What is Halogen-Free Material?
Halogen-free materials:According to the JPCA-ES-01-2003 standard, copper-clad laminates with chlorine (Cl) and bromine (Br) content less than 0.09% Wt (weight ratio) are defined as halogen-free laminates. (At the same time, CI + Br total amount ≤ 0.15% [1500PPM])
Halogen-free materials include: TUC’s TU883, Isola’s DE156, GreenSpeed? series, Shengyi’s S1165/S1165M, S0165, etc.
Why Ban Halogens?
Halogens refer to the group of chemical elements in the periodic table, including fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Currently, flame-retardant materials such as FR4 and CEM-3 primarily use brominated epoxy resins as flame retardants.
Research by relevant organizations indicates that halogen-containing flame-retardant materials (polybrominated biphenyls PBB: polybrominated diphenyl ethers PBDE) release dioxins (dioxin TCDD), benzofurans, and other highly toxic gases when burned, producing a large amount of smoke and unpleasant odors, which are carcinogenic and cannot be expelled from the human body, severely affecting health.
Therefore, the European Union’s legislation prohibits the use of six substances, including PBB and PBDE. Similarly, documents from the Ministry of Information Industry of China require that electronic information products marketed must not contain lead, mercury, hexavalent chromium, polybrominated biphenyls, or polybrominated diphenyl ethers.
It is understood that PBB and PBDE are no longer used in the copper-clad laminate industry, and brominated flame retardants other than PBB and PBDE, such as tetrabromobisphenol A and dibromophenol, are more commonly used, with the chemical formula CISHIZOBr4.
Although there are currently no legal regulations governing brominated flame retardants in copper-clad laminates, these brominated laminates release a large amount of toxic gases (brominated) when burned or during electrical fires; they also produce significant smoke. During hot air leveling and component soldering of PCBs, the materials may release trace amounts of hydrogen bromide due to high temperatures (>200°C); whether they also produce toxic gases is still under evaluation.
In summary, the negative consequences of using halogens as raw materials are significant, making the ban on halogens necessary.
Phosphorus and phosphorus-nitrogen compounds are primarily used. Phosphorus-containing resins, when burned, decompose under heat to form polyphosphoric acid, which has strong dehydrating properties, creating a char layer on the surface of the polymer resin, isolating the burning surface of the resin from contact with air, thus extinguishing the fire and achieving flame retardancy. Phosphorus-nitrogen compounds in polymer resins produce non-combustible gases during combustion, assisting the resin system in flame retardancy.
Characteristics of Halogen-Free Laminates
By replacing halogen atoms with P or N, the polarity of the molecular bond segments in epoxy resin is reduced to some extent, thereby improving the quality of insulation resistance and breakdown strength.
1) Material Water Absorption
Halogen-free laminates have lower water absorption than conventional halogen-containing flame-retardant materials because the nitrogen and phosphorus in nitrogen-phosphorus-based resins have fewer free electrons compared to halogens, resulting in a lower probability of forming hydrogen bonds with hydrogen atoms in water.
For laminates, low water absorption positively impacts the reliability and stability of the material.
2) Material Thermal Stability
Halogen-free laminates contain more nitrogen and phosphorus than conventional halogen-containing materials, resulting in increased molecular weight and Tg values. Under heat, their molecular mobility is lower than that of conventional epoxy resins, thus halogen-free materials have a relatively smaller thermal expansion coefficient.
Compared to halogen-containing laminates, halogen-free laminates have more advantages, and the trend of replacing halogen-containing laminates with halogen-free laminates is inevitable.
Laminate parameters may vary among different companies. For instance, in the case of Shengyi substrates and PP for multilayer boards, to ensure sufficient resin flow and good bonding strength, a lower heating rate (1.0-1.5°C/min) and multi-stage pressure application are required. Additionally, during the high-temperature phase, a longer duration is required, maintaining 180°C for over 50 minutes.
The following is a recommended set of pressing parameters and the actual heating conditions of the laminate. The tested laminate shows a bonding strength of 1.0N/mm between the copper foil and the substrate, and the board has not exhibited delamination or bubbling after six thermal shocks.
3) Drilling Processability
Drilling conditions are an important parameter that directly affects the quality of hole walls during PCB processing. Halogen-free copper-clad laminates, due to the use of P and N functional groups, increase molecular weight and enhance the rigidity of 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 standard FR-4 drilling parameters may not yield ideal results. When drilling halogen-free boards, some adjustments should be made under normal drilling conditions.
4) Alkaline Resistance
Generally, halogen-free laminates have poorer alkaline resistance than ordinary FR-4, so special attention should be paid during the etching process and rework process after solder masking. The immersion time in alkaline stripping solutions should not be too long to prevent the appearance of white spots on the substrate.
5) Halogen-Free Solder Mask Production
Currently, there are many types of halogen-free solder mask inks available on the market, and their performance is not significantly different from that of ordinary liquid photoresist inks. The operational procedures are also similar to those of ordinary inks.
Due to their lower water absorption and compliance with environmental requirements, halogen-free PCBs can meet the quality requirements of PCB boards, leading to an increasing demand for halogen-free PCBs.
Some Screenshots from Electronic Books
【Complete Set of Hardware Learning Materials Collection】
