Phenolics are a group of very hard materials manufactured by administering phenolic resin to layers of glass, paper or fabric under high pressure and temperature. It is important to know how to bond phenolics in order to enable their useful properties in various products.
Phenolics are most commonly used in:
- Printed circuit boards (PCBs), as phenolic paper.
- Aerospace applications, such as rocket bases, as phenolic tubes.
- High speed bearings, as phenolic glass.
- Density controls, as phenolic micro-balloons.
The PCBs made from phenolic paper are also known as FR-1 and FR-2, due to the material being flame resistant, with FR-1 having a rating of 130 °C and FR-2 having a rating of 105 °C. Phenolic tubes have excellent heat resistance, rigidity and strength. Phenolic glass has great electrical insulation and is resistant to thermal shocks, making it suitable for electromechanical purposes.
Generally, it is a relatively easy process to bond phenolics. However, different methods are used depending on the specifications of the application. For example, applications in PCBs require considering the heat resistance, electrical properties and thermal conductivity of the finished product. Similarly, applications in structural tubing depend on the impact, weathering and solvent resistances.
Since paper phenolics are the most common form of phenolics, only the specific properties of a typical sheet of FR-2 are explored in this article.
Main Properties of Paper Phenolics (FR-2)
- Permittivity: 4.5 at 1 MHz
- Maximum short-term working temperature: 105 °C
- Dissipation factor: 0.024-0.26 at 1 MHz
- Dielectric strength: 740 V/mil
Surface Preparation Tips for Phenolics
To obtain good strength bonding, ensure that phenolic surfaces are free of all traces of dust, oil, grease or dirt by cleaning them with acetone or alcohol wipes first. For optimum strength bonding, the phenolic surfaces can be gently abraded with an emery cloth, eliminating the oxide layer, then wiped with a solvent wipe to remove all loose particles. We recommend using a solvent wipe such as Wipemaster Wipe 9.
When using cyanoacrylates to bond phenolics (as explained below), apply a special primer such as the Permabond POP to the clean phenolic surfaces before an attempt at bonding.
For further information regarding surface preparations, read: What you need to know on bonding plastics.
Adhesives for Bonding Phenolic
Here we explore the best adhesives that can be used to bond phenolics and PCBs:
Permabond 820, a cyanoacrylate adhesive, gives rapid results for applications concerning high-temperature resistance, such as bonding electrical components on PCBs before soldering. It is colourless, easy to use, and offers adhesion to a large range of different substrates. The 820 is ideal for bonding to rubber, metal, and plastics. Beware that this product is not used with strong oxidizing materials or polar solvents as this could affect the bond strength.
Permabond 947, another cyanoacrylate adhesive, gives durable bonds and is ideal for use in low odour applications. As well as being more comfortable to work with, it is non-frosting and therefore offers good aesthetics to the final product. The 947 is suitable for a wide variety of different substrates as it bonds most materials. It is also very stable, and if refrigerated, provides optimum results for one year. Similarly, this product must also not be used with strong oxidizing materials or polar solvents as this would affect the bond strength.
Permabond UV683 and UV681, two UV curable adhesives, offer great bonds if light can reach the bonds. They are tack free and fast curing with low-power lamps. Their low viscosity and high temperature resistance make them ideal for coating microchips and wave-soldering, as well as protecting them against vibrational and environmental damage.