**What is a Halogen-Free Substrate**

Among the halogen-free PCB substrates: According to the JPCA-ES-01-2003 standard, copper-clad laminates containing chlorine (Cl) and bromine (Br) in amounts less than 0.09% by weight are defined as halogen-free copper-clad laminates. (Additionally, the total content of Cl + Br must be ≤0.15% [1500 ppm]).

Halogen-free materials include: TUC’s TU883, Isola’s DE156, the GreenSpeed series, Shengyi’s S1165/S1165M, S0165, and others.

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**Why Ban Halogen**

Halogen elements refer to those in the periodic table, including fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Currently, halogen-free, flame-retardant substrates used in PCBs, such as FR4 and CEM-3, are typically made with brominated epoxy resins.

Halogen-containing flame-retardant materials (e.g., Polybrominated Biphenyls (PBB) and Polybrominated Biphenyl Ethers (PBDE)) release harmful substances such as dioxins (TCDD), benzofurans, and other toxic compounds when burned or disposed of improperly. These materials generate significant smoke, a pungent odor, and release highly toxic gases, some of which are carcinogenic and do not easily break down in the human body, severely impacting health.

As a result, legislation now prohibits the use of six substances, including PBB and PBDE. China’s Ministry of Information Industry mandates that electronic information products sold on the market must be free of lead, mercury, hexavalent chromium, polybrominated biphenyls, or polybrominated biphenyl ethers.

It is understood that PBB and PBDE are no longer widely used in the copper-clad laminate industry. Instead, alternative brominated flame-retardant materials, such as tetrabromobisphenol A (TBBPA) and dibromophenol, are commonly used. The chemical formula for these compounds is C15H16Br4O2.

Although copper-clad laminates containing bromine-based flame retardants are not yet regulated by specific laws or standards, such materials can release large amounts of toxic bromine-containing gases and smoke during combustion or electrical fires. Additionally, during the PCB reflow soldering process, when the board is exposed to temperatures exceeding 200°C, small amounts of hydrogen bromide may be emitted. Whether this also results in the generation of other toxic gases remains under evaluation.


In summary, the use of halogens as raw materials has significant negative consequences, and it is essential to ban halogens. Phosphorus and nitrogen are the key elements. When phosphorus resin is burned, it decomposes under heat to form meta-polyphosphoric acid, which has strong dehydrating properties. This leads to the formation of a carbonized film on the surface of the polymer resin, isolating the burning surface from air and extinguishing the fire, thereby providing a flame-retardant effect. The polymer resin containing phosphorus and nitrogen compounds generates incombustible gases when burned, further contributing to the flame-retardant properties of the resin system.

**Features of Halogen-Free Sheets**

By replacing halogen atoms with phosphorus (P) or nitrogen (N), the polarity of the molecular bond segments in the epoxy resin is reduced, improving both the insulation resistance and the resistance to breakdown.

1) **Water Absorption of the Material**

The halogen-free sheet material, based on nitrogen-phosphorus oxygen reduction resin, has fewer electrons than halogen-based materials. Consequently, it has a lower tendency to form hydrogen bonds with water molecules, resulting in lower water absorption compared to conventional halogen-based flame-retardant materials. Lower water absorption improves the reliability and stability of the material, particularly in board applications.

2) **Thermal Stability of the Material**

The nitrogen and phosphorus content in halogen-free sheets is higher than the halogen content in conventional halogen-based materials. This leads to higher monomer molecular weight and a higher Tg value. As a result, the molecular mobility of halogen-free materials is lower when heated, which reduces their coefficient of thermal expansion. In comparison with halogen-containing boards, halogen-free boards offer distinct advantages, making the shift towards halogen-free materials a clear industry trend.

For laminating, parameters may vary by company. For example, when using Shengyi substrates and PP in multi-layer boards, ensuring full resin flow and strong bonding requires a lower heating rate (1.0-1.5°C/min) and multi-stage pressure fitting. A longer high-temperature stage of over 50 minutes at 180°C is recommended. Below is a suggested platen program with actual temperature rise data. The bonding strength between the copper foil and the extruded board substrate was 1.0N/mm, and after six thermal shocks, no delamination or air bubbles were observed.

3) **Drilling Processability**

Drilling conditions are critical as they directly impact the quality of the hole wall during PCB processing. Halogen-free copper-clad laminates, with their P and N functional groups, have higher molecular weight and stronger molecular bond rigidity, which enhances material strength. Additionally, the Tg of halogen-free materials is typically higher than that of conventional copper-clad laminates. As a result, using standard FR-4 drilling parameters may not yield satisfactory results. Drilling halogen-free boards requires some adjustments to the standard drilling conditions.

4) **Alkali Resistance**

Halogen-free sheets generally have lower alkali resistance compared to standard FR-4. Therefore, special attention is needed during the etching process and the rework process after applying solder masks. The immersion time in alkaline stripping solutions should be carefully controlled to avoid causing white spots on the substrate.

5) **Production of Halogen-Free Solder Mask**

There are currently various halogen-free solder mask inks available globally, and their performance is largely comparable to traditional liquid photosensitive inks. The operational process is essentially the same as with conventional inks.

Halogen-free PCBs exhibit low water absorption, meet environmental protection requirements, and have properties that meet the quality standards for PCB boards. As a result, the demand for halogen-free PCBs has risen.