PCB Tech - Discuss the halogen-free requirement of PCB now

What is a halogen-free substrate

Among the halogen-free PCB substrates: 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 and so on.

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Why ban halogen

Refers to the halogen elements in the periodic table of chemical elements, including fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). At present, the halogen-free, flame-retardant substrates required by PCBs, such as FR4, CEM-3, etc., are mostly brominated epoxy resins.

Halogen-containing flame-retardant materials (Polybrominated biphenyls PBB: Polybrominated biphenyl ethers PBDE) will emit dioxin (dioxin TCDD), benzene furan (Benzfuran), etc. when they are discarded and burned, causing a large amount of smoke and a difficult smell Smell, there is a highly toxic gas, which is carcinogenic, and cannot be discharged after being ingested by the human body, which seriously affects health.

Therefore, the law prohibits 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 this type of copper clad laminate containing bromine as a flame retardant is not regulated by any laws and regulations, this type of bromine-containing copper clad laminate will release a large amount of toxic gas (brominated type) and smoke during combustion or electrical fire. Large; When the PCB is leveled with hot air and PCB components are soldered, the board is 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.

Phosphorus and phosphorus and nitrogen are the mainstays. When phosphorous resin is burned, it is decomposed by heat to generate meta-polyphosphoric acid, which has strong dehydration property, so that a carbonized film is formed 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 be flame-retardant.

Features of halogen-free sheet

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.

1) Water absorption of the material

The halogen-free sheet material has fewer electrons than halogens in the nitrogen-phosphorus-based oxygen reduction resin. 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.

2) Thermal stability of the material

The content of nitrogen and phosphorus in the halogen-free sheet material is greater than the halogen content of ordinary halogen-based 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.

The lamination parameters may be different for different companies. Take the above-mentioned Shengyi substrate and PP as a multi-layer 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°C/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 show delamination or air bubbles after six thermal shocks.

3) Drilling processability

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 at the same time enhance 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 using ordinary FR-4 drilling parameters for drilling, the effect is generally not very satisfactory. When drilling halogen-free boards, some adjustments should be made under normal drilling conditions.

4) 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 solder mask, special attention should be paid to the immersion time in the alkaline stripping solution. To prevent white spots on the substrate.

5) Production of halogen-free solder mask

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.