When purchasing printed circuit boards, you should come across halogen-free circuit boards. You may have a question in your mind: what is halogen-free PCB? Why do PCB manufacturers produce it? This article will answer your doubts.
Halogen Free PCB
It is not difficult to understand from the literal meaning that halogen-free printed circuit boards refer to circuit boards that do not contain halogen materials or whose halogen content is negligible.
The standard for halogen-free printed circuit boards is that the content of chlorine or bromine must be less than 900 parts per million, and the halogen device’s content must be less than 1500 parts per million. Unlike lead-free PCBs, the halogen in halogen-free circuit boards is not a material but is a toxic gas. It is not only harmful to human health but also polluted environment. Therefore, we should select halogen-free materials in PCB manufacturing. It is good for the environment and human health.
What are the halogen elements?
We all know in middle school that halogen elements are a large category. It is a non-metallic element composed of bromine, chlorine, fluorine, iodine, and other elements. In the electronics industry, bromine and chlorine are often used in PCB materials, such as PCB laminates and prepreg layers. In particular, bromine has good flame retardancy and is used as a flame retardant for FR4 substrates. This property is crucial in many PCB applications.
Why is ‘halogen’ banned?
According to relevant research, halogen-containing flame retardant materials will release many toxic gases when burned, accompanied by an unpleasant odor, such as hydrogen halides, PBDE, and PBDF. When hydrogen halides come into contact with water vapor in the air, acid rain is produced. The sulfur dioxide in the acid rain is corrosive and will pollute the environment. PBDE and PBDF are carcinogens that affect people’s health. The EU RoHS regulations clearly state thatPBDE and PBB flame retardants are strictly restricted.
Halogen substitutes
Halogen-free circuits remove the reliance on halogen materials by replacing flame-retardant compounds and resins.
Most PCB manufacturers use phosphorus and phosphorus nitrogen to replace halogen PBB and PBDE.
Unlike halogens, phosphorus decomposes into polyphosphoric acid when it burns. Then, the polyphosphoric acid dries and hardens on the surface, forming a carbonized film that allows the surface of the insulating resin to come into contact with the air, achieving a flame-retardant effect. During the entire process, no harmful or combustible gases are generated, and it can also help extinguish the fire source through lack of oxygen.
Disadvantages of Halogen-Free PCB
Expensive:Halogen-free printed circuit boards require specially designed chemical resins, which are not as easy to produce as PBB and PBDE. So, companies that provide coatings usually charge higher fees.
Few manufacturers:More manufacturers provide traditional PCBs than halogen-free printed circuit boards, which means that you can buy the number of halogen-free circuits will be limited, so be cautious when looking for a manufacturer.
The manufacturing process is more inconvenient:Without halogen-free, the PCB may become more brittle and prone to white spots, making the manufacturing of halogen-free printed circuit boards more difficult, especially for manufacturing processes such as solder mask production and appearance processing.
Advantages of Halogen-Free PCB
Halogen-free circuit boards use environmentally friendly materials that are harmless to human health and beneficial to the environment. Halogen-free PCBs also have the following benefits:
Environmentally friendly:When handling halogen-free printed circuit boards, they will not produce harmful substances such as halogen compounds, reducing environmental pollution and complying with the halogen ban of EU countries.
Protect the health and safety of workers:Halogen compounds are harmful substances that release toxic gases that are harmful to the human body at high temperatures.
Therefore, using halogen-free materials can reduce the harmful gases generated during the welding and heat treatment process, which not only protects the health and safety of workers but also improves the safety of the working environment.
Improve circuit board insulation:Halogen-free insulation materials are low-cost and effective for disposable electronic products. Since halogens are replaced by phosphorus or nitrogen, the polarity of epoxy resin is reduced, thereby improving the circuit insulate resistance and reducing the breakdown force of the current.
Lower water absorption:For boards, low water absorption affects improving the reliability and stability of materials. Phosphorus nitrogen molecules have fewer electrons than halogens. It is not easy to combine with water molecules, which means that halogen-free circuit boards have a higher resistance to water absorption. Although the scale of halogen-free printed circuit boards is much smaller than that of PCBs, they can better cope with humid environments and situations.
High thermal stability:The nitrogen and phosphorus content in halogen-free circuit boards is higher than that of ordinary boards, which increases the monomer molecular weight and Tg. Under heating conditions, the molecular mobility of halogen-free materials is lower than that of traditional epoxy resin boards. Therefore, halogen-free circuit boards have a lower thermal expansion coefficient, enabling them to withstand higher temperatures. Because halogen-free PCB can withstand high temperatures, it is more suitable for high-power product design.
Recyclable:Recycling halogen-free circuit boards can fully and effectively utilize the metal materials in circuit boards, such as gold, copper, silver, etc. Recycling these metal materials can reduce the demand for scarce resources, drive the growth of related companies, and provide more employment opportunities for social talents.
