Characterization of laser surface roughening on copper using a Q-switched ND: YAG laser

Abstract

Laser surface roughening (LSR) is a new surface treatment. Copper is a good conductor and it is the key material to a lead frame in electronic packaging that serves mainly to support chip. However, LSR is still at an infant stage compared to chemical methods that are widely used to roughen the copper surface in modern material processing. The problem with chemical methods is that it cannot be controlled and involves waste or pollution which is not environmental friendly and requires high budget. Therefore, in attempt to shed light on this matter, a fundamental study is carried out by roughening copper surface using laser at different parameters including energies and number of pulses. LSR system is developed using a Q-switched Nd:YAG laser with fundamental wavelength of 1064 nm and pulse duration of 10 ns. In this process, the laser energies used in the range from 150 mJ until 600 mJ. An adhesion test was carried out on copper plates to study the relationship between the surface roughness and the adhesion force. A plate of a pure copper with a smooth surface was employed as a sample. When the Q-switched Nd:YAG laser is focused on the sample, the roughening copper surface formed and was analyzed using atomic force microscope (afm), field emission scanning electron microscope and high resolution x-ray diffractometer. Average roughness and root mean square roughness increase with laser energies and number of pulses. Therefore, the surface roughness processing is strongly dependent on the laser energies and number of pulses. The stronger the laser energies and number of pulses, the higher is the roughness of the copper surface. The scanning electron microscope results show that the microstructure drastically changed with the laser energies and number of pulses. Most of the surface roughness is free from porosity and crack even at increasing laser energies and number of pulses. This will give advantages in adhesive bonding. The adhesion test shows that when the surface roughness increases, the adhesion force also increases. These conditions may also give an advantage to the surface roughness with increasing the Q-switched Nd:YAG laser will provide a strong bond to the surface in order to improve the adhesion force. Therefore, LSR process can be a beneficial method for roughening the copper surface compared to existing methods.