Ultrashort Pulsed Laser Drilling of Printed Circuit Board Materials

Abstract

We report on a comprehensive study of laser percussion microvia drilling of FR-4 printed circuit board material using ultrashort pulse lasers with emission in the green spectral region. Laser pulse durations in the pico- and femtosecond regime, laser pulse repetition rates up to 400 kHz and laser fluences up to 11.5 J/cm2 are applied to optimize the quality of microvias, as being evaluated by the generated taper, the extension of glass fiber protrusions and damage of inner lying copper layers using materialography. The results are discussed in terms of the ablation threshold for FR-4 and copper, heat accumulation and pulse shielding effects as a result of pulse to pulse interactions. As a specific result, using a laser pulse duration of 2 ps appears beneficial, resulting in small glass fiber protrusions and high precision in the stopping process at inner copper layer. If laser pulse repetition rates larger than 100 kHz are applied, we find that the processing quality can be increased by heat accumulation effects.

Keywords: ablation threshold; heat accumulation; laser drilling; materialography; microvia; printed circuit board (PCB); pulse to pulse interactions; ultrashort pulse (USP) laser.

Figure 1

Laser-based fabrication of microvias in FR-4 PCB material and their technical specification based on the taper (ratio of the upper to lower diameter), glass fiber protrusion, and damage in the inner copper layer Cu${}_{\mathrm{damage}}$.

Figure 2

Setup for investigations of pulse to pulse interactions in the percussion drilling of microvias in FR-4 PCB material.

Figure 3

Evaluation of the microvia quality after laser percussion drilling process using (a) materialography and (b) optical microscopy.

Figure 4

Ablation thresholds for (a) copper and (b) FR-4 using laser fluences of F = 1.3–11.5 J/cm${}^2$, laser pulse durations of $\tau$ = 0.9 and 6 ps, and a number of pulses between 15 and 60.

Figure 5

(a) Glass fiber protrusion, (b) taper, (c) damage in inner copper layer and (d) diameter of 10 laser drilled microvias in FR-4 PCB material depending on the laser pulse duration of $\tau$ = 0.23, 1 and 2 ps using a laser pulse repetition rate of f = 50 kHz, a laser pulse energy of E = 10 µJ and a number of laser pulses between 52 and 65.

Figure 6

(a) Glass fiber protrusion, (b) taper, (c) damage in inner copper layer, and (d) diameter of 10 laser drilled microvias depending on the laser pulse energy of E = 15.8–45.8 µJ and laser pulse repetition rate of f = 5–400 kHz using a laser pulse duration of $\tau$ = 2 ps.

Figure 7

Selected cross sections of microvias in FR-4 PCB material at different laser pulse energies of E = 15.8–45.8 µJ and laser pulse repetition rates of f = 5–400 kHz using a laser pulse duration of $\tau$ = 2 ps, observing an decrease in taper for laser pulse repetition rates of more than 100 kHz.