Laser trimming

Laser trimming describes the manufacturing process of using a LASER to adjust the operating parameters of an electronic circuit.

The usual approach is to use a laser to burn away small portions of resistors, raising their value (resistance). The burning operation can be conducted while the circuit is being tested by automatic test equipment, leading to extremely accurate (appropriate) final values for the trimming resistor(s) (also known as AOT - Adjust on test).

Laser trimming is the controlled alteration of the attributes of a capacitor or a resistor by a laser beam. Selecting one or more components on the circuit and adjusting them with the laser achieves this. The alteration changes the resistor or capacitor value until the nominal value has been reached.

The resistance value of a film resistor is defined by its geometric dimensions (length, width, height) and the resistor material. A lateral cut in the resistor material by the laser narrows the current flow path and increases the resistance value. The same effect is obtained whether the laser changes a thick-film or a thin-film resistor on a ceramic substrate or an SMD-resistor on a SMD circuit. The SMD-resistor is produced with the same technology and normally is laser trimmed as well.

Trimmable chip capacitors are build up as multilayer plate capacitors. Vaporizing the top layer with a laser decreases the capacitance by reducing the area of the top electrode.

Passive trim is the adjustment of a resistor to a given value. If the trimming adjusts the whole circuit output (e.g. output voltage, frequency, switching threshold ...), this is called active trim. During the trim process, the corresponding parameter is measured continuously and compared to the programmed nominal value. The laser stops automatically when the value reaches the nominal value.

Trimming LTCC resistances in a pressure chamber

The passive trimmer uses a pressure chamber to enable resistor trimming in a single run. The LTCC boards are contacted by the assembly side and trimmed by the resistor side. This trimming concept needs requires no contacting places between the resistances, because the fine pitch adapter contacts at the other side on the places for the units. Therefore, the LTCC can be arranged more compactly and less expensively.

Function mode:
*The LTCC goes in the contacting unit.
*From the bottom side it comes a rigid needle adapter under the LTCC.
*From the top side come now a pressure chamber to the product and give a pressure from 1 to 4 bars to the LTCC.
*If we have the pressure we make an air flow, so that the trimming particles to be take off.
*The rigid needles contact the LTCC from the bottom side.
*The measuring technique measures resistances.
*Through the pressure glass the laser trims the resistances.

Advantages with this trimmer:
*Trimming of unlimited printed resistors in one step → 100% Alignment liberty.
*No contamination on board, adapter or in system.
*Contact area up to 101.6mm x 101.6mm (4" x 4").
*Automatic system compensation for temperature fluctuation.
*No need for dedicated trimming test pads.
*Using Rigid Needle Adapter: pitch 150 µm, pad size 70 µm.
*More than 500'000 contacting cycles with the adapters.
*Density < 280 points/cm².

Trimming potentiometers

Often designers use potentiometers, which would have to be adjusted during end testing until the desired function of the circuit has been reached. In many applications, the end user of the products does not allow potentiometers. Therefore manufacturers determine the needed resistance or capacitance values by measurement and calculation methods and afterwards solder the suitable component into the final PCB.

It is simpler to substitute the potentiometer or the adjust element with a trimable chip resistor or chip capacitor and the potentiometer adjusting screwdriver with the laser and active trimming. The achieved accuracy is higher, the procedure can be automated and the long term stability is better than at potentiometers or at least in the same region as replace-and-resolder chip components. Often the laser for the active trim could be integrated in existing measurement places at the customer factories.

Program from digital logic circuits

A similar approach can be used to program digital logic circuits. In this case, fuses are blown by the laser, enabling or disabling various logic circuits. An example of this is the IBM Power-4 processor chip where the chip contains five banks of cache memory but only requires four banks for full operation. During testing, each cache bank is exercised. If a defect is found in one bank, that bank can be disabled by blowing its programming fuse. This built-in redundancy allows higher chip yields than would be possible if all cache banks had to be perfect in every chip. (If no bank is defective, a fuse can be blown arbitrarily, leaving just four banks.)

External links

* [http://www.microcontact.ch Laser trimmers with a pressure chamber]
* [http://www.ls-laser-systems.com System manufacturer laser trimmers]
* [http://www.epp-germany.com Representative of GSI laser trimmers]
* [http://www.cmac.com/services/manufacture/substrates/Thick-filmHybrids.php C-MAC MicroTechnology laser trimming on thick-film hybrids]