2 Due to the excellent weldability, simplicity, and low cost of OSP (Organic Solderable Protective Film), it is considered the best surface treatment process. In this article, thermal desorption gas chromatography-mass spectrometry (TD-GC-MS), thermogravimetric analysis (TGA), and photoelectron spectroscopy (XPS) were used to analyze the relevant heat resistance performance of the new generation of high-temperature OSP films.
3 Gas chromatography detected small organic components that affect the solderability of high-temperature OSP membranes (HTOSP), and showed that the alkylbenzimidazole-HT in the high-temperature OSP membrane had almost no volatility. TGA data shows that the degradation temperature of HTOSP membrane is higher than the current industry standard OSP membrane.
4 XPS data shows that after 5 lead-free reflow soldering cycles, the oxygen content of high-temperature OSP only increases by about 1%. The above improvements are directly related to the industrial lead-free solderability requirements.
OSP films have been used in printed circuit boards for many years and are organometallic polymer films formed by the reaction of azoles with transition metal elements such as copper and zinc. Many studies have revealed the corrosion inhibition mechanism of azoles on metal surfaces. G.P. Brown successfully synthesized organometallic polymers of benzimidazole and copper(II), zinc(II), and other transition metal elements and described the excellent high-temperature resistance properties of poly(benzimidazole-zinc) by TGA. G.P. Brown’s TGA data show that the degradation temperature of poly(benzimidazole-zinc) is as high as 400°C in air and 500°C in a nitrogen protective atmosphere, while the degradation temperature of poly(benzimidazole-copper) is only 250°C. The recently developed new HTOSP film is based on the chemical properties of poly(benzimidazole-zinc) and thus has excellent heat resistance. OSP films are mainly composed of organometallic polymers and entrained organic small molecules, such as fatty acids and azoles, during deposition. Organometallic polymers provide the necessary corrosion resistance, copper surface adhesion, and OSP surface hardness. The degradation temperature of the organometallic polymer must be higher than the melting point of the lead-free solder to withstand lead-free processing. Otherwise, the OSP film will degrade after the lead-free process. The degradation temperature of OSP films largely depends on the heat resistance of the organometallic polymers. Another important factor affecting copper antioxidant activity is the volatility of azole compounds, such as benzimidazole and phenylimidazole. The small molecules of the OSP film evaporate during the lead-free reflow process, thus affecting the oxidation resistance of copper. The thermal resistance of OSP can be scientifically demonstrated using gas chromatography-mass spectrometry (GC-MS), thermogravimetric analysis (TGA), and photoelectron spectroscopy (XPS).
Analysis by gas chromatography-mass spectrometry
The copper panels tested were coated with: a) a novel HTOSP film; b) an industry-standard OSP film; and c) another industrial OSP film. About 0.74-0.79 mg of OSP film was scraped off the copper plate. Neither the coated copper plates nor the scraped samples were subjected to any reflow treatment. The H/P6890GC/MS instrument was used in this experiment, and a syringe without a barrel was used. Syringe-free syringes can desorb solid samples directly in the injection chamber. Syringe-free syringes can transfer samples from tiny glass tubes to the inlet chamber of a gas chromatograph. The carrier gas continuously brings volatile organics to the GC column for collection and separation. Placing the sample against the top of the column allows for efficient replication of thermal desorption. After there is enough sample to be desorbed, the gas chromatography starts to work. In this experiment, a RestekRT-1 (0.25 mm id×30 m, film thickness of 1.0 μm) gas chromatography column was used. The heating program of the gas chromatography column: after heating at 35°C for 2 minutes, the temperature was raised to 325°C, and the heating rate was 15°C/min. Thermal desorption conditions were: after heating at 250°C for 2 minutes. The mass/charge ratios of the separated volatile organic compounds were detected by mass spectrometry in the range of 10-700 daltons. The retention times of all small organic molecules were also recorded.
Thermogravimetric Analysis (TGA)
Likewise, a new HTOSP film, an industry-standard OSP film, and another industrial OSP film were coated on the samples, respectively. About 17.0 mg of OSP film was scraped from the copper plate as a material test sample. Neither the sample nor the film can be subjected to any lead-free reflow treatment before TGA testing. TGA tests were performed under nitrogen protection using a 2950TA from TA Instruments. The operating temperature was maintained at room temperature for 15 minutes and then increased to 700°C at a rate of 10°C/min.
