Processing blind holes in FR4 copper-clad boards to interconnect electronic components is an essential method in printing circuit boards. The quality of blind hole manufacturing is a crucial factor in determining device performance. Laser processing offers advantages such as high precision, no mechanical force, and flexible control, making it the primary method for machining blind holes in FR4 copper-clad boards. Recently, the application of ultrashort pulse lasers has reduced the thermal effects and improved drilling accuracy. However, current laser processing techniques still face challenges in controlling the taper of hole sidewalls and achieving sufficient hole depth. Sidewall taper is a crucial indicator for evaluating blind holes. To ensure reliable interlayer connections, the solder pad at the bottom of the blind hole should be exposed as much as possible. On the other hand, blind holes with uniformly inclined and smooth sidewalls are more conducive to subsequent metallization processes, thus improving the yield of finished products. Considering these conflicting requirements, the ratio of the bottom diameter to the top diameter of blind holes must be controlled within the range of 70%?90%. Furthermore, existing research primarily focuses on shallow blind holes, and the manufacturing of deep blind holes with depths exceeding 500 μm still presents significant challenges. In this study, we adopt five-axis laser scanning technology to avoid obstruction of the laser beam by the material surface and sidewalls in order to achieve sidewall taper adjustment and improve blind hole depth.
In this study, a five-axis laser scanning system with a wavelength of 1030 nm and a pulse width of 436 fs is employed to perform laser scanning. The experimental material is an FR4 copper-clad board, with the glass fiber composite material thickness of 925 μm and the copper thickness of 35 μm. The processing of blind holes adopts a layer-by-layer material removal method. The processing of each layer is divided into two steps. In step 1, the laser draws a spiral line on the material surface, and in step 2, the laser performs additional scanning around the hole circumference to increase the material removal rate at the sidewalls. Simultaneously, during the laser scanning of the two-dimensional pattern, the five-axis scanning system controls the tilt angle of the laser beam to avoid obstruction of laser energy by the hole sidewalls. After the scanning is completed for one layer, the laser focus moves downward to process the next layer of material until the blind hole processing is finished. Compressed air at a pressure of 1 bar(1 bar=105 Pa) is supplied coaxially. The processed results are observed using a laser confocal microscope.
Compared to laser repetition rate and scanning speed, the impact of laser pulse energy on the sidewall taper is more significant (Fig. 2). Modifying the line spacing of the spiral pattern can enhance the uniformity of material removal at the bottom of the hole and mitigate the influence of material anisotropy on the uniformity of material removal (Fig. 3). By adjusting the scanning strategy, continuous control over the sidewall taper and hole geometry dimensions can be achieved (Figs. 4 and 5). The hole sidewalls are straight with surface roughness (Sa) of less than 5 μm. The glass fiber composite material at the bottom of the blind hole is thoroughly removed, with a bottom roughness of less than 2 μm and good roundness. The damage depth to the copper layer at the bottom of the blind hole is less than 1 μm (Fig. 6).
This study investigates the femtosecond laser machining technology for deep blind holes in FR4 copper-clad boards using a five-axis laser scanning system. The research demonstrates that, compared with laser repetition rate and scanning speed, the variation in laser pulse energy has the most significant impact on the sidewall taper of blind holes. Matching the line spacing of the scanning pattern with laser processing parameters can improve the uniformity of material removal inside the holes. By adjusting the laser scanning strategy, the sidewall taper of blind holes can be controlled, allowing for continuous and adjustable diameter ratios between the bottom and entrance of the blind hole within the range of 70%?90%. It also enables the adjustment of the blind hole radius, with a maximum aspect ratio of 4.9∶1. Inspection of the drilling results at the bottom of the blind hole reveals complete removal of the glass fiber composite material, with a copper layer damage depth below 1 μm. This research achieves high-precision manufacturing of deep blind holes in FR4 copper-clad boards and continuous control of sidewall taper, enhancing the quality of blind hole machining and promoting the application and development of five-axis laser scanning technology.
