Trimming LTCC resistances

A pressure chamber is used in one type of passive trimmer to enable resistor trimming in a single run. On the assembly side, test probes touch the LTCC boards, which are then trimmed with a laser beam on the resistor side. Because the fine pitch adapter connects the component on the other side of where the trimming happens, this trimming method does not require any contact points between the resistances. As a result, the LTCC can be configured in a more compact and cost-effective manner.

Function mode:

The LTCC is mounted in the contact unit.

From the opposite side a rigid probe contacts the circuit.

From the top side the chamber is pressurized to 1 to 4 bars, with a controlled exhaust port to achieve air flow through the chamber.

As the resistance material is vaporized, the waste particles are removed in the air flow.

Advantages of this method:

Trimming of unlimited number of printed resistors in one step without obstruction from test probes.

No contamination on board, adapter or in system.

Density up to 280 points/cm².

The process of utilising a laser to change the operational parameters of an electronic circuit is known as laser trimming https://www.tridentet.com. A laser is used in one of the most common applications to burn away small parts of resistors, therefore increasing their resistance value. The circuit can be burned while being tested by automatic test equipment, resulting in the best possible final values for the resistor(s) in the circuit.

A film resistor’s resistance value is determined by its geometric dimensions (length, width, and height) as well as the resistor material. The laser makes a lateral cut in the resistor material, narrowing or lengthening the current flow route and raising the resistance value. Whether the laser modifies a thick-film or thin-film resistor on a ceramic substrate or an SMD-resistor on an SMD circuit, the impact is the same.

Passive trim is the adjustment of a resistor to a given value. If the trimming adjusts the whole circuit output such as output voltage, frequency, or 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.

Designers frequently employ potentiometers, which are tweaked during end-of-life testing until the circuit’s desired function is achieved. Potentiometers can drift, be mis-adjusted, or produce noise in many applications, therefore the end user would prefer not to have them. As a result, manufacturers use measuring and calculating methods to calculate the required resistance or capacitance values, then solder the appropriate component into the finished PCB; this procedure is known as “Select on Test” (SOT) and is highly labor-intensive.

The potentiometer or SOT portion can easily be replaced with a trimmable chip resistor or chip capacitor, and the potentiometer adjustment screwdriver can be replaced with laser trimming. The precision gained can be higher, the operation can be automated, and the long-term stability is better than potentiometers and at least as good as SOT components. The laser for active trimming is frequently integrated by the manufacturer into existing measurement systems.

Digital logic circuits can be programmed in a similar way. The laser, in this scenario, blows fuses, enabling or disabling various logic circuits.

The IBM POWER4 microprocessor is an example of this, as it has five banks of cache memory yet only requires four for full operation. Each cache bank is exercised during testing. If a flaw is discovered in one of the banks, the programming fuse for that bank can be blown to disable that bank. Because of the built-in redundancy, larger chip yields are feasible than if all cache banks in each chip had to be perfect. A fuse can be blown randomly if no bank is defective, leaving only four banks. Read More