Processing technology of PCB printed circuit board using UV laser equipment

UV Lasers 鈥?This is one of the most versatile and efficient technologies for processing printed circuit boards (PCBs). Beams from lasers typically offer a low-stress alternative to mechanical PCB processing methods such as milling or routing, but UV lasers offer an additional benefit not available with other laser sources, namely the ability to limit thermal stress. This is possible because most UV laser systems operate at low power levels. By utilizing a process sometimes called 鈥渃old ablation,鈥?beams from UV lasers produce a reduced heat-affected zone (HAZ), thereby minimizing burring, charring, and other negative effects of thermal stress typically associated with high-power lasers.

UV lasers have wavelengths shorter than visible light, making them invisible to the naked eye. While you may not be able to see the laser beam, it is these same short wavelengths that allow UV lasers to be precisely focused to create very fine circuit features while maintaining excellent positioning accuracy.

In addition to the short wavelength and lower workpiece temperatures, the high photon energy in UV light makes UV lasers ideal for processing a wide range of PCB material combinations, from standard materials such as FR4 to high-frequency ceramic composites to flexible PCB materials including polyimide.

The graph in Figure 1 shows the absorptivity of three common PCB materials for six lasers. These include an excimer laser (248nm wavelength), an infrared laser (1064nm), and two CO2 lasers (9.4渭m and 10.6渭m). The UV laser (Nd:YAG, 355nm) is one of the rare lasers that has high-quality absorption in all three material types.

The UV laser shows very high absorptivity when processing resins and copper, and also records decent absorption when processing glass. Of these major material groups, only the more expensive excimer laser (248nm) has better all-around absorptivity. This diversity of materials makes UV lasers well suited for a wide range of PCB applications across many industries, from creating the most basic board features, circuit traces, to performing advanced processes such as creating pockets for embedded chips.

UV systems process boards directly from CAD data, meaning any middlemen in the board creation process are eliminated. This, combined with the precise focusing capabilities of UV, allows UV systems to operate with high feature resolution and positioning repeatability.

Application 1: Surface Etching/Circuit Creation

UV lasers work quickly when creating circuits, etching surface patterns onto a board in just a few minutes. This makes UV the fastest method for PCB prototyping. As more prototyping labs are equipped with in-house UV laser systems, R&D departments are taking notice.

Depending on the optical calibration, the size of the UV laser beam can be in the 10-20渭m range, allowing for the creation of fine-line circuit traces.

While creating circuits using a laser beam is the fastest method for PCB prototyping, production-scale surface etching applications are often best left to chemical processes. That said, there are many companies with high-mix manufacturing environments that etch small to medium batches using UV systems.

Application 2: PCB Depaneling

UV laser cutting is an excellent choice for large or small-scale production for PCB depaneling, especially for flex or rigid-flex applications. Given the rise of flexible PC materials, depaneling to remove individual boards from a panel presents significant challenges. Mechanical depaneling methods such as V-scoring and wiring can easily damage sensitive and thin substrates, posing a problem for electronic manufacturing services (EMS) companies that depanel flex and rigid-flex applications. UV laser cutting not only eliminates the effects of mechanical stresses that occur during depaneling, such as burrs, deformation, and damage to circuit components, but also reduces the effects of thermal stresses that occur with other laser depaneling options, such as CO2 cutting.

Figure 2 shows the same flexible substrate (polyimide) cut using a CO2 laser (left) and a UV laser (right). Significantly more charring and burning was observed with the hot CO2 laser than with the UV laser, which, as mentioned earlier, utilizes a cold ablation process. In today's era of miniaturization, this reduction in pressure is of great significance. Considering the space saved by the reduced 鈥渃utting buffer鈥?means components can be placed closer to the edge of the circuit and more circuits can be mounted on each panel, maximizing efficiency and pushing the limits of flexible circuit processing.

Application 3: Drilling

Another application that takes advantage of the small beam size and low stress characteristics of UV lasers is through-hole drilling, including through-holes, micro-vias, and blind and buried vias. UV laser systems drill holes in boards by focusing a vertical beam directly through the substrate. Depending on the material used, holes as small as 10渭m can be drilled.

One area where UV is particularly effective at drilling holes is in multi-layer applications. Multi-layer PCBs are held together with laminate materials that are heat pressed together. These so-called 鈥減repreg鈥?materials are known to delaminate, especially when using hotter laser sources. However, the relatively stress-free nature of the UV laser eliminates this problem. In this cross section, a 4-mil hole is drilled in a 14-mil multi-layer board. The application consists of Cu on flexible polyimide with no delamination between layers. When it comes to the low-stress nature of the UV laser, it also raises a larger issue: the ability to improve yield figures. Yield is the percentage of usable boards that have been removed from a panel.

Throughout the manufacturing process, boards can be damaged in a number of ways, including cracked solder joints, broken components, or delamination. Any of these factors can result in boards being deposited in the production line鈥檚 trash bin rather than in a shipping box. UV lasers greatly reduce, if not completely eliminate, these detrimental effects, providing a rapid return on investment in the form of high production yields.

Application Four: Deep Engraving

Another application that demonstrates the versatility of UV lasers is deep engraving, which comes in many forms. Using controls in the laser system software, the beam can be set to perform controlled ablation, which is the ability to cut to the desired depth in the material, stop, travel and complete the necessary processing, and then move to another depth and obligation. Various deep applications include pocket creation, which can be used to embed chips, and scraping, which removes organic materials from metals. Figure 3 shows a scraping application performed by a UV laser. Here, you can see the clean cut provided by the laser beam, as well as the undamaged metal surface beneath the removed organic material.

As shown in Figure 4, a UV laser can also be used to create multiple steps in a substrate substrate. In this polyethylene material, the laser was set to create a step at a depth of 2 mils, another at 8 mils below that, and another at 10 mils below that. This illustrates the full user control that a UV laser system provides. Deep engraving, like drilling, is an application that can be effectively handled by a UV laser in small, medium, or large volumes.

Conclusion: One way to rule them all

What鈥檚 truly remarkable about UV lasers is that they can accomplish all of the above applications in one step. What does this mean for those manufacturing circuit boards? Instead of completing each application on different equipment using competing processes and methods, the entire part can be processed in one go.

This streamlined production scheme helps eliminate quality control issues that arise as boards transition from process to process. UV鈥檚 debris-free ablation qualities also mean that post-processing cleaning methods are not required.

Combine this streamlined approach with the low-stress, material-versatility properties of UV light, and it鈥檚 easy to see why UV lasers are growing in popularity as a method for processing circuit boards. Soon, UV won鈥檛 just be a technology you shouldn鈥檛 ignore 鈥?it鈥檒l be something you can鈥檛 miss.