Achieving Selective Cleaning on Semiconductors Packaging Using Atmospheric Pressure Plasma

Abstract

Surface cleaning on PCB substrate prior to assembly and packaging processes results in more reliable removal of surface contaminants. Plasma is a well-known and a key process in packaging industries for dry etching, photoresist removal, surface cleaning, etc. A non-thermal plasma generated at atmospheric pressure has been developed for this purpose with potential advantages of cleaner surface, controllability and high throughput process, in comparison to chemical cleaning or vacuum plasma.In this study, commercial plasma indicator is used as the substrate where the polymer with circle-shape (5 mm in diameter) is coated on rectangular polyethylene terephthalate (PET) substrate. Atmospheric pressure plasma (APP) with compressed dry air gas source is applied to clean this polymer-coated in selective area at maximum 145 °C in less than 1.6 s of plasma exposure time. Optical microscopy (O/M) analysis is conducted to observe the surface cleaning results. The cleaning performance is characterised with colour fade-out area on polymer coating with respect to the selective area defined by a mask. Image-J processing is also performed to quantitatively measure the fade out area with two categories: 1) clean, the area with polymer coating is completely fade out; 2) partial-clean, gradient or diffusion between fade-out and non-fade-out area.In this study, it is proved that selective cleaning with APP system is able to reach a spatial resolution of submillimetres, of which a scale threshold for applications on semiconductors packaging. In addition, our results indicate that cleaning performance is a function of working distance (WD) and plasma exposure time. Clean area is more dominant when the working distance is shorter, and when the scanning time is prolonged. It is found that when the working distance increases, the ratio of clean-area decreases and the partial-clean area increases. The prolonged exposure time results in increasing of clean-area. However, the partial-clean area remains nearly constant during multiple scanning. Furthermore, the local temperature follows a parabolic rate with exposure time, and has insignificant difference among working distance. Hypothesis of the mechanism will be discussed.