Views：982 Date：2020-05-08

What is halogen-free substrate

According to the JPCA-ES-01-2003 standard: the copper (C1) and bromine (Br) contents are less than 0.09% Wt (weight ratio), which is defined as halogen-free copper clad laminate. (At the same time, the total amount of CI + Br ≤0.15% [1500PPM])

Why ban halogen?

Halogen refers to halogen elements in the periodic table of chemical elements, including fluorine (F), chlorine (CL), bromine (Br), and iodine (1). At present, the flame retardant base material, FR4, CEM-3, etc., the flame retardant is mostly brominated epoxy resin. Among the brominated epoxy resins, tetrabromobisphenol A, polymerized polybrominated biphenyls, polymerized polybrominated biphenyl ethers, and polybrominated diphenyl ethers are the main fuel barriers of copper clad laminates. They have low cost and are compatible with epoxy resins. However, studies by relevant institutions have shown that halogen-containing flame retardant materials (polymerized polybrominated biphenyls PBB: polymerized polybrominated biphenyl ethers PBDE) will emit dioxin (TCDD), benzofuran (Benzfuran), etc. The amount of smoke is large, the smell is unpleasant, the highly toxic gas is carcinogenic, and cannot be discharged after ingestion. It is not environmentally friendly and affects human health. Therefore, the EU initiated a ban on the use of PBB and PBDE as flame retardants in electronic information products. The same document from the Ministry of Information Industry of China requires that by July 1, 2006, electronic information products put on the market must not contain lead, mercury, hexavalent chromium, polymerized polybrominated biphenyls or polymerized polybrominated biphenyl ethers.

The laws of the European Union prohibit the use of six substances such as PBB and PBDE. It is understood that PBB and PBDE are basically no longer used in the copper clad laminate industry, and most of them use bromine flame retardant materials other than PBB and PBDE, such as tetrabromo Bisphenol A, dibromophenol, etc., its chemical formula is CISHIZOBr4. This kind of copper clad laminate containing bromine as a flame retardant is not regulated by any laws and regulations, but this type of bromine-containing copper clad laminate will emit a large amount of toxic gases (bromide type) when burning or electrical fires, and the amount of smoke is large ; When the PCB is used for hot air leveling and component welding, the plate is affected by high temperature (> 200), and it will also release a trace of hydrogen bromide; whether it will also produce dioxins is still under evaluation. Therefore, FR4 sheets containing tetrabromobisphenol A flame retardant are not currently prohibited by law and can be used, but cannot be called halogen-free sheets.

Principle of halogen-free substrate

For now, most of the halogen-free materials are mainly phosphorous and phosphorous nitrogen. Phosphorus-containing resin decomposes by heating to produce meta-polyphosphoric acid, which has strong dehydration properties, which forms a carbonized film on the surface of the polymer resin, and isolates the resin combustion surface from contact with air to extinguish the fire and achieve a flame retardant effect. The polymer resin containing phosphorus and nitrogen compounds generates non-combustible gas when it is burnt, which helps the flame retardant of the resin system.

Characteristics of halogen-free plates:
1 Insulation of materials
P or N is used to replace the halogen atom, thus reducing the polarity of the molecular bond segment of the epoxy resin to a certain extent, thereby improving the qualitative insulation resistance and resistance to breakdown.
2 Material water absorption
Halogen-free plates because the N and P foxes in the nitrogen-phosphorus-based oxygen-reducing resin have fewer electrons than halogens, and the probability of forming hydrogen bonds with hydrogen atoms in water is lower than that of halogen materials, so the water absorption of the materials is lower Conventional halogen flame retardant materials. For the board, the low water absorption has a certain effect on improving the reliability and stability of the material.
3 Thermal stability of materials
The content of nitrogen and phosphorus in halogen-free plates is greater than that of ordinary halogen-based materials, so its monomer molecular weight and Tg value have increased. In the case of heat, the molecule's ability to move will be lower than that of conventional epoxy resins, so the coefficient of thermal expansion of halogen-free materials is relatively small.
Experience in producing halogen-free PCB
Lamination
Lamination parameters may vary from company to company. Taking the above-mentioned Shengyi substrate and PP as multi-layer board, in order to ensure the full flow of the resin and make the bonding force good, a lower plate heating rate (1.0-1.5 ℃ / min) and multiple sections Pressure coordination, in addition to the high temperature stage requires a longer time, 180 ℃ for more than 50 minutes. The following is a recommended set of platen program settings and the actual temperature rise of the sheet. The extruded board detects that the bonding force between its copper foil and the substrate is 1.ON / mm, and the board after the graph shows no delamination or bubbles after six thermal shocks.
Drillability
Drilling condition is an important parameter, which directly affects the hole wall quality of PCB during the processing. Halogen-free copper clad laminates use P and N series functional groups to increase molecular weight and strengthen the rigidity of molecular bonds, which also enhances the rigidity of the material. At the same time, the Tg point of halogen-free materials is generally higher than ordinary copper clad laminates. The drilling parameters of FR-4 are generally not ideal. When drilling halogen-free plates, some adjustments should be made under normal drilling conditions.
Alkali resistance
Generally, the halogen-free plate has poorer alkali resistance than the ordinary FR-4. Therefore, special attention should be paid to the etching process and the rework process after solder mask. The immersion time in the alkaline stripping solution should not be too long. To prevent white spots on the substrate.
Halogen-free solder mask production
There are many kinds of halogen-free solder resist inks currently on the market, and their performance is not much different from ordinary liquid photosensitive inks. The specific operations are basically similar to ordinary inks.
Conclusion:
Halogen-free PCB board has lower water absorption and adapts to environmental protection requirements, and can also meet the quality requirements of PCB board in other properties. Therefore, the demand for halogen-free PCB board has been increasing.