PCB Technical - PCB drilling and ultraviolet light technology

There are currently four types of lasers used to make PCB micro-vias: CO 2 lasers, YAG lasers, excimer lasers and copper vapor lasers. CO2 lasers are typically used to produce holes of about 75μm, but because the beam reflects off the copper surface, it is only suitable for removing the dielectric. The CO 2 laser is very mess-free, cheap, and requires no maintenance. Excimer lasers are the best choice for producing high-quality, small-diameter holes, with a typical aperture value of less than 10μm. These types are most suitable for high-density array drilling of polycarbonate substrates in microBGA devices. The development of copper vapor lasers is still in its infancy, but it still has advantages when high yields are required. The copper vapor laser can remove the dielectric and copper, but it will bring serious problems during the production process, which will make the airflow only produce products in a restricted environment.

The most common laser used in the printed circuit board industry is the Q-switched Nd: YAG laser with a wavelength of 355nm in the ultraviolet range. This wavelength can melt most metals (Gu, Ni, Au, Ag) when drilling holes in printed circuit boards, with an absorption rate exceeding 50% (Meier and Schmidt, 2002), and organic materials can also be melted. The photon energy of the ultraviolet laser can be as high as 3.5-7.5 eV, which can break the chemical bond during the melting process, the part is through the photochemical action of the ultraviolet laser, and the part is through the photothermal action. These functions make UV lasers the first choice for printed circuit board industry applications.

The YAG laser system has a laser source that provides an energy density (flow rate) exceeding 4J/cm 2, which is necessary for the copper circulation of the surface of the microvia. The energy density required for the melting process of organic materials is only about 100 mJ/cm 2, such as epoxy resins and polycarbonates. In order to operate accurately in such a wide spectrum range, it is necessary to control the laser energy very accurately and precisely. The drilling process of the micro-via requires two steps. The first step is to open the copper foil with a high-energy density laser, and the second step is to remove the dielectric with a low-energy density laser.

When the wavelength of the laser is 355nm, its typical spot diameter is about 20μm. When the pulse time is less than 140ns, the frequency of the laser is between 10-50kHz, and the material will not generate heat at this time.

The scanner/reflection system is controlled by the computer to locate the laser beam, and the telecentric lens is used to focus, so that the beam can be drilled at an accurate angle. The scanning process generates a vector pattern through software to compensate for material and design deviations. The scanning area is 55 x55mm. This system is compatible with CAM software and supports all commonly used data formats.

The laser system was proposed by the German Mis LPKF. The base of its mechanical design is made of hard granite, and its surface polishing accuracy is not less than 3μm. The table support is placed on the gas bearing and controlled by the linear engine. The positioning accuracy is controlled by a glass ruler, and its repeatability is guaranteed to be within ± 1μm. The workbench itself is equipped with an optical sensor, which can accurately adjust the position of the laser at different reflection points to compensate for optical distortion and long-term drift. After adjustment, a series of correction data generated by the software can cover the entire scanning area. The drift scale compensation takes about 1min to operate. Any changes in the substrate, such as position deviation from the reference, can be detected by a high-resolution CCD camera and compensated by software control.

This system is very suitable for the production of prototypes, because it can be drilled and configured, from flexible to rigid printed circuit boards can be used, including metal polymers, such as solder resist, protective layer, dielectric, etc. Raman et al. introduced the most advanced solid-state ultraviolet laser system and its application in the production of high-density interconnected microvias.

Lange and Vollrath explained the various applications of the ultraviolet laser system (micro-line drilling 600 system) in drilling, shaping and cutting. The system can drill holes and micro-vias, the copper layer hole diameter is reduced to 30μm, and can perform single-step manipulation for a certain range of substrates. This system can also produce printed circuit board outer conductors with a minimum width of 20μm, Its production capacity greatly exceeds that of photochemistry. The production speed of this system can be as high as 250 drills, and it can accept all standard inputs, such as Gerber and HPGL. Its operating area is 640mm x 560mm (25.2in x 22in), and the maximum material height is 50mm (2in), which can be used for most common substrates. The base of the machine table and its guide rails are made of natural granite, with an accuracy of ±3μm. The worktable is driven by a linear drive and supported by an air bearing; its position is controlled by a glass ruler with heat compensation, and its accuracy is ± iμm. The installation of the substrate on the console is done by vacuum equipment.