A model for self-defocusing in laser drilling of polymeric materials

Abstract

A numerical thermal model is presented for laser microvias drilling in multilayer electronic substrates with Nd:YAG ͑YAG denotes yttrium aluminum garnet͒ and CO 2 lasers. Such substrates have different optical properties such as the refractive index and absorption coefficient at these two laser wavelengths, resulting in different drilling mechanisms. Since the skin depth of the polymer is large for both the lasers, volumetric heating is considered in the model. As soon as a small cavity is formed during the drilling process, the concave curvature of the drilling front acts as a concave lens that diverges the incident laser beam. This self-defocusing effect can greatly reduce the drilling speed as predicted by the model. This effect makes the refractive index of the substrate at different wavelengths an important parameter for laser drilling. The model was used to calculate the laser ablation thresholds which were found to be 8 and 56 J / cm 2 for the CO 2 and Nd:YAG lasers respectively. Due to the expulsion of materials because of high internal pressures in the case of Nd:YAG laser microvia drilling, the ablation threshold may be far below the calculated value. A particular laser beam shape, such as pitch fork, was found to drill better holes than the Gaussian beam.

Laser Ablation

“…The calculation results based on eqn [43] and eqn [44] are shown in Figure 39 and Figure 40, respectively. The comparison shows that the effect of non-Fourier heat conduction cannot be ignored.…”

“…However, for polymers, phase transitions (e.g., solid / melt / vapor) and chemical degradation often occur at high temperatures (39). Generally, both polymers and ceramics have a deeper heat penetration, so the heat transport theoretical model considering a volumetric heating for drilling is more accurate than a model only considering surface heating (41)(42)(43). Lawrence et al confirmed that for CO 2 laser, the absorption length for Al 2 O 3 /SiO 2 -based refractory material is three orders of magnitude greater than most of metals (38,40).…”

Microwave Drilling with Litz Wire using a Domestic Applicator

“…The area of microwave drilling on various materials like concrete, glass, ceramics, basalts, silicon, and bones are well envisaged and explored [9] - [12]. The application of bone drilling in biomedical engineering, electronics industry and aeronautical engineering involves the majority of recent drilling applications for small size holes [13] - [18].…”