Laser Trimming Systems for electronic circuits

What is laser trimming?

Lasers have been used for many years for trimming resistor value or other circuit parameters. Laser trimming replaces the need for SOT (select-on-test) or potentiometers, and offers many improvements over abrasive trimming of resistors. The latter methods are slower, more labour intensive than laser trimming, more expensive and probably less stable and reliable. Lasers can be used to trim thick film resistors, thin film resistors and capacitors. Passive resistor trimming refers to altering a resistor to a specific value; active resistor trimming also known as functional trimming changes a circuit parameter to a target value; this may be voltage, current, frequency. Laser trimmers are used in the manufacture of resistors, oscillators, op-amps, proximity switches, current sensors. Laser trimming is used extensively in the manufacture of automotive parts, proximity sensors, pressure sensors and oscillators.

Laser trimming is a way of achieving precision adjustment of the attributes of an electronic circuit. A laser removes material from the appropriate component and thus adjusts its value. Laser trimming is faster, more economical and more accurate than other methods such as select-on-test, using potentiometers or abrasive trimming. A lateral cut into an SMD or hybrid resistor increases its resistance. Removing part of the top electrode of a capacitor decreases its capacitance. There are two fundamental types of laser trimming; passive trimming is where a resistor or capacitor is trimmed to a specific value. Active or functional trimming refers to adjustment of a circuit parameter such as voltage, current or frequency. For a more detailed technical explanation see the Technology section of this web site.

What is a laser trimmer?

A laser trimming system speeds up, automates and increases the accuracy of your trim application. A laser trimming system which can trim resistors, capacitors or inductors consists of (i) a laser with a high beam quality and small beam diameter (20 to 40 microns) (ii) an industrial PC with software to develop applications programs (iii) a galvanometer consisting of two mirrors which control the beam movement (iv) instrumentation to measure the circuit parameter (v) a prober may be used to make contact with the circuit and (vi) a step & repeat table may be required to trim multiple circuits. LS engineers build a system from modules, customised to your requirements, providing optimum cost and performance.  LS also supplies modules for integration into OEM systems or production lines, as well as complete stand-alone systems with instrumentation, prober and automated handling.

How does a laser trimming system work?

For trimming, the circuit board is placed under the positioning unit and contacted electrically. The beam deflection unit moves the laser beam to the component to be trimmed and trims it until the programmed nominal value is reached. A lateral laser cut in a resistor narrows the current flow path, thereby increasing the resistance. Removing part of the top electrode of a capacitor decreases the capacitance. During the trim the relevant parameter is measured continuously or sequentially via IEEE bus and compared with the nominal value. When the parameter reaches the nominal value the trim cut stops automatically.  If the parameter cannot be measured continuously the trim is done step by step. Resistance, voltage, current, frequency – with suitable measuring equipment any output parameter can be trimmed. Various types of cut and the appropriate selection of laser parameters enable a precise trim of the circuit board, with excellent long-term stability.

Advantages of laser trimming over potentiometers are, lower component cost, smaller space requirement on pcb, higher reliability, automated trimming during manufacture, faster and less liable to error and re-work. Advantages of laser trimming compared to select-on-test resistors are, less labour intensive, faster, no re-work. Advantages of laser trimmers over abrasive trimmers are higher speed, greater accuracy, cleaner, lower running costs.

Lower running costs

Laser resistor trimming systems used to be very expensive to buy and expensive to run. Laser resistor trimming systems required three phase mains supplies to power the huge arc lamps necessary to stimulate the YAG laser rod. So much heat was produced that a continuous flow of cooling water was required. Today laser trimmers are diode-pumped, are powered by a single phase supply and are air-cooled. Running cost are negligible. Overall the cost and time savings plus the increase in reliability of the electronic system can easily exceed the capital outlay.

Examples of laser trim cuts on resistors and capacitors

‘L’ cut into a thick film resistor

‘Serpentine’ cut into a thick film resistor

‘Shave’ cut into smd capacitor

Software

The LS software runs under Windows. The programs can be generated flexibly in a flow chart. Simple, configurable interfaces permit the seamless integration of external systems as well as the connection to higher level host computers. Parameter and command items using configurable dialog boxes facilitate programming and reduce programming time.

Industrial laser systems for trimming resistors, capacitors, hybrid and other electronic circuits