What is halogen-free PCB? Why is PCB halogen-free required?
01 What is halogen-free PCB?
Halogen-free PCB refers to printed circuit boards whose halogen content meets specific limit standards. According to the JPCA-ES-01-2003 standard (JPCA refers to the abbreviation of the Japan Electronic Circuit Industry Association): copper-clad boards with chlorine (Cl) and bromine (Br) content less than 0.09% Wt (weight ratio) are defined as halogen-free PCBs. (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. ...
02 Why ban halogen?
Halogen: refers to the halogen elements in the periodic table of chemical elements. In the periodic table of chemical elements, the periodic system VIIA group elements refer to the halogen elements, including fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). At present, flame retardant substrates, such as FR4 and CEM-3, are mostly made of brominated epoxy resins.
Research by relevant institutions shows that halogen-containing flame retardant materials (polymerized polybrominated biphenyls PBB: polymerized polybrominated diphenyl ethers PBDE), when discarded and burned, will release harmful substances such as dioxin (TCDD) and benzofuran. This substance produces a large amount of smoke, has a bad smell, and has highly toxic gases that can cause cancer. Once ingested by the human body, it cannot be discharged, which seriously affects health.
Halogen-free boards achieve environmental protection and non-toxicity of PCBs by replacing or reducing these harmful elements.
Therefore, EU law prohibits the use of six substances such as 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, polymerized polybrominated biphenyls or polymerized polybrominated diphenyl 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 are more commonly used, such as tetrabromobisphenol A, dibromophenol, etc., whose chemical formula is CISHIZOBr4. Although there are no laws and regulations to regulate this type of copper-clad laminate containing bromine as a flame retardant, this type of bromine-containing copper-clad laminate will release a large amount of toxic gas (brominated type) and smoke when it burns or an electrical fire occurs; when the PCB is hot-air leveled and the components are soldered, the board is affected by high temperature (>200°C) and will also release a trace amount of hydrogen bromide; whether it will also produce toxic gases is still under evaluation.
In summary. The negative consequences of using halogens as raw materials are huge, and it is necessary to ban halogens.
03 Principle of halogen-free substrates
At present, most halogen-free materials are mainly phosphorus-based and phosphorus-nitrogen-based.
When phosphorus-containing resins are burned, they are decomposed by heat to form partial phosphoric acid, which has a strong dehydration property, forming a carbonized film on the surface of the polymer resin, isolating the burning surface of the resin from contact with the air, extinguishing the fire, and achieving a flame retardant effect. Polymer resins containing phosphorus and nitrogen compounds produce non-combustible gases when burned, which assists the resin system in flame retardancy.
04 Characteristics of halogen-free PCB
1. Insulation of materials
Due to the use of P or N to replace halogen atoms, the polarity of the molecular bond segment of epoxy resin is reduced to a certain extent, thereby improving the insulation resistance and anti-puncture ability of the material.
2. Water absorption of materials
Halogen-free boards have fewer electron pairs of N and P in nitrogen-phosphorus-based epoxy resins than halogens, and the probability of forming hydrogen bonds with hydrogen atoms in water is lower than that of halogen materials. Therefore, the water absorption of halogen-free boards is lower than that of conventional halogen-based flame retardant materials. For boards, low water absorption has a certain impact on improving the reliability and stability of materials.
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3. Thermal stability of materials
The content of nitrogen and phosphorus in halogen-free boards is greater than that of halogen in ordinary halogen-based materials, so the molecular weight and Tg value of the monomers are increased. Under heating, the mobility of its molecules will be lower than that of conventional epoxy resins, so the thermal expansion coefficient of halogen-free materials is relatively small. Compared with halogen-containing boards, halogen-free boards have more advantages, and it is also a general trend for halogen-free boards to replace halogen-containing boards.
05 Precautions for halogen-free PCB production
Lamination: Lamination parameters may vary due to different companies' boards. Take the above-mentioned Shengyi substrate and PP as a multi-layer board. In order to ensure the full flow of the resin and good bonding strength, a lower board heating rate (1.0-1.5℃/min) and multi-stage pressure matching are required. In addition, a longer time is required in the high temperature stage, 180℃ is maintained for more than 50 minutes.
The following is a recommended set of press program settings and actual board heating conditions. The bond strength between the copper foil and the substrate of the pressed board is 1.ON/mm. After six thermal shocks, the board after electroplating has no delamination or bubbles.
Drilling processability: Drilling conditions are an important parameter that directly affects the hole wall quality of the PCB during processing. Halogen-free copper clad laminates use P and N series functional groups to increase the molecular weight and enhance the rigidity of the molecular bonds, thereby 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. Therefore, the drilling effect is generally not very ideal when drilling with the drilling parameters of ordinary FR-4.
When drilling halogen-free boards, some adjustments need to be made under normal drilling conditions.
Alkali resistance: Generally, the alkali resistance of halogen-free boards 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 fact that the immersion time in the alkaline film stripping liquid should not be too long to prevent the appearance of white spots on the substrate.
Halogen-free solder mask production: There are many kinds of halogen-free solder mask inks currently available on the market, and their performance is not much different from that of ordinary liquid photosensitive inks. The specific operation is basically the same as that of ordinary inks.
Halogen-free PCB boards have lower water absorption and meet environmental protection requirements. Other performances can also meet the quality requirements of PCB boards. Therefore, the demand for halogen-free PCB boards has been increasing.
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