technology is inexpensive; thick-film resistors are produced by screen-printing a special paste (a mixture of glass and metal oxides) on to a ceramic substrate. Depending on the size of the chip, hundreds or thousands of parts may be processed simultaneously. The screen printing process generally yields resistors within 5% to 20% of the required value, so manufacturers calibration-trim the resistors by laser to get the right value without slowing resistor production.

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There are different laser trim designs and shapes which present different manufacturing challenges and that yield different strengths and weaknesses.

The process of laser trimming thick-film resistors involves pulsing a round laser beam into the cured thick-film material. The laser light vaporizes the material under and around the beam. Removal of resistor material slightly raises the resistance value. Subsequent pulses remove more material and further raise the value of the resistance.

The simplest and most obvious trim shape is the single plunge or straight cut. Of all the trim shapes it can be made the fastest but is also less precise than some other trim designs. The single-plunge trim is challenging for high precision because the further the cut into the element, the greater the effect. This trim type isn&#;t practical if the initial element value is far below the desired value because it will be difficult to adjust the resistance value without overshoot. However, a single-plunge trim is economical when the initial resistance is close to the final resistance value&#;trimming doesn&#;t take long and it is relatively easy to control the resistance value. But the trim speed for single-plunge trim tends to be somewhat slower than for other types though overall trim time is less.

Pulse-handling resistors should have little if any trimming. Current crowding effects&#;peaks in the current density&#;arise in the area around the trim, and that is the failure point for thick-film resistors under pulse conditions. So pulse-handling resistors are generally printed to close to their desired resistance. The single plunge is a effective trim shape for these types of resistors, but it is often too imprecise for general purpose and commodity chip resistors.

The double-plunge cut provides much more precision than single-plunge cuts. The first cut brings the resistance value close to the desired final value. The second cut is in an area with less current flow, so it changes the resistance much less, allowing for precise resistance control. The second trim length is kept between 50% to 80% of the length of the first cut to ensure stable electrical performance and continuous power handling. The precision for this trim shape depends only on the second cut, so the machine trim speed can be faster than for the single plunge. Many thick-film resistors are trimmed in this manner, especially those with smaller elements.

L trim is another common trim shape. This shape can yield a precision resistance while also providing a greater range of resistance value adjustment. The initial trim into the part stops when the resistance value begins to change rapidly. Then the laser changes direction and proceeds toward the opposite termination.

The second part of the trim allows for precision control because the current crowding arises only along the first part of the trim, so the second part causes much less change in resistance. L trimming can usually take place at the same speed as a double-plunge trim. The L trim is generally regarded as a more stable trim than single or double plunge but requires a somewhat larger resistor area for effective value calibration. Most thick-film resistors currently on the market will have either a plunge or L cut trim.

Serpentine cuts may be used to adjust resistance to a value significantly higher than the printed resistance. Trims are spaced equally apart and continue until the resistance value is reached. This trim type will have higher parasitic noise, lower overall stability, poorer pulse performance. And the resistance material must be long enough to permit adequate value adjustment. Thus serpentine cuts are most effective for high resistances where power and current handling requirements are irrelevant.

Serpentine cuts are also occasionally used for adjusting metal plate sense resistors. These resistors generally use resistive material comprised of exotic alloys such as manganese-copper or nickel-chromium-aluminum to obtain special properties such as low inductance or low thermal EMF. Even when timed with serpentine patterns, their element mass and pulse capability stays roughly the same despite the length of the trims and the substantial amount of material removed.

Top hat trims&#;so named because the resistor material topology resembles the outline of a top hat&#; provide the best accuracy and stability for high voltages and high resistance values. Top hat trims require a precise serpentine resistance element and are best for larger chip sizes. The serpentine resistance pattern dramatically increases both the voltage capability and resistance value of the element. The top hat portion of the element provides a wider area for the laser trim, allowing precision value control without harming the resistor&#;s voltage capability. Most precision high-voltage resistor series utilize the top hat type of trim.