Photoelectron Spectroscopy (XPS)
Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), is a chemical surface analysis method. XPS measures the chemical composition of the coating surface at 10 nm. The HTOSP film and the industry standard OSP film were coated on the copper plate and then subjected to 5 lead-free reflows. The HTOSP film before and after reflow treatment was analyzed by XPS; the industry standard OSP film after 5 lead-free reflows was also analyzed by XPS, and the instrument used was VGESCALAB Mark II.
Through Hole Solderability Test
Through-hole solderability testing is performed using Solderability Test Boards (STBs). A total of 10 solderability test board STV arrays (4 STVs per array) were coated with a film thickness of approximately 0.35 μm, of which 5 STV arrays were coated with HTOSP film and the other 5 STV arrays were coated with an industry-standard OSP film. The coated STVs are then subjected to a series of high-temperature, lead-free reflow treatments in a solder paste reflow oven. Each test condition included 0, 1, 3, 5, or 7 consecutive reflows. There were 4 STVs per membrane for each reflow test condition. After the reflow process, all STVs are processed for high temperature and lead-free wave soldering. Through-hole solderability can be determined by inspecting each STV and counting the number of correctly filled through-holes. The criterion for through-hole acceptance is that the solder fill must be filled to the top of the plated through-hole or to the upper edge of the through hole. Each STV has 1196 through holes: 10mil holes-Four grids, 100 holes each grid square and round pads; 20mil holes-Four grids, 100 holes each grid square and round pads; 30mil holes-Four grids, 100 holes each grid square, and round pads.
Test Solderability by Tin-Dipping Balance
The solderability of the OSP film can also be determined by a dip tin balance test. Apply the HTOSP film on the tin-dipping balance test samples, after 7 times of lead-free reflow, Tpeak=262°C. The reflow process was performed in air using a BTUTRS combined with an IR/convection reflow oven. Wetting balance testing was performed in accordance with IPC/EIAJ-STD-003A Section 4.3.1.4, using the “Robotic Process Systems” automated dip balance tester, EF-8000 flux, no-clean flux, and SAC305 alloy solder.
Welding Bond Strength Test
Weld bond strength can be measured by shear force. The BGA pad test board (0.76mm in diameter) was coated with HTOSP films with thicknesses of 0.25 and 0.48 μm and subjected to three times of lead-free reflow treatment at 262°C. And soldered to the pads with matching solder paste, the solder balls are SAC305 alloy (0.76mm diameter). Shear tests were performed with a DagePC-400 adhesion tester at a shear rate of 200 μm/sec.
Results and Discussion
Gas Chromatography-Mass Spectrometry
Gas chromatography-mass spectrometry can detect the volatility of organic components in OSP films. Different OSP products in the industry contain different azoles including imidazoles and benzimidazoles. Alkylbenzimidazoles for HTOSP membranes, alkylbenzimidazoles for standard OSP membranes, and phenylimidazoles for other OSP membranes volatilize when heated in a gas chromatography column. Since organometallic polymers do not evaporate, gas chromatography-mass spectrometry cannot detect metal-polymerized azoles. Therefore, gas chromatography-mass spectrometry can only detect azoles and other small molecules that do not react with metals. Small molecules that are less volatile are generally retained longer under the same heating and gas flow conditions in a GC column. The residence time of the alkyl benzimidazole for the standard OSP membrane and the phenyl imidazole for the other OSP membrane was 19.0 min, illustrating the volatility of the HT alkyl benzimidazole.
Among the three OSP films, the HTOSP film contained fewer impurities. Organic impurities in OSP films can also affect the film’s solderability during reflow processing and cause discoloration. It was reported by Koji Saeki that due to the lower copper ion density on the OSP membrane surface, the polymerization reaction at the surface was weaker than that at the bottom of the membrane. The authors of this paper believe that unreacted azoles still remain on the surface of the OSP film. During the reflow process, more copper ions move from the bottom to the surface layer of the film, thereby providing an opportunity to react with unreacted azole compounds in the surface layer, thereby preventing copper oxidation. The alkylbenzimidazole-HT used in the HTOSP film is less volatile and therefore has a better chance of reacting with the copper ions moving from the lower layer to the surface layer, thereby reducing the oxidation of copper during the reflow process. XPS can show the transfer of copper ions from the lower layer to the surface layer, thereby reducing the oxidation of copper during the reflow process.
