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1. What is halogen free PCB(HF PCB) substrate
According to jpca-es-01-2003 standard: PCB CCL with chlorine (C1) and bromine (BR) content less than 0.09% wt (weight ratio) is defined as halogen-free copper clad laminate. (at the same time, total CI + br ≤ 0.15% [1500ppm])
2. Why should halogen be banned in PCB
Halogen refers to the halogen group elements in the periodic table of chemical elements, including fluorine (f), chlorine (CL), bromine (BR) and iodine (1). At present, flame retardant FR4 substrate, FR4, CEM-3, etc., flame retardant is mostly brominated epoxy resin. In brominated epoxy resin, tetrabromobisphenol A, polybrominated biphenyl, polymerized polybrominated diphenyl ether and polybrominated diphenyl ether are the main blocking fuel for copper clad laminates, which are low cost and compatible with epoxy resin. However, studies by relevant institutions have shown that halogen containing flame retardant materials (polybrominated biphenyl PBB: polybrominated diphenyl ether PBDE) will emit dioxin dioxin (TCDD) and benzfuran (benzfuran) when they are burned on fire. They have a large amount of smoke, bad smell, high toxic gases, carcinogenic, and can not be discharged after ingestion, which is not environmentally friendly and affects human health. Therefore, the EU initiated a ban on PBB and PBDE as flame retardants in electronic information products. According to the same document issued by the Ministry of information industry of China, the electronic information products put into the market shall not contain lead, mercury, hexavalent chromium, polybrominated biphenyls or polybrominated diphenyl ether. Six substances, such as PBB and PBDE, are prohibited by EU law. It is understood that PBB and PBDE are basically not used in the copper clad laminate industry. Bromine flame retardant materials other than PBB and PBDE are mostly used, such as tetrabromobisphenol A, dibromophenol, etc., whose chemical formula is cishizobr4. There are no laws and regulations on this kind of copper clad laminate containing bromine as flame retardant. However, this kind of copper clad laminate containing bromine will release a large amount of toxic gas (brominated type) during combustion or electrical fire. When PCB is used for hot air leveling and component welding, the board will also release a small amount of hydrogen bromide due to the influence of high temperature (> 200); whether dioxins will also be produced is still under evaluation. Therefore, FR4 board containing tetrabromobisphenol A flame retardant is not prohibited by law and can be used, but can not be called halogen free PCB board.
3. Principle of halogen free PCB(HF PCB) substrate
At present, most of halogen free PCB materials are mainly phosphorus and phosphorus nitrogen series. When the phosphorus containing resin is burned, it will decompose into metapolyphosphoric acid, which has strong dehydration property, forms carbonization film on the surface of polymer resin, insulates the burning surface of resin from contacting with air, extinguishes the fire and achieves flame retardant effect. The high polymer resin containing phosphorus and nitrogen compounds produces non combustible gas during combustion, which helps the resin system to be flame retardant.
4, Characteristics of halogen free PCB(HF PCB)
4.1) insulation of halogen free PCB materials
Due to the substitution of halogen atoms with P or N, the polarity of the molecular bond segment of epoxy resin is reduced to a certain extent, so as to improve the insulation resistance and puncture resistance of epoxy resin.
4.2) water absorption of halogen-free PCB materials
halogen free PCB(HF PCB) board is less likely to form hydrogen bond with hydrogen atom in water than halogen material because the lone pair electrons of N and P in nitro phosphorus redox resin are less than that of halogen material, so the water absorption of halogen-free PCB material is lower than that of conventional halogen-based flame retardant material. For the plate, low water absorption has a certain impact on improving the reliability and stability of the material.
4.3) thermal stability of halogen free PCB materials
The content of nitrogen and phosphorus in Halogen-free PCB is higher than that in ordinary halogen-based PCB, so the monomer molecular weight and TG value are increased. In the case of heating, its molecular movement ability will be lower than that of conventional epoxy resin, so the thermal expansion coefficient of halogen-free PCB material is relatively small.
