Laser trimming

As laser trimming or laser trimming, we describe a method of adjustment ( trimming) of components by laser- induced material changes.

By laser ablation resistors in thick film, thin film circuits and integrated circuits to the exact resistance value ( passive balance) or the function of the circuit ( active balance ) can be adjusted. Furthermore, mechanical oscillator (eg tongues of musical boxes, accordions) are compared to the exact resonant frequency.

Laser trimming of resistors

In particular, thick-film resistors have the technology, large tolerances. By removal of resistive material ( cuts in the film ) with a focused laser beam, the cross section of the resistor is reduced or increases its effective length. Thus, the electrical resistance increases. The method can be applied only if the initial electric resistance is too low.

During the trimming process, the respective output signal is continuously measured and compared with the target value. When reaching the set value of the laser cutting is automatically stopped.

Originates the output signal at the active camera from a comparator or a threshold, it can be directly used for switching off the laser. A typical example would be a proximity switch.

Are the Passive alignment measurement and data transfer too slow, can also be adjusted gradually, ie it is alternately trimmed and measured. The step size is the closer, the signal is the target value, the smaller is selected, in order to avoid an adjustment. Typical examples are also frequency comparisons, since the frequency measurement by frequency count of principle is often too slow.

Active filters often require close-tolerance capacitors. The cutoff frequencies of the RC elements can be corrected in some cases, an active adjustment of the resistors. Meanwhile, there are also laser-trimmable chip capacitors.

Also SMD resistors on circuit boards can be laser trimmed. It vorgetrimmte chip resistors are used, or it is an active calibration is carried out so as to avoid the use of potentiometers.

Types of lasers

For matching sections and sive effect for trimming thick, thin and SMD resistors and capacitors SMD - Q-switched solid-state lasers.

Most are so pumped with arc lamps or diode lasers, Nd: YAG laser. To achieve smaller foci and for better absorption they are often doubled in frequency, so that they radiate in the infrared instead of (1064 nm), green ( 532 nm). The pump with diode lasers leads over lamp-pumped systems to better and more stable beam quality.

The pulse mode ( Q-switching ) is done with repetition rates of 100 Hz to a few 10 kHz. The pulse durations were 100 ns, so that the heat affected zone remains low. This, and the generally very high beam quality ( that means less focus diameter ) have a positive effect on the long-term stability, process reliability and matching precision.

Sectional shapes

Are carried out transverse to the resistance path straight cuts from the side. At high alignment accuracy is used the so-called L- section, which consists of a cross - cut and an adjoining, longitudinal section of the resistance path. The latter causes less change in resistance per cut length and thus allows a finer adjustment.

For capacitors, the top electrode is removed. It has to be achieved by suitable absorption depth and / or the pulse energy and pulse duration, in that the underlying dielectric is not damaged.

Long term stability

The areas around the laser cuts lead to reduced long-term stability of the resistors, since compensation processes take place and lead to changes in environmental conditions. Therefore matched resistors are often protected by a layer of varnish. The exact focus of the laser has a significant influence on the stability.

Adjustment with pressure chamber

Pressure chambers are used when matching circuit boards and other large flat circuit boards. A lowered onto the board compressed air chamber with this seal is similar to the contact pressure of the metering needle ( rigid needle adapter) in order to prevent a deflection of the board. This is necessary at the same time contact of many points in order to prevent a deviation of the focus position ( defocus ) of the laser.

The contacts are made from below, the laser beam comes from above. He passes through an anti-reflective window into the chamber. In addition, the chamber is flushed with compressed air, whereby the combustion taken away and contamination of the circuit carrier is prevented.

• All resistors can be trimmed in a passage → 100 % adjustment freedom Great time savings for the whole trimming process → unique handling
• Boards come after trimming finished out → no intermediate handling
• There is a per adapter board is only required
• Unique Händling in the engine = less stress for boards

• To test the existing gold surfaces can be used
• The rigid needle adapter set up contact on 100 micron structures
• When checking pitch distances can be resolved by 200 microns
• Large test points omitted, whereby the boards can be made smaller
• No Laserabbrand on adapter, as this is covered by board

Active adjustment of circuits

Laser trimming of integrated circuits takes place, for example, in FET operational amplifiers, in order to reduce the manufacturing tolerances of the offset voltage.

Balance of mechanical oscillators

Known is the balancing of tongues, which are used for tone generation in musical boxes and accordions. It can, in contrast to resistors made ​​a drain and a Aufgleich - depending on whether the mass is removed at the end or the spring constant is changed in the region of the bending stress.