Laser applications in the electronics industry
One of the main users of laser materials processing is the electronics industry, which is one of the world’s biggest industrial sectors
According to Statista, in 2023 the Consumer Electronics market alone is worth $1,028bn. Of this, the largest segment is telephony – which has a 2023 market volume of around $498bn.
The growth and evolution of the electronics sector is showing no signs of slowing, thanks in part to a continuing drive towards smaller, more energy-efficient and more environmentally friendly electronic products. This means there is an increasing use of lasers for a range of tasks within electronics manufacturing. Machining work carried out by lasers includes drilling very small holes in printed circuit boards (PCBs), altering the performance of electronic components by trimming them down in size, trimming complete circuits down, and machining features into silicon wafers. In addition, for quality control and traceability, lasers are used to mark various parts including electronic components, PCBs, switches and connectors.
Not only do lasers work much faster and use less power when carrying out machining tasks compared with mechanical methods, they eliminate the need for consumables during marking processes. The latter has the additional environmental gain of eradicating the use of potentially hazardous chemicals and inks. Another advantage of using laser-based technologies in the manufacture of circuits is that lasers can process a wide range of materials including ceramics, composites, metals and plastics, which are all found within the electronic devices we use daily.
Laser drilling of PCBs
For PCBs to be turned into usable electronics, components and connections between them must be added. This requires tiny holes to be drilled that are used for the copper “tracks” that transfer signals between PCB layers. These holes are known as “vias” and are copper-plated after drilling.
As electronic devices become smaller, PCBs are increasingly being miniaturised and contain ever more densely packed components. When the vias required are less than 0.15mm in diameter it is not possible to use mechanical drilling because drill bits smaller than this tend to break, so a laser must be employed instead.
For slightly larger holes, mechanical drilling can have the advantage over laser drilling in that it creates consistent holes that are not tapered, unlike the thimble-shaped holes blown by lasers. Lasers can also leave ragged edges to the holes. But on the plus side, lasers can ablate a wider range of materials and hole sizes than mechanical drills, are non-contact (meaning no tool wear), and can be faster. Also, mechanical drills usually go all the way through the board, whereas laser drilled “micro-vias” (vias less than 150 micrometres in diameter that enable more complex and denser packed PCBs) only reach between one or two electrical layers within the PCB.
Lasers that can be used for ablating holes in PCBs include, for example, fibre lasers, UV-YAG lasers and CO2 lasers: the choice of laser being dependent on the material that needs to be machined. Nanosecond lasers are generally used for flexible PCBs, with short-pulsed lasers being ideal for drilling the FR4 class of PCB – which has an epoxy resin-glass fabric composite as its base material – because they don’t tend to burn the composite.
Laser trimming of components and circuits
Another application of lasers by the electronics industry is for trimming material away from a component to change its operating parameters. For example, a widespread laser trimming task is burning away some of the material that thin-film and thick-film resistors are made from to alter the 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, 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 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.
The miniaturisation of PCBs has led to them being more densely packed, meaning that the process used to cut them from panels needs to be highly accurate and must create a narrow kerf. Lasers from firms such as Coherent can be used for these tasks (Image: Coherent)
While continuous-wave (CW) CO2 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 (10-12s) and femtoseconds (10-15s) long.
For the depannelling of PCBs, in other words the separating of individual PCBs from a panel, CO2 lasers or ultraviolet (UV) lasers can be used. This process involves ablating the PCB material layer by layer until separation occurs.
CO2 lasers have the advantage over mechanical milling and sawing processes that they are non-contact, but they do generate a sizeable heat-affected zone (HAZ). Although slower than CO2 lasers, using UV-based diode-pumped solid-state (DPSS) lasers instead reduces the size of the HAZ and creates substantially less debris.
Keeping this type of processing contact-free reduces costs because there is little wear and tear on the material being ablated or on the cutting technology. The accuracy of laser cutting also reduces material wastage, and enables panels to be fitted together readily during assembly stages.
Marking for traceability
The PCBs that all electronics are built around are just one of the components of electronic equipment that must be marked for traceability and quality control. This is so that if an error is identified during a production process, the batch of PCBs that the faulty board comes from can be readily found.
These marks can consist of bar codes, data matrix codes, serial numbers or text. Lasers are ideal for this task since the marks must fit into a tiny space and be applied on materials that are sensitive to force and heat.
In some cases, such as in aerospace applications, the marks will be subject to harsh environmental conditions, so using laser marking as opposed to marking with inks is preferential because it is less likely to erode or fade over time. Laser marking is also a very quick process, which speeds up production by enabling large quantities of parts to be marked in a short timeframe.
Lasers with wavelengths in the infrared, UV and green regions of the spectrum can all be used for marking wafers, while minimising the amount of surface damage to the wafer. Such damage is undesirable as it could subsequently act as a dust trap.
Because semiconductor materials used in electronic hardware must be marked with a machine-readable code, the precision and speed of pulsed lasers makes them the perfect choice for this task in which the surrounding material must be protected.
The pulsed lasers used for marking create a change in the surface of the material they are focussed on without causing excess heat input, which is what also makes them particularly suited to creating visible marks. The type of surface change is dependent on not only the pulse duration but also the wavelength of the laser and the irradiance of the beam.
Electronic connectors can also be laser-marked, as can the flame-retardant plastics used for some electronic components including switches. Flame-retardant plastics need to be marked for traceability and this can be carried out effectively using UV lasers, because their wavelength is absorbed well by these materials. The laser induces a photochemical reaction on the surface of the component, which creates the mark.
Marking is also useful for assembling electronic parts as well as for their testing and ongoing maintenance, so the fact that lasers are suitable for marking ceramics, metals and plastics makes them ideal for widespread use in the electronics industry.
Looking to the future
With chips continually shrinking, and an ever increasing demand for more consumer electronics products, the need for lasers within the electronics sector is set to grow.
As PCBs get smaller for example, the holes in them will also need to reduce in size – the creation of which is a task that can only be accomplished by laser drilling.
Lasers are also increasingly being used in the production of displays for devices such as smartphones and tablets. Laser scribing, for instance, can be used in the manufacturing of blue LEDs, as well as for photovoltaic cells for solar panels.
Laser processing is helping enable an emerging generation of displays made from microLEDs for products such as watches and augmented reality headsets (Images: Shutterstock/Ground Picture + khoamartin)
In the meantime, laser processing is helping enable an emerging generation of displays made from microLEDs for products such as watches, TVs and augmented reality (AR) headsets.
During manufacturing, the microLEDs – which at only a few micrometres thick are around a hundred times smaller than standard LEDs – require releasing from the wafer that they are grown on.
So far, the precision and energies of deep UV (DUV) excimer lasers are proving to be perfect for this task.
Lead image: Shutterstock/raigvi