2-Thick Film TechnologyScreen-printing-
Screen-printing is the process of transferring an ink through a patterned woven mesh screen or stencil using a squeegee.
After allowing a period of time after printing for settling of the ink to occur, each layer of ink that is deposited is usually dried at a moderately high temperature (50 to 200 °C) to evaporate the liquid component of the ink and fix the layer temporarily in position on the substrate so that it can be handled or stored before final processing. For inks based on polymers and some solder pastes that cure at these temperatures this may be the final step that is required. Some inks also require curing by exposure to UV light.
For many of the metal, ceramic and glass inks used in thick film processes a high temperature (usually greater than 300 °C) firing is required to fix the layers in position permanently on the substrate.
Laser trimming of resistors
After firing, the substrate resistors are trimmed to the correct value. This process is named Laser trimming. Many chip resistors are made using thick film technology. Large substrates are printed with resistors fired, divided into small chips and these are then terminated, so they can be soldered on the PCB board. With laser trimming two modes are used; either passive trimming, where each resistor is trimmed to a specific value and tolerance, or active trimming, where the feedback is used to adjust to a specific voltage, frequency or response by laser trimming the resistors on the circuit while powered up.