Microwave printed-circuit board (PCB) prototypes can be quickly and cost-effectively produced in-house by means of any number of commercial mechanical milling machines or chemical etching. A faster and more accurate approach, however, is to perform the etching and machining by laser. That is the approach used in the new ProtoLaser 100 system from LPKF Laser & Electronics (Wilsonville, OR), which can mill and drill both laminated ceramic materials and soft substrate materials such as polytetrafluoroethylene (PTFE) circuit-board materials. The system generates circuits directly from computer-aided-design (CAD) and computer-aided-manufacturing (CAM) software and is compatible with a wide range of layout file formats, including Gerber, HPGL, and DXF files.
The ProtoLaser 100 system (Fig. 1) represents a high-speed laser-based PCB prototyping system that can also be used for production runs. The system can be used to create high-resolution lines and spacing in high-frequency PCBs in a fraction of the time of a mechanical milling/drilling system (Fig. 2). It can also be used in conjunction with a mechanical system to form a full-featured PCB prototype production center.
The new system overcomes a shortcoming of earlier laser systems: the fast and efficient removal of large areas of conductive layers from standard FR 4 to soft PTFE substrates. Traditionally, these areas are removed from a laminate to leave behind circuit traces and other structures on the substrate. Laser ablation of large areas was known as a very slow process and the quality of final results was usually not acceptable. Laser was therefore normally applied only to produce the finest features on the boards and the remaining parts of the boards were produced by standard chemical methods. This has resulted in highly increased complexity and price of the production of the boards.
The ProtoLaser 100 overcomes this limitation using a single laser to produce complete RF and high-density circuits in a single production step. The same laser used to form precise lines and high-frequency circuit features is used in a different operating mode to remove the unwanted sections of conductive-layer material. Traditionally, laser-based systems have employed different laser hatching algorithms for removal of large conductive areas from a laminated substrate. By this approach, subsequent laser channels are lying closely together or partially overlapping to achieve ablation of complete hatched area. The main problem with this technique, however, is that ablated conductor material (commonly copper) is deposited to the left and/or right of the channel structured by the laser, since the conductive material does not completely vaporize from the surface of the laminated substrate. When channels overlap or even approach each other, the extra material tends to be deposited back into the newly formed channel, forming irregularities in the channels and channel sidewalls (Fig. 3) that result in uneven high-frequency performance. In some cases, the laser energy can cause overheating and damage to a sensitive PCB substrate.
The new drilling and milling system features a proprietary laser source, designed and manufactured by LPKF. The ProtoLaser 100's infrared (IR) dye-pumped laser provides the type of structuring control not associated with traditional laser sources, using a fine 25-µm (1-mil) diameter laser beam to achieve minimum track width of 50 µm (2 mils) and minimum track spacing of 25 µm (1 mil). The laser operates at a wavelength of 1064 nm, pulse repetition frequencies (PRFs) of 10 to 100 kHz, and minimum laser pulse length of 35 ns at a PRF of 30 kHz.
With a ProtoLaser 100, PCB production takes place in two phases: the laser-structuring phase and the laser-rubout phase. In the structuring phase, CAD software guides the laser beam to outline all the features of the final circuitry. During this stage, the conductive areas which are to be ablated from the board are divided into many small subareas or stripes (Fig. 3). All generated lines are then structured by the laser. Laser-beam parameters (beam diameter on the working field, laser wavelength, pulse frequency, energy, and length) are carefully chosen in a way that isolation channels are reliably structured in the conductive layer without significantly damaging the substrate.
The width of the stripes depend on the parameters used later in the rubout process; however, after the structuring phase is completed, the established subareas are no longer in direct electrical and thermal contact with each other. Thermal contact only remains via the substrate, a poor thermal conductor.
In the rubout phase, laser parameters are changed so that the laser beam no longer reaches the threshold for the ablation of the conductive layer (average power remains high but peak power is sharply reduced). The laser beam is then guided along the stripes and its energy is absorbed in the conductive layer (Fig. 4). Since the structured channels do not allow transversal diffusion of the absorbed energy into the surrounding material, the temperature of the illuminated stripes increases dramatically and eventually the adhesion force between the conductive layer and the substrate diminishes significantly. By applying an adequate external force, the complete stripe is removed as a single piece from the substrate. The mechanism that allows the stripe removal is a combination of the strong temperature raise at the metal-insulator interface and a mechanical shock originating from the expansion of the metal layer.
The advantages of the new rubout method are obvious. Since the conductive-material stripes are removed as single solid pieces, no particles are deposited on the substrate during the stripe-removal process, resulting in a substrate that is clean and undamaged. The width of the stripe and the speed of the laser beam moving along the stripe depend on the applied laser power but, in general, the process speed exceeds the conventional laser-rubout methods by at least a factor of 10.
The ProtoLaser 100 system has a work area measuring 420 × 380 mm (16.5 × 14.9 in.). The laser beam covers a scan field measuring 100 × 100 mm (4 × 4 in.) and works with materials as thick as 150 mm (6 in.). It can be used with a variety of substrate materials, such as FR3, FR4, ceramic-filled soft substrates (TMM and RO4000), and PTFE substrates. The system operates with a wide range of CAD/CAM file formats, including Gerber, HPGL, Sieb & Meier, Excellon, DXF, Barco, and ODB++ files. The system itself measures 2,000 × 1350 × 2150 mm (78.7 × 53.1 × 84.6 in.) and weighs between 600 and 700 kg (1320 and 1540 lbs.).
The ProtoLaser 100 can be combined with the company's ProtoMat series of mechanical milling and drilling PWB prototyping CNC machines for extended functionality in a prototyping or small-production environment. Since the machines operate without hazardous chemicals, they can be operated even in environmentally sensitive areas. LPKF Laser & Electronics, 28220 SW Boberg Rd., Wilsonville, OR 97070; (503) 454-4200, FAX: (503) 682-7151, e-mail: [email protected], Internet: www.lpkfusa.com.