Halogens in polychlorinated bipheny

If you ask most designers where the halogen elements in a PCB are found, it is doubtful they would tell you. Halogens are commonly found in brominated flame retardants (BFR), chlorinated solvents and polyvinyl chloride (PVC). Halogens are clearly not dangerous in every form or concentration, and there are no health problems with holding PVC pipes or drinking tap water. If you were to burn that tube and inhale chlorine gas released when the plastic breaks, that might be a different story. This is the main problem with halogens in electronics. They can be published at the end of the PCB life cycle. So, where exactly do you find halogens in the circuit board?

As you know, PVC is not only used for piping, but also for wire insulation, and therefore may be a source of halogens. Chlorinated solvents can be used to clean PCBS during manufacturing. BFR is used for PCB laminates to reduce the risk of board fire. Now that we have examined the main source of halogens in the circuit, what should we do about it?

Halogen free PCB

Like RoHS lead-free requirements, halogen-free standards require CM to use new materials and manufacturing methods. Just like any standard “halogen-free” specific limit set by various organizations. The IEC definition of halogens does not contain chlorine and bromine less than 900 PPM and total halogens less than 1500 PPM, while RoHS has its own limitations.

Now why quote “halogen-free”? This is because meeting the standards does not necessarily guarantee that your board is halogen-free. For example, IPC prescribes tests for detecting halogens in PCBS, which typically detect ionic bonded halogens. However, most of the halogens found in the flux are covalently bound, so the test cannot detect them. This means that to make a truly halogen-free sheet, you need to go beyond standard requirements.

If you’re looking for a specific source of halogens, one is TBBPA, which is the BFR commonly used in laminates. To eliminate this starting point, you need to specify halogen-free laminates, such as active phosphorus base laminates. Your flux and solder may also introduce halogens into the PCB, so you will also need to discuss with CM what alternatives may exist there. It can be painful to use new materials and technologies on boards, but halogen-free circuits have some advantages. Halogen-free PCBS generally have good heat-dissipation reliability, which means they are better suited for the high-temperature processes required for lead-free circuits. They also usually have lower permittivity if you want to preserve signal integrity.

Halogen-free board design

The advantages of halogen-free boards come at the cost of increased complexity not only in the manufacturing process but also in the design. A good example is halogen-free solders and fluxes. Halogen-free varieties can sometimes alter the solder to flux ratio and cause scratches. This is where the solder merges into a large ball rather than being distributed throughout the joint. One way to solve this problem is to better define the pad with a blocking film. This will edge up the solder paste and reduce defects.

Many new materials have their own design quirks, and you may need to contact the manufacturer or do some research before using them. Halogen-free boards are on the rise, but by no means universal. You should also talk to your CM to see if they have the ability to manufacture PCBS from halogen free materials.

As time goes by, we seem to find that the more and more materials we use every day pose health risks to us. That’s why organizations like IEC develop halogen-free board standards. Remember where halogens are usually found (BFR, solvent, and insulation), so if you need halogen-free, you know which halogens to replace. Different standards allow for different amounts of halogens, and certain types of halogens may or may not be detected. You need to do research beforehand to understand the location of problem areas on the PCB. Once you know which material to use, it is best to check with the manufacturer and CM to determine the best way forward. You may need to adjust the design or work with CM on certain manufacturing steps to ensure that your board is successfully completed.