In scan cut trims, resistor material is typically removed from each edge of the resistor equally. Scan cuts are generally used when the adjustment range is small and high precision is required. This trim process is significantly slower than others and requires overlapping subsequent laser trim scans to ensure all material from each trim is completely removed. The best stability comes from starting and finishing scan trims in the conductors on the ends of the element. This type of trim requires materials that are compatible with this trim operation. Thus this type of trim is not commonly used because of unique material requirements and of the time and control required.

Laser trimming effects

It should be noted that the basic rule for trimming is that the trim itself should be minimized. For thick-film resistive elements, the process of laser trimming has detrimental effects which can&#;t be overcome. As the laser moves through the element, the material adjacent to the laser trim permanently changes. The thick-film material heats up in proportion to the distance from the laser trim. As this material cools, microcracks develop because the cured thick-film material conducts heat relatively poorly. These microcracks lead to poorer TCR (temperature coefficient of resistance), poorer lifetime stability, higher parasitic noise, and increased VCR (voltage coefficient of resistance).

Also at the edge of the trim a dielectric glass layer forms. The material at the edge is referred to as laser slag and has a non-uniform appearance. The dielectric glass layer helps to stabilize resistive material at the trim edges and prevents it from falling into or across the trim line.

The effect of trimming becomes evident in graphs of resistance vs. VCR and resistance change vs. operating time. Generally speaking, trimming can cause significant degradation of VCR and resistance change over the life of the part.

All in all, each trim shape has its advantages relative to the design goals of specific thick-film resistor series. Manufacturers minimize the trim length to ensure the best possible TCR, overall stability, and lowest noise.

Laser Trimming Machines

Laser trimming equipment we carry

What is laser trimming?

Laser trimming is a manufacturing process that uses a laser to modify the operating parameters of an electronic component or a circuit by reducing the quantity of the component&#;s material incrementally.

A typical application of laser trimming is in adjusting the resistance of an unnecessary thin-firm or thick-film resistor by cutting away a smaller proportion of the resisting material.

This trim or cut increases the resistance of a component by narrowing or expanding the resistive material&#;s current path. Measuring the active resistance value of the material resistor while the trimming process continues is an accurate way of establishing the final results.

Besides, specific capacitors can be accurately laser trimmed to achieve an accurate capacitive. This is usually achieved by removing the upper layer on a multilayer capacitor to decrease its capacitance by reducing the top electrode area.

How does laser trimming work?

Laser trimming technology has many applications such as cutting metal plates. It also makes it possible to cut tiny holes and intricate shapes.

The laser trim process on stainless steel, mild steel, and aluminum plate is accurate, yields accurate cut quality, and produces a small heat affect zone and a small kerf width.

The laser beam comprises a column of highly intense light of a mono color or wavelength. For instance, of the CO2 laser, the wavelength is part of the Infra-Red light spectrum, thus making it invisible to the naked human eye.

The beam is about ¾ inch in its diameter as it passes from the resonator, emitting it through the beam path. The beam can be bounced in various directions using several mirrors and beam benders before focusing on a plate.

The focused laser beam passes through a nozzle before it hits the plate. Also, it flows through the nozzle right before it comes into contact with the plate. Besides, compressed gas also flows through the nozzle, for instance, Nitrogen or Oxygen.

A unique lens is used to focus the beam or even a curved mirror, which happens in the laser cutting gear head.

The beam is accurately concentrated such that the shape of the focus spot and the energy density around the spot is precisely round, centered from the nozzle, and consistent.

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When a giant laser beam is focused down a single pinpoint, the heat density generated is exceptionally high. Take, for example, using a magnifying glass to concentrate the sun&#;s rays on a single tip of a reef to start a fire.

Now consider focusing over 6 KWatts of energy onto a single spot and how the spot becomes. The extreme power density causes rapid heating, melting, melting, and complete or partial vaporization of the heated material.

When trimming mild steel, the laser beam heat is enough to create a standard oxy-fuel heating process since the laser cutting gas is pure oxygen, just like any other oxy-fuel torch.

