What is a Halogen Free PCB?
As the name suggests, PCBs free of halogens are halogen-free PCBs. Here halogen refers to chlorine, fluorine, iodine, astatine, and bromine. Halogens are very harmful to life. To generate a halogen-free PCB, you always need a halogen-free lamination of copper clad.
Halogen in PCBs
Halogens are not a substance or a chemical in themselves. Halogen belongs to the related elements and is in the periodic table in group 17 (VII). These are F (fluorine), Cl (chlorine), Br (bromine), I (iodine), and At (astatine). Astatine is not used in electronic products as it is a radioactive material. So, for PCBs, only four halogens are of concern.
These halogens had several uses in PCBs. For example, chlorine acts as a protective covering in polyvinyl chloride wires or flame retardants. Chlorine also acts as a solvent for cleaning computer chips. Similarly, bromine finds its use as a flame retardant and as a sterilizer.
Iodine is a common disinfectant, while fluorine is a part of drinking water to promote teeth health.
Earlier, copper-clad used PBB and PBDE as flame retardants, but they were soon banned. However, many halogen-containing PCBs still use bromine flame retardants such as standard FR4 PCBs and CEM-3 PCBs. This bromine-based flame retardant is primarily brominated epoxy resin.
What Levels are Considered Halogen Free?
As per IEC 61249-2-21 and JPCA-ES-01-2003 standards, the halogen-free copper-clad should have less than 900 parts per million of chlorine and bromine or 0.09 wt% individually. Also, the whole of chlorine and bromine must be less than 1500 parts per million or within 0.15% wt.
However, it is important to be aware because there are several standards for halogen presence in the market. Independent manufacturers set their halogen levels as halogen-free manufacturing is not needed legally.
Halogen-Free Board Design
In general, nothing exists as a proper halogen-free PCB. Halogens are present in the circuit boards, and you can find them in the most unexpected places. Take the example of green circuit boards. To make green circuit boards halogen-free, you need to remove the green substrate from the solder mask. Similarly, epoxy resins contain chlorine, but they are essential to protect PCBs. You may also find halogen in concealed elements such as glass sizes, curing agents, wetting, or resin accelerators.
Additionally, there are some drawbacks to not using halogens. For example, the defective ratio of solder to flux without halogen can result in graping. However, you can fix this by defining pads with a solder resist called solder mask.
Thus, you need to work with your PCB manufacturer to get a clear picture of halogens in your PCBs. However, most manufacturers are ready for this transparency and make PCB circuits with minimal or no halogen. However, not all makers are fully equipped and ready to do this.
It is better that you discuss everything with the manufacturer and getaway out to create a PCB by avoiding unnecessary halogens.
Making 100% halogen-free PCs is challenging; you can make one with permitted halogen levels by IEC and RoHS.
Why use halogen-free PCB?
There are several advantages of using a halogen-free PCB that includes:
- Better electrical insulation: Halogen-free PCBs have lesser polarity as they use less polar Nitrogen and Phosphorus. As a result, halogen-free PCBs can bear greater electric shocks
- Small thermal coefficient: as phosphorus and Nitrogen replace the halogens, they give larger molecular weight to the circuit boards. As a result, the molecules are less mobile than standard epoxy resin. As a result, halogen-free printed circuit boards have high thermal stability and lesser thermal expansion than halogen-containing PCBs.
- Small moisture absorption rate: Epoxy resins with phosphorus and Nitrogen have lesser chances of forming hydrogen bonds with hydrogen atoms. However, epoxy resins with halogens do form hydrogen bonds in water. Thus, halogen-free PCBs absorb less moisture than halogen PCBs.
- Less toxic: as compared to halogen-packed PCBs, halogen-free PCBs pose lesser health risks to the technicians and individuals using PCBs.
- Safe for the environment: Halogen-free circuit boards create lesser risk for the environment. For areas where PCB recycling is impossible, it is easier to dispose of these lower halogen compounds.
- Affordable and effective: halogen-free PCBs are a cost-effective option for single-use electronics.
Halogen-free PCB materials
Halogen-free PCB substrates mean halogen-free FR4 materials. Here FR refers to the flame retardant and 4 means epoxy resins. In the halogen-free FR4, Phosphorus and Nitrogen replace halogens. Further, the ink for the solder mask is also halogen-free.
Halogen-free FR4 materials are Greenspeed series, ShengYi’s S1550G, S1165, S1165M, S6015, Panasonic R15 series, Ventec’s VT-447, and many more.
The halogen-free PCB manufacturing process
The basic steps of Halogen-free PCB manufacturing are the same that of standard PCB and include the following steps:
- Designing the PCB
- Design Review and Engineering Questions
- Printing the PCB Design
- Printing the Copper for the Interior Layer
- Etch the Inner Layers or Core to Remove Copper
- Layer Alignment
- Automated Optical Inspection
- Laminating the PCB Layers
- PCB Plating
- Outer Layer Imaging
- Outer Layer Etching
- Outer Layer AOI
- Solder Mask Application
- Silkscreen Application
- Finishing the PCB
- Electrical Reliability Test
- Profiling and Route Out
- Quality Check and Visual Inspection
- Packaging and Delivery
However, the process is different in some steps, such as:
- As the halogen-free FR4 materials have higher mechanical strength than normal FR4 and CEM-3 materials, you need more drill power to drill holes on the circuit boards in the drilling step.
- Halogen-free FR4 materials are less resistant to alkali than other FR4 materials. So, the time for soaking in alkaline stripping solution is less during the etching process.
- Usually, there is no lead in halogen-free PCBs. The lead assemblies need 260 degrees Celsius for reflow soldering, which is 20 degrees higher than lead PCB assemblies.
The electronics industry always promotes the use of environmentally friendly electronics. As halogens are toxic to the environment, there is a massive demand for halogen-free PCBs.
The law has not made it mandatory to use halogen-free PCBs. However, the concerned organizations are trying to drop these hazardous elements from PCBs.