Thermogravimetric Analysis (TGA)
Thermogravimetric analysis (TGA) measures the mass change of substances due to temperature changes, and can perform effective quantitative analysis of mass changes. In the experiment of this paper, thermogravimetric analysis is a simulation method of lead-free reflow under nitrogen protection, which is used to analyze the volatilization of small molecules and the degradation of macromolecules of the OSP film during the lead-free reflow process of the OSP film under nitrogen protection. The TGA results showed that the degradation temperature of the industry standard OSP film was 259°C, while that of the HTOSP film was 290°C. Although the degradation temperature of poly(benzimidazole-zinc) is as high as 400°C, the actual degradation temperature of the HTOSP film cannot reach a high temperature of 400°C due to the presence of poly(benzimidazole-copper) in the film. Since the chemical composition of the industry-standard OSP film is poly(benzimidazole-copper), the degradation temperature of the film is low, only 259°C. Interestingly, another HTOSP film had two degradation temperatures, 256°C and 356°C, respectively. The reason is that this OSP film may contain iron, or due to the gradual decomposition of poly(phenylimidazole-iron). The TGA results obtained by F. Jian and his colleagues showed that poly(imidazole-iron) also has two degradation temperatures, 216°C and 378°C, respectively.
Photoelectron Spectroscopy
Photoelectron spectroscopy utilizes the analytical methods of photoionization and energy dispersion of emitted photoelectrons to study the composition and electronic state of the sample surface. The binding energy points of oxygen (1s), copper (2p), and zinc (2p) are shown in the XPS spectrum as 532-534eV, 932-934eV, and 1022eV, respectively. This technique can quantitatively analyze the surface composition of the outer 10 nm of the sample. By analysis, the HTOSP film contains 5.02% oxygen and 0.24% zinc before lead-free reflow treatment. After five times of lead-free reflow, the oxygen and zinc contents of the HTOSP film were 6.2% and 0.22%, respectively. After 5 lead-free reflows, the copper content increased from 0.60% to 1.73%. The reason for the increase of copper ions may be that the copper ions in the lower layer migrate to the surface layer during the reflow process. E.K. Chan and others also performed industry standard surface analysis of OSP films using photoelectron spectroscopy. Before any reflow treatment, the oxygen content was 5.0%, and then the oxygen content increased to 9.1% and 11.0% after 1 and 3 conventional SnPb reflows in air, respectively. It was also reported that the oxygen content of SnPb increased to 6.5% after nitrogen protection and reflow. In this experiment, photoelectron spectroscopy showed that the oxygen content of the industry standard OSP film increased to 12.5% after 5 lead-free reflows. Therefore, before and after 5 lead-free reflows, the oxygen content increased by 7.5%, which was greater than the 1.2% increase in oxygen content of the HTOSP film. The soldering performance of copper depends to a large extent on the degree of copper oxidation and the strength of the flux used. Therefore, the oxygen content measured by XPS is a good indicator of the heat resistance of OSP films. Compared with the industry-standard OSP film, HTOSP has better heat resistance. After 5 lead-free reflows, the discoloration test showed that the HTOSP film had basically no discoloration, while the industry-standard OSP film had obvious discoloration. The discoloration test results were consistent with the XPS analysis results.
Solderability Test
Wetting tin balance tests show that after multiple lead-free reflows, the through-hole solderability of the HTOSP film is higher than that of the current industry-standard OSP film. This is consistent with the heat resistance of the HTOSP film. With the increase of lead-free reflow times, T. (time to zero) will gradually increase, but the tin-wetting force will gradually decrease. However, the HTOSP film maintained its excellent solderability after 7 cycles of lead-free reflow. Shear testing showed that the shear force gradually increased and reached a point of 25N. Since the shear force depends on the cross-section of the shear, the results will vary depending on the shape of the solder ball and the gap between the shear and the pad. The authors of this paper believe that the shear force is not limited by the thickness of the OSP film as long as the copper surface is adequately protected against copper oxidation.
In conclusion
1. Alkylbenzimidazole-HT volatility for HTOSP membranes compared to other OSP membranes tested.
2. Degradation temperature of HTOSP film compared to other OSP films tested.
3. After 5 times of lead-free reflow, the oxygen content of HTOSP film increased by only 1%, while that of the industry-standard OSP film increased by 7.5%. At the same time, the HTOSP film basically does not change color.
4. Due to the excellent heat resistance of the HTOSP film, after more than 3 times of lead-free reflow, it still provides excellent solderability in the through-hole test and the tin dip balance test.
5. HTOSP film can provide high reliability solder joints, shear tests can prove this reliability on PCB board.
