Laser trimming
As laser trimming or laser trimming , a method of is called adjustment (trimming) of components by laser-induced material changes.
Resistances in thick - film and thin-film circuits and on integrated circuits can be matched to the exact resistance value ( passive calibration ) or the function of the circuit ( active calibration) by laser ablation .
Furthermore, mechanical vibrators (for example the tongues of music boxes and accordions) can be adjusted to the exact resonance frequency.
Laser resistance adjustment
Thick-film resistors in particular have large tolerances due to their technology . By removing resistance material (incisions in the layer) with a focused laser beam , the cross-section of the resistor is reduced or its effective length is increased. This increases the electrical resistance . The procedure can only be used if the original electrical resistance is too low.
During the trimming process, the respective output signal is constantly measured and compared with the nominal value. When the target value is reached, the laser cut is automatically stopped.
If the output signal comes from a comparator or a threshold switch during active calibration, it can be used directly to switch off the laser. A typical example of this are proximity switches.
If the measurement and the data transmission are too slow during passive adjustment, adjustment can also be carried out step by step, i.e. that is, it is trimmed and measured alternately. The closer the signal comes to the setpoint, the smaller the selected step size in order to avoid overbalancing. Typical examples of this are also frequency adjustments , since the frequency measurement by frequency counting is often too slow due to the principle involved.
Active filters often also require capacitors with tight tolerances. In some cases, the limit frequencies of their RC elements can be corrected by actively balancing the resistors. However, there are now also laser-trimmable chip capacitors.
Also SMD resistors on printed circuit boards can be laser trimmed. Pre-trimmed chip resistors are used or an active calibration is carried out in order to avoid the use of potentiometers .
Laser types
Q-switched solid-state lasers are used for adjustment cuts and removal to adjust thick, thin-film and SMD resistors as well as SMD capacitors .
Mostly these are Nd: YAG lasers pumped with arc lamps or diode lasers . To achieve smaller foci and for better absorption, these are often doubled in frequency so that they shine green (532 nm) instead of in the infrared (1064 nm). Pumping with diode lasers leads to a better and more stable beam quality compared to lamp-pumped systems.
The pulsed operation (Q-switching) takes place with repetition frequencies of approx. 100 Hz to several 10 kHz. The pulse durations are around 100 ns, so the heat affected zone remains small. This and the usually very high beam quality (that means a small focus diameter) have a positive effect on long-term stability, process reliability and calibration accuracy.
Cut shapes
Straight cuts are made from the side across the resistance track. In the case of high calibration accuracy, the so-called L-cut is used, which consists of a transverse incision and an adjoining cut running along the resistance track. The latter causes a smaller change in resistance per cut length and therefore allows a finer adjustment.
In the case of capacitors, the top electrode is removed. A suitable absorption depth and / or pulse energy as well as the pulse duration must ensure that the dielectric below 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 here and environmental influences lead to changes. Therefore balanced resistors are often protected by a layer of lacquer. The exact focusing of the laser has a significant influence on the stability.
Adjustment with pressure chamber
Pressure chambers are used to align circuit boards and other large-area, flat circuit boards. A compressed air chamber with a seal, lowered onto the board, compensates for the contact pressure of the measuring needles ( rigid needle adapter ) in order to prevent the board from bending. This is necessary with simultaneous contacting of many points in order to avoid a deviation of the focus position (defocusing) of the laser.
The contact is made from below, the laser beam comes from above. He enters the chamber through an anti-reflective window. In addition, compressed air flows through the chamber, which removes the burn and prevents contamination of the circuit board.
- procedure
- Adapter supports the board from below
- Pressure chamber moves from above onto the board and seals
- Counterpressure to compensate for the contact is built up
- Flow sucks off dirt and burned-off particles
- Contact pins contact the substrate
- Resistances are measured on their values
- Laser trims the printed resistors until they reach their set point
- Advantages of balancing in the pressure chamber
- All resistors can be trimmed in one pass → 100% freedom from adjustment
- Great time savings for the entire trimming process → one-time handling
- Blanks come out finished after trimming → no more intermediate handling
- Only one adapter per board is required
- One-time handling in the machine = less stress for circuit boards
- Effective suction from the laser burn-off prevents board contamination
- There are no longer any need for large test points between the resistors
- Contacting from below on the gold side
- The existing gold surfaces can be used for testing
- The rigid needle adapters used can contact structures on 100 µm
- Pitch distances of 200 µm can be resolved during testing
- There are no large test points, which means that the boards can be made smaller
- No laser erosion on the adapter as it is covered by the circuit board
Active adjustment of circuits
Laser trimming of integrated circuits found for example in FET - operational amplifiers instead, to reduce their manufacturing tolerances of the offset voltage.
Adjustment of mechanical transducers
The balancing of tongues that are used to generate sound in music boxes and accordions is known. In contrast to resistors, an adjustment and compensation can be carried out - depending on whether the mass is removed at the end or the spring constant is changed in the area of the bending load.
Individual evidence
- ↑ http://laz.htwm.de/ Laserinstitut Mittelachsen.