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The purpose of this paper is to investigate the effects of H2 flow rate on improving the solder wettability of oxidized‐copper with liquid lead‐free solder (96.5Sn‐3Ag‐0.5Cu) by Ar‐H2 plasmas. The aim was to improve the solder wettability of oxidized copper from 0 per cent wetting of copper oxidized in air at 260^oC for 1 hour to 100 per cent wetting of oxidized‐copper modified by Ar‐H2 plasmas at certain H2 flow rates and to find correlations between the surface characteristics of copper and the solder wettability with liquid lead‐free solder.

To reduce the copper oxides on the surfaces of oxidized‐copper for improving solder wettability with liquid lead‐free solder, this study attempted to apply Ar‐H2 plasmas to ablate the copper oxides from the surfaces of oxidized‐copper by the physical bombardment of the Ar plasmas and to reduce the surfaces of oxidized‐copper by the chemical reaction of H2 plasmas with the surfaces of oxidized‐copper.

The solder wettability of oxidized‐copper was found to be highly dependent on the surface characteristics of the copper. The values of polar surface free energy and dispersive surface free energy on the surfaces of oxidized‐copper modified by Ar‐H2 plasmas were close to those values of solid lead‐free solder, which resulted in improved solder wettability with liquid lead‐free solder. Auger spectra indicated that the Ar‐H2 plasma modification was used to remove the copper oxides from the surfaces of oxidized‐copper.

The surface characterization of copper surfaces is typically determined by expensive surface analysis tool such as Auger Electron Spectroscopy (AES). This paper reports the results of a study of a promising technique called the sessile drop test method, for examining the surface free energies such as total surface free energy, polar surface free energy and dispersive surface free energy on the surfaces of copper to clarify how the solder wettability of oxidized‐copper with liquid lead‐free solder was enhanced by Ar‐H2 plasmas.

An investigation has been performed on the improved solder wettability of oxidized aluminum (Al) with lead-free solder (96.5Sn-3.5Ag) using Ar-H₂ plasmas. The lead-free solder wettability was raised from 62.2 per cent wetting for Al oxidized in air at 250 C for 4 h to 98.4 per cent wetting of oxidized Al modified by Ar-H₂ plasmas at a certain H₂ flow rate. This study aims to gain insight on the surface characteristics of Al affecting the solder wettability with a liquid lead-free solder.

Ar-H₂ plasmas at certain H₂ flow rates are intended to reduce Al oxides on the surfaces of oxidized Al substrates both by physical bombardments via Ar plasmas and chemical reductions with H₂ plasmas, while Al substrates are exposed in Ar-H₂ plasmas to improve the solder wettability with a liquid lead-free solder.

Surface characteristics of oxidized Al substrates have been identified to play key roles for enhanced lead-free solder wettability using Ar-H₂ plasmas. A decrease in polar surface free energy and an increase in dispersive surface free energy on the surfaces of oxidized Al substrates are exploited to advance the lead-free solder wettability. Decreased composition ratios of O to Al, detected by X-ray photoelectron spectroscopy (XPS) for oxidized Al substrates, are crucial for improved lead-free solder wettability.

XPS is typically used to analyze the surface compositions of Al oxides. To provide a rapid and non-expansive method to identify the surfaces of Al substrates prior to soldering to assure lead-free solder wettability, this study proposes a measurable skill, a so-called sessile drop test method, to investigate surface free energies such as total, polar and dispersive surface free energy on the surfaces of Al substrates, to illuminate how the lead-free solder wettability of oxidized Al is improved by Ar-H₂ plasmas.

The purpose of this paper is to investigate the efficiencies of argon (Ar), oxygen (O₂) and O₂ followed by Ar (O₂→Ar) plasma treatments in terms of contaminant removal and wire bond interfacial adhesion improvement. The aim of this study is to resolve the “lifted ball bond” issue, which is one of the critical reliability checkpoints for light emitting diodes (LEDs) in automotive applications.

Ar, O₂ and O₂→Ar plasma treatments were applied to LED chip bond pad prior to wire bonding process with different treatment durations. Various surface characterization methods and contact angle measurement were then used to characterize the surface properties of these chip bond pads. To validate the improvements of Ar, O₂ and O₂→Ar plasma treatments to the wire bond interfacial adhesion, the chip bond pads were wire bonded and examined with a ball shear test. Moreover, the contact resistance of the wire bond interfaces was also measured by using four-point probe electrical measurements to complement the interfacial adhesion validation.

Surface characterization results show that O₂→Ar plasma treatment was able to remove the contaminant while maintaining relatively low oxygen impurity content on the bond pad surface after the treatment and was more effective as compared with the O₂ and Ar plasma treatments. However, O₂→Ar plasma treatment also simultaneously reduced high-polarity bonds on the chip bond pad, leading to a lower surface free energy than that with the O₂ plasma treatment. Ball shear test and contact resistance results showed that wire bond interfacial adhesion improvement after the O₂→Ar plasma treatment is lower than that with the O₂ plasma treatment, although it has the highest efficiency in surface contaminant removal.

To resolve “lifted ball bond” issue, optimization of plasma gas composition ratios and parameters for respective Ar and O₂ plasma treatments has been widely reported in many literatures; however, the O₂→Ar plasma treatment is still rarely focused. Moreover, the observation that wire bond interfacial adhesion improvement after O₂→Ar plasma treatment is lower than that with the O₂ plasma treatment although it has the highest efficiency in surface contaminant removal also has not been reported on similar studies elsewhere.

The purpose of this paper is to investigate the end point of the plasma cleaning process, in order to save valuable production time and also to prevent the destruction of expensive devices through overheating.

Post plasma cleaning analysis using dynamic contact angle (DCA) analysis and video microscope observations have been conducted on plasma treated Entec coated copper, hot air solder levelled (HASL) and electroless nickel immersion gold (ENIG) surfaces in order to determine the effect of surface finish on plasma process times. Post plasma cleaning and lead-free wave soldering analysis of the three surface finishes have been evaluated.

The DCA results indicate that the end-point for plasma cleaning of Entec coated copper is achieved within one to three minutes (indicated by a low contact angle hysteresis). Further cleaning after three minutes may lead to surface degradation and poor solder wettability. This is consistent with the results of the video microscope observations which show well soldered component leads and pads with good solder coverage for copper surface finish treated at a short plasma process time (one minute) and poorly soldered component leads and pads for a surface treated at a long process time (ten minutes). Similar work conducted on HASL and ENIG finishes show better results (well soldered component leads and solder pads) for longer plasma process times of five to ten minutes.

This paper indicates that plasma process time determines the wettability and solderability of the treated samples. It shows that the plasma conditions including the process time must be optimised and characterised for every application in order to avoid damage to expensive devices. The findings also give the confidence to implement plasma cleaning of lead-free (99.3Sn/0.7Cu) solder PCBs for fluxless soldering.