When trimming aluminum or stainless steel, the laser beam is used to melt the material while the laser cutting gas blows off the molten metal pieces of the kerf. When cutting using a CNC laser cutter, the laser beam source/ head is adjusted over the metal being cut, thus achieving the desired cut shape.

Typically, a capacitive height is used to control the system such that a very accurate distance is maintained between the plate being worked upon and the nozzle end.

This distance is equally important since it determines the focal point of the plate surface. Raising or lowering the focal point from above the plate surface greatly impacts the cut quality.

Other parameters that can affect the cut quality, such as a stable laser bean, reliable and extremely accurate cutting process, are properly controlled.

Passive trim vs active trim

Laser trimming can be done in two ways: Active and Passive. Passive trimming involves adjusting a single component such as a capacitor or a resistor to a specific value.

If the trimming changes the entire circuit output, such as its frequency, voltage, or attenuation, this is described as an active trim. During the trimming process, the circuit output performance is actively monitored.

Once the desired output is achieved, the trimming process is automatically shut off. The process variability arises from the laser power based on the component level, laser spot size, wavelength, or pulse duration of the laser emitter.

Electrical contact is required to the component circuit to ensure feedback measurement in both active and passive trim. This is usually done through a dedicated probe card that uses either pressure pins or spring blades.

What is the significance of laser trimming?

One advantage of laser trimming over mechanical cutting is that it has an easier work holding and reduced contamination of your workpiece( no cutting edges get contaminated by the resulting cut material.

Precision cuts and High accuracy. This is because the cutting laser beam doesn&#;t degrade during the cutting process. Besides, laser trimming uses a very powerful laser and extremely small laser, focusing the beam of light on the material&#;s surface.

No need to modify or replace tooling gears hence lower costs. Laser trimmers are economical to use even in limited run-projects. This is particularly true since the laser cutter does not need to use custom-build tools or modifications for your projects.

Easier to use. It would be best to have a laser cutter, material cut, and a schematic to load into your computer. This cuts down the overall cost even when working on small batch projects.

Can handle any complex job. There is no job complexity when it comes to the laser cutter. The high-powered laser beam can work even on the very narrow section of the material being cut, thus resulting in little or no warping and distortion.

Less wastage and high utilization of the material. Laser trimmers can utilize a significant percentage of your material, thus maximizing the overall usable components.

Why is trimming important to semiconductor manufacturing?

Besides, trimming is a useful approach for the semiconductor industry when establishing various devices in a mono wafer device design, also known as device derivatives or options in general terms.

It is incorporated with a metal fuse, a poly resistor Zener called analogue trim cuts.
Improving semiconductors through trimming. Laser alignment involves target modification of electronic circuit properties through link blasting or laser cuts.

For this reason, the alternative component is selected and processed using a laser. Lateral trimming into a resistor increases its resistance value.

When it comes to capacitors, removing the upper cover electrode can significantly reduce capacitance.

Trimming LTCC resistances in a pressure chamber

Some passive trimmer utilizes a specialized pressure chamber to facilitate resistor trimming in a single phrase.

In this case, the LTCC boards are contacted using test probes on an assembly side and cutting done by a laser beam arising from the resistor side.

This cutting method does not require contact points between the two resistances due to the pitch adapter contact with the component on the other side of where the cutting occurs. This means it is possible to arrange LTCC less expensively and more compactly.

Advantages of this trimming method

• You can trim an unlimited number of resistors without hindering regular test probes.
• No contamination of the adapter, board, or the trimming system
• It has a laser beam density of up to 280 points/cm²

Trimming potentiometers

Trim potentiometers, also known as trim pots, tune, adjust and calibrate circuits. It is a type of adjustable potentiometer or variable resistor. These trim pots are used to calibrate circuits and equipment immediately after manufacturing.

Trim pots are not meant to be adjusted or seen by the device&#;s user. They are usually mounted directly on the circuit board and adjusted using a small knob or screwdriver. Some have expandable shafts that can be adjusted using fingers.

Designers use adjustable potentiometers during the end testing of a unit to determine the ideal performance units of a circuit. However, some end-users opt not to have potentiometers since they can easily drift, easily mis-adjust or sometimes develop noise.

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