Halogen-free PCBs refer to circuit boards fabricated using halogen-free raw materials, which are classified as eco-friendly materials. By substituting or reducing the use of halogen-containing materials, these PCBs ensure environmental safety and non-toxicity. According to the JPCA - ES - 01 - 2003 standard, a copper-clad laminate is defined as halogen-free when the chlorine (Cl) and bromine (Br) contents are each less than 0.09% Wt (weight ratio) and the total amount does not exceed 0.15% (1500 PPM). Common halogen-free materials include TUC's TU883 and Isola's DE156, among others.
The flame-retardant mechanism of halogen-free materials primarily relies on phosphorus-containing resins and phosphorus-nitrogen compounds. When phosphorus-containing resins burn, they decompose to generate metaphosphoric acid, which possesses strong dehydrating properties. This enables the formation of a carbonized film on the surface of the polymer resin, isolating the burning resin surface from air contact and thereby extinguishing the fire to achieve flame retardancy. Meanwhile, phosphorus-nitrogen compounds produce non-combustible gases during combustion, assisting the resin system in flame retardancy.
Halogen-free laminates use P or N to replace halogen atoms, which reduces the polarity of the molecular bond segments in the epoxy resin to a certain extent, thereby enhancing the material's insulation resistance and breakdown voltage resistance.
In nitrogen-phosphorus-based epoxy resins used in halogen-free laminates, the number of lone pair electrons on N and P is relatively lower compared to halogen materials. Consequently, the probability of forming hydrogen bonds with hydrogen atoms in water is lower than that of halogen materials, resulting in lower water absorption compared to conventional halogen-based flame-retardant materials. For laminates, low water absorption positively impacts the improvement of material reliability and stability.
The nitrogen and phosphorus contents in halogen-free laminates are higher than the halogen contents in ordinary halogen-based materials, leading to an increase in their monomer molecular weights and glass transition temperatures (Tg values). Under heating conditions, the molecular mobility of halogen-free laminates is lower than that of conventional epoxy resins, and their coefficients of thermal expansion are relatively smaller.
During the production of halogen-free PCBs, adjustments to the heating rate and pressure are required during the lamination process to ensure adequate resin adhesion.
In the drilling process, considerations must be given to the material's rigidity and Tg value, and parameters should be adjusted according to alkaline conditions to prevent damage to the substrate.
The operation of halogen-free solder mask inks is similar to that of ordinary inks, but special attention is required (the original text has an unclear "1" here, which may need further clarification in practical translation scenarios).
Halogen-free PCBs generate fewer harmful substances during production and use, meeting environmental requirements. Halogen-containing flame-retardant materials (such as PBB and PBDE) produce toxic substances like dioxins and benzofurans during combustion, posing severe threats to human health. Additionally, the combustion process releases a large amount of toxic gases and dense smoke, causing environmental pollution. In contrast, halogen-free materials do not produce toxic gases during combustion, which is beneficial for environmental protection and human health.
Halogen-free PCBs generally exhibit good thermal dissipation and reliability, making them suitable for high-temperature processes required by lead-free circuits. With a relatively low dielectric constant, they can maintain signal integrity. Furthermore, their low water absorption rate enables them to meet environmental requirements and satisfy the quality demands of PCBs.
Halogen-free PCBs are widely used in fields such as telecommunications, computing, and aviation, particularly suited for high-temperature processes required by lead-free circuits. As market demands for environmental protection and performance continue to rise, the demand for halogen-free PCBs is increasing. In the automotive sector, manufacturers of automotive rigid-flex PCBs are increasingly favoring halogen-free PCBs to meet both environmental and performance considerations, which represents an inevitable trend in industry development.
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