(Invited, Digital Presentation) Finite Element Analysis of Temperature Distribution on Laser Enhanced Electroless Plating by Continuous Wave Laser
Authors: Suzuki Tsunehisa , Uwano Shotaro , Fujii Tetsuya , Nomura Mitsuyoshi , Gouya Kenji , Muraoka Jun-ichi 
Published: December 2024

Laser enhanced electroless plating is a technique to deposit metal locally by irradiating a laser beam onto a substrate immersed in a plating solution. Since micro-sized metal deposits can be fabricated, it is expected to be applied to 3D-MID and high-definition metal 3D printers. We have performed Laser enhanced electroless plating of iron substrates in electroless nickel (Ni) plating solution using a continuous-wave (CW) laser and clarified the change in the properties of Ni deposits as a function of laser irradiation time. However, the temperature distribution on the substrate surface, the relationship between temperature change and Ni deposition form, deposition temperature conditions, and deposition mechanism have not been clarified.

In this study, to clarify the relationship between the substrate surface temperature and Ni deposition conditions during Laser enhanced electroless plating, we first measured the temperature distribution and temperature change on the backside of the substrate during Laser enhanced electroless plating using a thermal microscope. Next, the temperature distribution on the surface of the substrate was estimated from the measured temperature distribution on the backside of the substrate using finite element analysis, and the Ni deposition conditions were discussed and the analytical model was validated based on the temperature distribution on the surface of the substrate and the properties of Ni deposits. Furthermore, using the validated analytical model, we analyzed the change in the substrate temperature distribution in response to changes in laser output power.

A 980 nm CW multimode fiber laser with a laser power of 10-20 W was irradiated for 300 s through a quartz window onto a steel substrate immersed in the plating solution. A medium-high phosphorus electroless Ni-P solution was kept at 24 °C and circulated at a flow rate of 0.3 L/min using a pump. The iron substrate was coated with a black acrylic coating approximately 30 μm thick on the back side for temperature measurement and fixed over a single-crystal germanium window (2 mm thick). A thermal microscope (Apiste FSV-2000, wavelength range: 8-14 μm) was used to measure the temperature of the backside of the substrate during laser irradiation through the germanium. The temperature measured through germanium was corrected for the effects of reflection and attenuation using a coefficient obtained from the temperature measured using a thermocouple.

The temperature distribution analysis was performed using finite element analysis software (COMSOL Multiphysics®). The analytical model consists of a 300 µm-thick iron substrate (20 mm diameter), a 30 µm-thick acrylic coating film, and a 2 mm-thick single-crystal germanium, each in contact with the other, without considering the contact thermal resistance at each interface. The surface of the substrate is assumed to dissipate heat to the plating solution, and the backside of the substrate is assumed to dissipate heat to the atmosphere through the coating film and the germanium window, respectively. The heat transfer coefficients were determined as forced convection and natural convection, taking into account the flow rate and velocity, respectively. A heat source with a Gaussian distribution with a spot diameter of 150 μm was placed at the center of the substrate surface. Considering the laser attenuation in the plating solution and the reflection on the substrate surface, the heat quantity was adjusted so that the maximum value of the measured and analyzed temperatures on the backside of the substrate coincided at a laser power of 20W. The adjusted heat value was approximately 10 % of the laser power. Using this analytical model, the temperature distribution on the substrate surface was estimated from the measured temperature on the backside of the substrate when the laser was irradiated at 20 W output power. In addition, the heat quantity at an output of 10-15W was also set to 10 % of the laser power, as in the case of 20W, and the temperature distribution on the backside and surface of the substrate was estimated.

The temperature distribution on the surface of the substrate was estimated from the measured temperature distribution on the backside of the substrate using finite element analysis, and compared with the range of Ni precipitation by Laser enhanced electroless plating. The measured and analyzed temperatures were almost identical for the temperature distribution on the backside of the substrate and the temperature variation with irradiation time. Furthermore, from the comparison between the range of Ni deposition and the analytical temperature distribution on the substrate surface, it is estimated that Ni is deposited when the substrate surface temperature is above 80°C. This result is consistent with the Ni deposition conditions of electroless Ni plating in general.

Figure 1