With the development of high-performance hand-held gadgets such as cell phones, high-density interconnection printed circuit board (HDI PCB) has been playing a vital role in the electronics industry. Unlike conventional PCB, HDI PCB uses not only plated-through holes (PTH) but also microvias formed by laser drilling to achieve continuity between the top and bottom or to intermediate layers. The reason why uses laser drilling for microvias is that only laser drilling can attain precisely controlled depth drilling in small sizes and blind hole structures. On a thin flat glass reinforcement, laser drilling can drill 2.5 to 3 mils microvias, and on an unreinforced dielectric (without glass), 1 mil microvias can be drilled.
How laser drilling works?
Laser drilling uses a laser beam with wavelengths ranging from deep infrared to ultraviolet for microvias. Although the speed of laser drilling is lower than mechanical drilling, the cost of mechanical drilling exceeds the benefits of laser drilling over time. Also, laser drilling can efficiently drill dense vias in circuits on multilayer boards. However, a major issue with laser drilling is the limited drilled depth of the focused laser beam, especially for reliable and repeatable drilling. For example, when a single lens is used to focus the beam, the depth at which the laser can drill is restricted due to the limited depth of focus. Due to the small size of the vias, the laser beam undergoes Fresnel diffraction, which impacts the intensity of the beam in extremely deep vias.
Laser glass drilling technology
By beam shaping technology, the laser beam is projected onto the material surface, which absorbs beam energy to break chemical bonds. The released steam will generate a recoil pressure, applying a downward force on the remaining molten material as well as forcing the molten material to flow out of the hole.
There are mainly 4 methods for laser drilling: single pulse, percussion, trepanning and helical drilling. Figure 1 below shows different drilling methods.
Figure 1: Four Laser Drilling Methods
In the single pulse method, a single pulse of the laser beam is fired at the material to form the desired hole. Both the laser source and the work material remain stationary in this method.
Unlike the single pulse method, the percussion method utilizes a series of laser pulses to repeatedly shoot at the work material. The Percussion method can create deeper and more precise holes with smaller diameters than the single pulse method. This is because, between the laser beam and the work material, there is no relative movement.
Trepanning refers to the method of guiding the laser beam around a predetermined trajectory. The trajectory is the center of the hole to be drilled. It is worth noting that the diameter of the microvias to be cut is larger than the diameter of the laser beam. Therefore, the accuracy of the laser drilling microvias is dependent on the movement of the beam.
In the helical drilling method, the laser beam moves along a spiral path. At the same time, the laser beam rotates on its axis concerning the work material. “A dove prism controls the movement of the laser beam” (Sagar, 2021).
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