Improve market competitiveness:Nowadays, more and more consumers like environmentally friendly and safe electronic products, so halogen-free circuits can attract more consumers, thereby improving the market competitiveness of enterprises. Enterprises that use halogen-free materials can reflect their concern for the environment and incarnate their concern for the health of consumers and product quality, which is conducive to the long-term development of enterprises and can also improve brand reputation.
Quote for Halogen-free PCB Fabrication
Halogen-free materials
Halogen is a material mainly used in laminate production or prepreg layers. Sometimes, manufacturers use these substances in the solder paste used when assembling PCBs. Halogen compounds are replaced with safer materials in halogen-free circuits, as described below.
• Halogen-free laminate
In halogen-free PCB, laminate is used for the substrate. Usually, it is halogen-free FR4, where FR stands for flame retardant material. Below are the alternatives for both flame retardant and resin.
Flame retardants:
Several brominated compounds are used in traditional PCBs, such as PBB, PBDE, TBBPA, and HBCD. In halogen-free printed circuit boards, these compounds are replaced by the following halogen-free flame retardant additives.
• Aluminum (aluminum hydroxide, aluminum diethyl phosphinate)
• Phosphorus (phosphorus-based FR4 materials, phosphorus-based flame retardants)
• Magnesium hydroxide
• Ceramics
• Melamine derivatives
Epoxy resin:
FR4 halogen-free laminates use epoxy resin. It is typically a thermosetting polymer that provides mechanical stability and insulation. Traditionally, this resin contains halogens. In halogen-free boards, the resins used are usually the following:
• Benzoxazine resin
• Bismaleimide triazine
• trifunctional epoxy resin
• Bisphenol Epoxy Vinyl Ester
• Cyanate
• Polyimide
• Halogen-free flux
Halogenated solder paste or flux is used to assemble circuit boards. This solder paste contains a bromine activator and helps remove oxidation during the soldering process while also providing sanding properties. In the halogen-free solder, flux has been replaced by safer options.
Halogen-free PCB manufacturing
The manufacturing process of halogen-free boards is similar to that of conventional PCB manufacturing, except that halogen-free materials replace halogen materials. So, a few key steps are described here.
• Material preparation
The substrate, copper foil, and solder mask should all use halogen-free materials, such as epoxy resin or polyimide.
• Laminated
Lamination is bonding each layer of the circuit into a whole using the adhesiveness of the PP sheet. Copper foil, bonding sheet (prepreg sheet), and other materials are overlapped according to the process requirements.
• Drilling
Drilling conditions are critical to the quality of PCB hole walls. Halogen-free copper-clad laminates require adjustment of drilling parameters compared to ordinary copper-clad laminates due to their higher material rigidity.
• Alkali resistance
Generally speaking, the alkali resistance of halogen-free sheets is worse than that of ordinary glass fibers, so extra care is required during the etching process and the rework process after solder masking. The soaking time in the alkaline stripping solution should not be too long to avoid white spots on the substrate. It is recommended that when reworking halogen-free solder mask boards, it is best to make the first board, find the best parameters, and then rework in batches.
• Solder Mask
There are a variety of halogen-free solder mask inks available on the market. Their performance is similar to ordinary liquid photosensitive inks, and their operation methods are similar to traditional PCBs.
Halogen Test Methods
There are several analytical techniques to measure halogen content:
Combustion Ion Chromatography: Burn the sample to release halogen for detection
X-ray Fluorescence Spectroscopy: Using X-rays to stimulate Halogen emission
Headspace analysis: Detection of halogen gases released during heating
Wet Chemistry: Solution-Based titration methods
Total Organic Halogen Analysis: Measurement of total organically bound Halogens
Halogen Test Standards
This standard tests both substrates and finished PCBs. The relevant industry testing standards include:
• IPC-4101– Verifying Halogen content in base laminates
• IEC 61249-2-21 – Testing PCB laminates for halogen after manufacture
• UL 746A – Determination of Halogens in laminated panels by flammability
• IEC 61249-2-41 – Testing for total halogen content in laminating resins
Application of halogen-free circuit boards
With the popularity of electronic products, PCB has occupied various fields. Halogen-free printed circuit boards has also increasingly appeared in the public eye. Its common uses are as follows:
• High-end computers, robots, and large server farms
• Cell phones and other wireless devices
• Signaling and transportation equipment
• Consumer electronics such as TVs and game consoles
• Traffic applications such as GPS and on-board computer equipment
• Power stations and substations
• Laboratory
• Distribution and sorting rooms
The development prospects of halogen-free
Halogen-free printed circuit boards is safer, more environmentally friendly, and more reliable than traditional circuit boards. It will be the industry standard and the future development trend of various manufacturers. In order to develop safer and more economical halogen-free sheets, more and more companies will invest heavily in halogen-free sheet research. We can expect that more low-priced halogen-free boards will appear in the market in the future.
