Abstract: In order to meet the urgent requirements of the electronics industry for the elimination of lead, the printed circuit board (PCB) industry is shifting the final surface treatment from hot-air flat spray tin (tin lead eutectic) to other surface treatments, including organic protective film (OSP), silver precipitation, tin precipitation and chemical nickel precipitation. OSP membrane is considered to be the best choice due to its excellent weldability, simple process and low operating cost.
OSP(Organic weldable protective film) is considered to be the best surface treatment process due to its excellent weldability, simplicity of process and low cost.
In this paper, thermal desorption, gas chromatography-mass spectrometry (TD-GC-MS), thermogravimetry (TGA) and photoelectron spectroscopy (XPS) were used to analyze the thermal properties of the new generation of high temperature resistant OSP films. Small molecular organic components affecting weldability in high temperature resistant OSP films (HTOSP) were determined by gas chromatography, and the alkyl benzimidazole-HT in high temperature resistant OSP films showed minimal volatility. The TGA data showed that HTOSP membrane had higher degradation temperature than the current industry standard OSP membrane. XPS data showed that the oxygen content of high-temperature OSPs increased by only about 1% after 5 lead-free reflow cycles. The above improvements are directly related to the requirements of industrial lead-free weldability.
OSP membranes have been used in circuit boards for many years as organomaterials formed by the reaction of azole compounds (azole) with transition metal elements, such as copper and zinc. Many studies [1,2,3] have revealed the corrosion inhibition mechanism of azole compounds on metal surfaces. G.p.bowen  has successfully synthesized organometallic polymers of benzimidazole with copper (II), zinc (II) and other transition metal elements, and has described by TGA the excellent high temperature resistance of poly (Benzimidazole-zinc). G.P.Brown's TGA data show that poly (benzimidazole-zinc) degrades at up to 400℃ in air and 500℃ in nitrogen-protected atmospheres, while poly (Benzimidazole-copper) degrades at only 250℃. The recently developed new HTOSP membranes are based on the chemical properties of poly (benzimidazole-zinc) to provide optimal heat resistance.
OSP films are mainly composed of organometalllic polymers and small organic molecules, such as fatty acids and azole compounds, enjoined during deposition. Organometalllic polymers provide the necessary corrosion resistance, copper surface adhesion, and OSP surface hardness. The degradation temperature of the organometalllic polymer must be higher than the melting point of the lead-free solder to withstand the lead-free process. Otherwise, the OSP film will degrade after being treated with a lead-free process. The degradation temperature of OSP films largely depends on the heat resistance of organometalllic polymers. Another important factor affecting copper oxidation resistance is the volatility of azole compounds, such as benzimidazole and phenylimidazole. The small molecules of OSP film will evaporate during the lead-free reflux process, thus affecting the oxidation resistance of copper. The heat resistance of OSPs can be scientifically demonstrated using gas chromatography-mass spectrometry (GC-MS), thermogravimetry (TGA), and photoelectron spectroscopy (XPS).
1. Analysis by gas chromatography-mass spectrometry
The copper plates for these tests were coated with: a) the new HTOSP film; b) industrial standard OSP film; c) Another kind of industrial OSP film. Scrape about 0.74-0.79mg of OSP film from the copper plate. The coated copper plates and scraped samples have not undergone any reflux treatment. H/P6890GC/MS instrument and needle-free syringe were used in this experiment. A needle - less syringe can desorb solid samples directly in the sampling chamber. The needle - less syringe can transfer the sample from the small glass tube to the gas chromatography inlet chamber. Carrier gas can continuously bring volatile organic compounds to the gas chromatograph column for collection and separation. The sample is placed close to the top of the column so that thermal desorption is effectively repeated. When enough samples were detached, gas chromatography began.
In this experiment, RestekRT-1(0.25mmid×30m, film thickness 1.0μm) gas chromatography column was used. Heating procedure of gas chromatographic column: After 2 minutes of heating at 35℃, the temperature starts to rise to 325℃ at a rate of 15℃ / min. The thermal desorption condition is: after heating at 250℃ for 2 minutes. The mass/charge ratio range of the isolated volatile organic compounds is 10-700 Daltons. Retention times of all organic small molecules were also recorded.