5. Production of halogen free PCB experience
At present, a large number of PCB suppliers have developed or are developing halogen-free PCB copper clad laminates and corresponding semi cured chips. As far as we know, pcl-fr-226 / 240 of polyclad, del04ts of Isola, s1155 / s0455, South Asia, Hongren ga-hf and Panasonic GX series. In 2008, ipcb has begun to use polyclad's pcl-fr-226 / 240 board for the production of mobile phone battery board. This year, ipcb has developed the production of Shengyi s1155 halogen free PCB board and Multilayer PCB board, and the halogen-free PCB board of South Asia is also in trial use. At present, the use of halogen-free plate has accounted for 20% of our total plate consumption.
5.1) lamination of halogen free PCB(HF PCB)
Lamination parameters may vary from company to company. Take the Shengyi substrate and PP as the multilayer board as an example. In order to ensure the full flow of resin and good bonding force, it requires a lower heating rate (1.0-1.5 degree Celsius / min) and multi-stage pressure coordination. In addition, it requires a longer time in the high-temperature stage, and maintains at 180 degree Celsius for more than 50 minutes. The following is a set of recommended platen program settings and the actual sheet temperature rise. The bonding force between the copper foil and the substrate is 1.on/mm. After six times of thermal shock, there is no delamination and bubble phenomenon on the extruded plate.
5.2) drilling machinability of halogen-free PCB
Drilling condition is an important parameter, which directly affects the quality of hole wall in PCB machining process. Because P and N series functional groups are used in the copper clad laminate of halogen-free PCB, the molecular weight is increased and the rigidity of molecular bond is enhanced. At the same time, the TG point of halogen-free material is generally higher than that of ordinary copper clad laminate. Therefore, the drilling effect of ordinary FR-4 drilling parameters is not very ideal. When drilling halogen-free plate, some adjustments should be made under normal drilling conditions. For example, our company adopts Shengyi s1155 / s0455 core board and PP four layer board, and its drilling parameters are not the same as normal drilling parameters. When drilling halogen-free plate, the speed of rotation is 5-10% higher than the normal parameters, while the speed of feeding and withdrawing is 10-15% lower than that of normal parameters, so that the roughness of the hole drilled is small
5.3) alkali resistance of halogen free PCB(HF PCB)
Generally, the alkali resistance of halogen-free PCB board is worse than that of common FR-4. Therefore, in the etching process and rework process after solder mask, special attention should be paid to that the immersion time in alkaline stripping solution should not be too long to prevent white spots on the substrate. In the actual production, our company has suffered a loss: the halogen-free plate that has been soldered and solidified needs to be backwashed due to some problems. However, it is still in the ordinary FR-4 backwashing mode, soaking in 10% NaOH at 75 degree Celsius for 40 minutes. As a result, all the white spots on the substrate are washed out, and the soaking time is shortened to 15-20 minutes. This problem no longer exists. Therefore, for halogen-free PCB board rework, it is best to do the first board to get the best parameters, and then batch rework.
5.4) solder mask fabrication of halogen free PCB(HF PCB)
At present, there are many kinds of halogen-free PCB solder resist ink on the market, and its performance is not different from ordinary liquid photosensitive ink, and the specific operation is almost the same as ordinary ink.
6, Conclusion
Due to the low water absorption and environmental protection requirements, halogen-free PCB can also meet the quality requirements of PCB in other properties. Therefore, the demand for halogen-free PCB is increasing. In addition, the major board suppliers have invested more funds in the research and development of halogen-free PCB substrate and halogen-free pp. I believe that in the near future, low-cost halogen-free PCB will be put into the market immediately. Therefore, ipcb put the trial and use of halogen free PCB on the agenda, worked out a detailed plan, and gradually expanded the share of halogen-free PCB in ipcb, so that ipcb was ahead of the market demand.
