For sophisticated precision micromachining applications, ultraviolet (UV) wavelength lasers have proven their value in a number of industries. The key factor for using UV laser technology as a solution is its capability of precisely ablating a wide range of materials at a high speed cost effectively.
Also shorter wavelengths enable stricter focusing, which is advantageous for processing high precision, small features in a non-contact manner.
Lasers are used in a range of PCB manufacturing processes such as depanelling, drilling, profiling (cutting), laser direct imaging (LDI), marking and repair, trimming.
Laser technology is a non-contact process hence eliminates mechanical stress on the material completely. Micro-cracking and burr formation in the material is also avoided. The fine focus attainable with UV lasers can remove small material volumes, minimizing ejected material deposits on the circuits.
Precision micromachining possible with UV lasers enables more circuits to be arranged on one panel, improving the net usable area. UV wavelengths are also absorbed by a range of materials in PCBs from polyimide to copper films offering a all-in-one flexibility kind of solution.
For instance, the high beam intensity possible with finer focus UV can remove copper, while low beam intensity possible by minimising laser power can cut dielectric material without damaging the bottom copper layer.
Cutting and Drilling in Flex PCB Manufacturing
Flexible PCB technology is moving towards miniaturization: thinner substrate materials and smaller hole sizes for both blind via holes and through vias, with concurrent increases in feature density.
It is not possible to achieve these small dimensions with longer wavelength lasers or mechanical methods. UV wavelengths help focus the beam to a considerably small spot size for drilling the needed hole dimensions in the order of Ø100µm down to a few tens of microns in diameter.
Figure 2 shows holes drilled in a Flex PCB panel, comprising a 25µm-thick polyimide (PI) layer sandwiched between two 12µm-thick copper (Cu) layers, using Spectra-Physics Talon® 355-15 laser.
Both Ø30-µm blind vias and Ø100-µm through-vias were drilled.
Figure 1. Schematic of Flex PCB panel layers: 25 µm of polyimide sandwiched between 12 µm copper layers.
Figure 2. Holes drilled in Flex PCB panel: Ø30-µm blind via (top); Ø100-µm through-via (bottom).
It is possible to drill these vias at very high speeds restricted not by the laser but by the accuracy and speed of the galvanometer-based scanner. Other than drilling, laser cutting is also used in flex PCB manufacturing. Whether contoured profile cutting or straight line singulation, high speed cutting with good quality is required.
For this process, the Talon UV laser was also tested with an optimized parameter set yielding a cutting speed of 215mm/sec as shown in Figure 3. This was possible with a 15µm kerf width, and microscope inspection shows good quality, with minimal oxidation or other heat affected zone in the copper.
Figure 3. Cu-PI-Cu cutting at 215 mm/sec with Talon 355-15 laser.
Cutting Rigid FR4 PCB Panels
Another key manufacturing process which can be addressed with UV lasers is cutting rigid, thick PCB panels having fiberglass-based polymer composites such as FR4. Cutting may be required for singulation or depaneling of completed devices from the larger PCB panel or for making contoured profile cuts.
Figure 4 shows a FR4 piece of 0.445mm thickness cut into a 10mm square using a 15W Talon laser. The considerable shorter nanosecond pulse width and variable pulse frequency enables controlled heat input into the material. Carbon debris formation has been reduced with careful process optimization.
As with many applications, there is a tradeoff between throughput and quality. For high throughput, continuous rapid scanning cuts through the material are performed in few seconds, but with slightly higher carbonization.
On the other hand for excellent quality cuts the process speed is reduced by introducing cooling delays between scans, yielding a reduced output but lower carbonization. The net speed at which the piece was cut was 7.5mm/sec.
Figure 4. 10 mm square cut from 0.445 mm thick FR4 panel (left); magnified corner (right).
Products: TALON 355-15
Talon is a new family of green diode pumped solid state (DPSS) Q-switched lasers that deliver a superior combination of reliability, performance, and cost. Based on Spectra-Physics’ It’s in the Box™ design, Talon 355-15 has the laser and controller integrated in a single, compact package, Talon 355-15 uses field-proven technology to output >15W and >300µJ per pulse of UV with a broad repetition rate range of 0 to 500 kHz, high pulse-to-pulse stability and excellent TEM00 mode quality for tens of thousands of operating hours. The Talon laser is specifically designed for micromachining applications in a 24/7 manufacturing environment where system uptime is crucial.
This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.
For more information on this source, please visit Spectra-Physics.