Laser Induced Selective Activation For Subsequent Autocatalytic Electroless Plating

The subject of this PhD thesis is “Laser induced selective activation for subsequent autocatalytic electroless plating.” The objective of the project is to investigate the process chains for micro structuring of polymer surfaces for selective micro metallization. Laser induced selective activation (LISA) is introduced and studied as a new technique for producing 3D moulded interconnect devices (3D-MIDs). This technique enables the metallization of polymer surface modified by laser and subsequently activated by a PdCl2/SnCl2 system. Various technologies exist on an industrial level for manufacturing MIDS, e.g. LDS®, 2-shot injection moulding, etc. Compared with them, LISA shows economic and environmental advantages. First, the range of available polymers is wide, and no pretreatment such as filler premix is need. Commercial polymer such as polyethylene and polystyrene, polycarbonate, ABS, etc. can be directly used in the LISA process. Second, in the wet steps, no chromic acid or other similar toxic compounds are used. The principle of the PdCl2/SnCl2 activation system is explained based on previous researchers’ studies. Investigations were conducted as to how the laser tracks keep the activation colloids based on three hypotheses. The first hypothesis is that laser machining leads to chemical changes of the polymer, which results in chemical bonding with the activation colloids. Chemical changes on the laser track were investigated by XPS or FTIR spectroscopy, but no evidence shows that chemical bonds exist. However, it is still not excluded that chemical bonding is part of the mechanism. The second hypothesis is that the laser track has a stronger attraction work to the activation solution. This is proved by a calculation using van Oss et al., theory based on contact angle measurement. The third hypothesis is that the activation and rinsing process can be described by diffusion. This hypothesis is proved using Fick’s diffusion laws combined with the short-time-plating experiment. The influence of laser parameters on the surface structure is investigated for Nd:YAG, UV, and fiber lasers. The mechanism of the surface structure formation reason is discussed and the Nd:YAG laser parameters’ effect on the structure of the laser track is investigated. There is a trend showing that the height of the laser track increases with the laser energy output. A characterization method for the porous surface based on the bearing area curve and its parameters is propsed. Comparison of two activation methods based on plating velocity was performed. It was found that the plating velocity is independent of the height of the laser track, as long as the structure is sponge-like and continuous. The adhesive strength of the copper coating is related to the laser structure. High laser energy input will lead to high laser track, on which the copper coating has a stronger adhesion than on the track made by lower energy, even though the coatings have similar thickness. The LISA method was tested in the case of an antenna. Two simple antenna shapes, planar dipole, and planar loop were studied, as well as a co-planar stripline. The antennas made by LISA exhibited impedance characteristics expected from the respective antenna shapes and comparable to antennas made by PCB technology.