Why is there a halogen-free requirement for PCB now?
What is a halogen-free substrate?
According to the JPCA-ES-01-2003 standard: Copper clad laminates with chlorine (C1) and bromine (Br) content less than 0.09% Wt (weight ratio) are defined as halogen-free copper clad laminates. (At the same time, the total amount of CI+Br≤0.15%[1500PPM]).
Halogen-free materials include: TUC's TU883, Isola's DE156, GreenSpeed? series, Shengyi's S1165/S1165M, S0165, etc.
2. Why should we ban halogen?
Refers to the halogen elements in the periodic table of chemical elements, including fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Currently, flame retardant substrates, FR4, CEM-3, etc., flame retardants are mostly brominated epoxy resin.
A Studies by related institutions have shown that halogen-containing flame-retardant materials (polybrominated biphenyls PBB: polybrominated diphenyl ether PBDE) will emit dioxins (dioxin TCDD), benzofuran (Benzfuran), etc. when they are discarded and burned. The smoke is large, the smell is unpleasant, there is a highly toxic gas, which is carcinogenic, and cannot be discharged after the body is ingested, which seriously affects health.
Therefore, EU laws prohibit the use of six substances including PBB and PBDE. The Ministry of Information Industry of China also requires that electronic information products put on the market must not contain substances such as lead, mercury, hexavalent chromium, polybrominated biphenyls or polybrominated biphenyl ethers.
It is understood that PBB and PBDE are basically no longer used in the copper clad laminate industry. Bromine flame-retardant materials other than PBB and PBDE, such as tetrabromobisphenol A, dibromophenol, etc., are mostly used. The chemical formula is CISHIZOBr4. Although there are no laws and regulations for such bromine-containing copper clad laminates as flame retardants, such bromine-containing copper clad laminates will release a large amount of toxic gas (brominated type) and smoke when burning or electrical fires. Large; When the PCB is leveled with hot air and components are soldered, the plate will be affected by high temperature (>200), and a small amount of hydrogen bromide will be released; whether it will also generate toxic gas is still under evaluation.
In summary. The use of halogen as a raw material has huge negative consequences and it is necessary to ban halogen.
3. The principle of halogen-free substrate
For now, most of the halogen-free materials are mainly phosphorus-based and phosphorus-nitrogen-based. When phosphorous resin is burned, it is decomposed by heat to generate meta-polyphosphoric acid, which has strong dehydration property, which forms a carbonized film on the surface of the polymer resin, which insulates the burning surface of the resin from contact with air, extinguishes the fire, and achieves a flame retardant effect. The polymer resin containing phosphorus and nitrogen compounds generates incombustible gas when burned, which helps the resin system to flame retardant.
4.The characteristics of halogen-free plates.
1 )Material insulation
Due to the use of P or N to replace the halogen atoms, the polarity of the molecular bond segment of the epoxy resin is reduced to a certain extent, thereby improving the qualitative insulation resistance and resistance to breakdown.
2) Water absorption of material
The halogen-free sheet material has fewer electrons than halogens in the nitrogen-phosphorus reducing resin, 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 material is lower than Conventional halogen-based flame retardant materials. For the board, low water absorption has a certain impact on improving the reliability and stability of the material.
3) Thermal stability of the material.
The content of nitrogen and phosphorus in the halogen-free sheet material is greater than the content of halogen in common halogen materials, so its monomer molecular weight and Tg value have increased. When heated, its molecular mobility will be lower than that of conventional epoxy resins, so the coefficient of thermal expansion of halogen-free materials is relatively small.
Compared with halogen-containing plates, halogen-free plates have more advantages, and it is also a general trend to replace halogen-containing plates with halogen-free plates.
5. Experience in producing halogen-free PCB
Laminating parameters may be different due to different companies' plates. Take the above-mentioned Shengyi substrate and PP as a multilayer board. In order to ensure the full flow of the resin and make the bonding force good, it requires a lower heating rate (1.0-1.5℃/min) and multi-stage Pressure fitting requires a longer time in the high temperature stage, and maintains at 180°C for more than 50 minutes. The following is a recommended set of platen program settings and the actual temperature rise of the plate. The bonding force between the copper foil and the substrate of the extruded board was 1.ON/mm, and the board after drawing electricity did not appear delamination or air bubbles after six thermal shocks.
2) Drilling workability
The drilling condition is an important parameter, which directly affects the quality of the hole wall of the PCB during processing. The halogen-free copper clad laminate uses P and N series functional groups to increase the molecular weight and the rigidity of the molecular bonds, thereby also enhancing the rigidity of the material. At the same time, the Tg point of halogen-free materials is generally higher than that of ordinary copper clad laminates, so ordinary Drilling with FR-4 drilling parameters is generally not ideal. When drilling halogen-free boards, some adjustments should be made under normal drilling conditions.
3 )Alkali resistance
Generally, the alkali resistance of halogen-free sheets is worse than that of ordinary FR-4. Therefore, in the etching process and the rework process after the solder mask, special attention should be paid to the immersion time in the alkaline stripping solution. To prevent white spots on the substrate.
4)Halogen-free solder mask production
At present, there are many kinds of halogen-free solder mask inks launched in the world, and their performance is not much different from that of ordinary liquid photosensitive ink. The specific operation is basically the same as that of ordinary ink.
Halogen-free PCB boards have low water absorption and meet the requirements of environmental protection, and other properties can also meet the quality requirements of PCB boards. Therefore, the demand for halogen-free PCB boards has increased.