ELECTRONICS
component’s resistance value. Cutting into the resistor material makes it more difficult for current to flow through it, thereby increasing the resistance of that component. The exact value of the resistance can be measured while this trimming is taking place, enabling very precise components to be produced. Similarly, some types of
capacitor can have their capacitance altered by laser trimming. For example, if the uppermost layer of a multilayer capacitor is taken away, the capacitance is decreased. This is because the value of capacitance is related to the plate area, and trimming part of the outer plates enables fine tuning of the area.
The trimming of individual
components like this is known as passive trimming, while trimming such that the entire output of a circuit is altered, changing for instance its voltage or frequency
output, is called active trimming. As with passive trimming, active trimming can be monitored during the process. This enables the laser to be turned off as soon as the measured output of the circuit reaches the required value. Different materials and types of trim can be achieved by using variations in laser power and spot size, wavelength, and
pulse duration. Fibre and CO2 lasers in the infrared region of the spectrum are suitable for trimming resistors, for example. Such lasers can also machine the ceramics used to make some capacitors and other electronic components: these materials are brittle and therefore susceptible to cracking if sawed or drilled mechanically.
Cutting into silicon wafers and separating PCBs Silicon wafers themselves can also be cut by lasers,
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allowing features like pockets and channels as small as 20 micrometres to be produced. In some instances however, the ‘cut’ may not actually go all the way through the wafer. A micromachining technique known as laser scribing can be used instead, particularly in high- volume production. In laser scribing, a laser beam
is scanned across wafers or substrates to create ‘scribe lines’ which are subsequently used to break the wafer or substrate. The fine control that using a laser can give to the scribing process
“The fine control that a laser can give to the scribing process enables a very well-defined break, devoid of micro-cracks”
enables a very well-defined break, devoid of micro-cracks, to be made. The scribe-then-break technique is a much quicker process than laser cutting right through a substrate. While continuous-wave (CW)
lasers with wavelengths in the 9.4 and 10.6µm range have long been the workhorse of choice for laser scribing, they have the disadvantage of producing a lot of heat. This leads to unwanted effects including micro cracks and localised melting. These effects can also occur when nanosecond pulsed solid-state lasers are used. But thermally induced stresses can be reduced by processing using ultrafast lasers, which emit pulses in the order of picoseconds (10-12 femtoseconds (10-15
CO2 s) and s) long.
For the depannelling of PCBs, in other words the separating of individual PCBs from a panel, CO2
lasers or ultraviolet g THE 2023 GUIDE TO LASER SYSTEMS LASER SYSTEMS EUROPE 17
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