Precise Cutting & Drilling of Thin or Flexible Materials
For cutting or drilling of PCB materials, UV laser systems work with a laser power of just a few watts. Flexible circuits have become a standard in consumer electronics, the automotive industry, and robotics. High-precision cutting operations, and flexible process control by LPKF UV laser technology is the first choice for drilling and cutting of thin PCBs.
Today’s laser systems are largely maintenance-free and have long-lasting laser sources. LPKF laser systems utilize an extraction unit and cause no harmful emissions. The process does not need tools nor masks, and control can be accomplished with state-of-the-art intuitive CAM software.
Another advantage is the unrivaled flexibility. The supplied CAM software controls the laser cutting parameters. The contour data are imported from the CAD layout. Libraries provide optimized laser parameters for the cutting process. A process only needs to be generated once and then can just be loaded and executed again. This ensures that laser cutting is suitable both for mass production and for production with high variance.
UV lasers are suitable for depaneling and marking of rigid, rigid-flex, and flexible PCB substrates and backing foils. UV laser cutting systems show their strengths in drilling of fine holes or singulation of boards in the SMT industry.
Improved cutting quality by UV laser?
CO2 laser systems with a wavelength of approx. 10.6 μm can be produced relatively cheaply and are available in power classes of up to several kW. However, they input a large amount of heat in the cut edges and cause carbonization of the edge regions.
The UV laser works with a different laser wavelength of 355 nm. The laser beam can be focused easily with optical elements. For UV laser cutting, powers of less than 20 Watts are focused on a spot with a size of just 20 μm – generating an energy density that is higher than that found on the surface of the sun.
UV Laser drilling in the PCB Production
Holes in boards are needed for interconnections in a multilayer or to connect the front and back sides of a double-sided PCB. Later the holes are are plated for electrical connections.
LPKF has introduced a new production system for drilling of flexible PCB materials. The LPKF MicroLine 5000 laser system has a large working area of 533 x 610 mm and can be equipped with reel-to-reel automation.
For drilling a hole, the laser pokes a hole in the center of the hole and then cuts out the contour. It does this more precisely than any other process can: the system can drill holes with a minimum diameter of a mere 20 μm and a high aspect ratio in organic or inorganic substrates. This precision is needed for flexible PCB materials, IC substrates, and HDI PCBs.
Depending on the thickness of the PCB material, one or more cuts along the desired contour are required to penetrate the substrate. The thinner the material, the faster the cutting process.
If the number of laser pulses emitted is less than that needed to penetrate the material, the substrate is engraved. In this way, unique part codes can be scribed onto the material to enable 100% traceability.
The pulse energy of the laser only acts on the material for a millionth of a second and hence causes no significant thermal damage to the component assembly – the thermal effects are already indiscernible just a few micrometers away from the kerf. An extensive study dealing with the influence of different parameters on thermal effects can be downloaded for free on the LPKF website. Only 100 microns near the cutting channel the thermal influence did not reach the temperature needed for soldering – the parts on the PCB stay safe.
Traces and solder joints close to the edge remain undamaged, and no burrs are created. UV laser cutting requires practically no cutting channel space on the board and no keep-out areas.
Cutting Thin Foils
A similar process is used for coverlays for protection of traces on flexible printed circuits. Coverlays are often made of polyimide and adhesive with a thickness of 25 μm or 12.5 μm and are very sensitive to deformation. Individual regions have to be removed from the coverlays, e.g., as solder pads, for later mounting or contacting.
These thin foil materials cannot stand mechanical loads. For thin, flexible substrates, a vacuum table ensures that the substrate remains flat, the laser cuts the apertures without any mechanical stress.
In rigid-flex PCBs, rigid layers and flexible constituents are laminated together as multilayers. Multiple different laser parameters may be used on a single board to account for the differences in substrate material and thickness.
Additionally, due to the delicate nature of the UV wavelength, very accurate micromachining can be used to create pockets or channels in any substrate material with the ability to stop at internal layers.
The gentle processing has a direct effect on the number of good parts produced – as long as the integrated vision system is used to detect distortions from previous processes. Thanks to the position detection and correction feature, substrates with distortions in the x or y direction or with rotations in the system can be processed. The position detection references fiducials or distinct board geometries and adapts the cutting line to the actual conditions. In this way, previously critical precursors become process-compatible good parts.
Depaneling of Populated PCBs and FPCs
At the end of the production chain, there are multiple electronic assemblies on one panel. These assemblies must be singulated either using a full-section cut or by cutting through tabs.
There are various techniques available for this. Sawing, stamping, and contour milling are only options for simple PCBs. More complex electronic circuits and thin substrates are sensitive to mechanical stress, dust, and geometric deviations. Three diagrams evaluate the possible methods in consideration of different aspects.
For cutting of complete contours, LPKF recommends a maximum thickness of up to 1.6 mm, depending on the laser source used. Through multiple cuts, considerably thicker materials can be handled – for delicate and high-cost assembled boards, the longer cutting times are outweighed by the improved reliability and quality.
For a tab cut, the laser cuts break-out tabs produced in a preceding routing process. This cutting operation can be performed right up to components situated close to the edge and is also cost-effective for thick boards.
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LPKF UV laser systems provide the flexibility, reliability and high precision that customers demand. By reducing product changeover time, increasing accuracy and eliminating tooling costs these systems simply produce a better product at a lower cost.