PCB Glue for Electronics – All You Need to Know

This article provides a detailed overview of the various types, applications, and selection tips for PCB glue used in electronics. In addition, we have expanded the discussion to include the specific requirements for optical industry PCBA.

By comparing traditional electronic PCBA with optical PCBA, particularly in terms of bonding processes and material selection, manufacturers can achieve precise control and optimal performance in their applications.

What is PCB Glue for Electronics?

PCB glue is a bonding agent for circuit board manufacturing, assembly, and repair processes. It joins circuit board layers, attaches flexible sections, and bonds components. It can also coat a PCB, forming a protective layer on the surface.

The glue is typically a resin, such as epoxy or acrylic. It also usually includes a hardener and sometimes fillers to achieve specific properties. The properties include heat and current conduction, heat resistance, etc.

In addition to these general functions, PCB glue plays an important role in specialized fields such as optical PCBA. In these applications, the glue must meet extra requirements: it should have high optical transparency and low yellowing to ensure that it does not interfere with optical signal transmission.

Uniformity of the adhesive layer is also crucial, as variations in thickness can affect the alignment and performance of sensitive optical components.

Below, we discuss their different types based on material and characteristics.

PCB Glue Types

Circuit board glue can be of many types based on their materials. We can also classify them by their unique functions and characteristics. With these variations in mind, the following adhesives are available today.

Epoxy Adhesives

Epoxy-based adhesives are the most popular, owing to their superior characteristics.  For instance, they form robust bonds, possess high Tg temperatures, and have good optical characteristics. PCB epoxy glue also excellently withstands environmental extremes.

Acrylic Adhesives

Acrylic adhesives cure in UV light, with some hardening under visible light. They offer good adhesion properties, which make them applicable in many situations. Furthermore, these adhesive types are available in many forms, from flexible and gel to hardened forms.

Polyurethane Adhesives

These adhesives are also flexible, like silicone. They provide good vibration resistance and sealing properties, making them popular for PCB conformal coatings. They also shield parts from environmental damage. On the downside, they withstand heat less.

Silicone Adhesives

Silicone adhesives are flexible types with excellent sealing and impact resistance properties. They are available in various options, from electrically conductive to high-temperature variants. In addition to bonding, manufacturers use this glue to coat PCBs to protect against the elements.

Cyanoacrylate Adhesives

Many people call this adhesive “super glue.” It hardens instantly and usually does not require specialized curing. Super glues are mainly suitable for temporary bonding. On the downside, they have poor moisture and exhibit brittleness.  

Pressure Sensitive Adhesives

Pressure-sensitive adhesive responds to pressure. These resins are essential when bonding heat-sensitive parts, limiting temperature elevation. They also suit uses where heat cannot penetrate, and UV-curing won’t work.

Electrically Conductive Adhesives

These contain conductive particles that allow them to make electrical contact. They replace solder, especially when assembling heat-sensitive boards. That’s because their curing temperature is much lower than soldering.

Electrically conductive adhesive is either isotropic or anisotropic. Isotropic types conduct electricity in all directions, while anisotropic versions are only conductive in one direction.

Thermally Conductive Adhesives

These are resins filled with metallic or inorganic particles to make them thermally conductive. Metallic fillers, such as silver, make them electrically conductive as well. Inorganic fillers include ceramics and mineral-based materials.  

Thermally conductive adhesives help dissipate heat while also bonding components. They are helpful in high-power circuits and often bond heat sinks. LED circuit boards also use these adhesives for thermal relief.

UV Cured Adhesives

UV-curable adhesives designed for optical applications set rapidly under UV light, minimizing thermal impact on sensitive optical components. Their fast curing time, combined with high transparency and minimal optical distortion, makes them ideal for applications requiring precise alignment and minimal downtime.

Low Viscosity Optical Adhesives

Formulated to achieve ultra-thin, uniform layers, low viscosity adhesives help maintain precise spacing and alignment between optical elements. Their controlled flow properties ensure that the adhesive layer remains consistent, reducing the risk of optical aberrations or misalignment.

Optically Transparent Epoxy Adhesives

These epoxies are specifically formulated to offer excellent clarity and low yellowing over time. They combine robust mechanical strength and thermal resistance with the optical properties necessary for applications such as bonding optical sensors, laser modules, and display components.

PCB adhesive application

Resource: https://youtu.be/OajeP-wiUus?si=9oHUYAAh8Cm2t8BD

What are the Uses of PCB Glue for Electronics?

PCB glue has various uses in fabrication, assembly, and rework steps. They can provide structure, insulate, conduct heat, or attach components, among other functions. Here is a list of these uses during manufacturing and after.

PCB Fabrication Uses

• Laminate Bonding: manufacturers use it to join PCB laminates. This process creates multilayer circuit boards. The adhesive is usually a pressure-sensitive type.
• Copper Layer Bonding: the adhesive also sometimes helps bond the copper layer(s) to the laminate.
• Rigid Flex PCBs: the resin bonds flexible PCB sections to rigid boards. This process creates rigid-flex boards.

PCB Assembly Uses

• Component Attachment:  PCB component glue attaches SMT parts before soldering. It prevents them from falling over.
• Component Mounting:  adhesives can permanently mount components. It replaces solder and is usually electrically conductive.
• Flip Chip Under-fill: underfilling provides mechanical stabilization. The adhesive is often epoxy since it has a good flow that fills every gap.
• Thermal Management: thermally conductive adhesives find valuable applications in high-power circuit boards where they help transfer heat. Manufacturers often use them to attach heat sinks.
• Electrical Insulation: non-conductive adhesives isolate circuit sections or components, preventing problems like short circuits.

Post Assembly Uses

• Sealing and Encapsulation: this helps shield parts in harsh conditions, protecting against dirt, moisture, and solvents. They also prevent scratching and other damage.
• Cable and Wire Management: it secures wires for routing or space savings. It keeps them stationary, preventing damage.
• Repair and Rework: PCB glue for repair typically fills gaps, provides isolation, or fixes delaminated sections. You can also use it to attach components, among other needs.
• Edge Bonding: they stabilize sensitive parts and prevent vibrations and shock damage. The adhesives are high-strength types for endurance and durability.

What glue is Safe for Electronics?

The best adhesive for PCB applications is arguably epoxy. This resin has a low outgassing, making it less harmful. Epoxy glue for PCB use also has a high dielectric strength, produces strong bonds, and resists chemicals and moisture.

The resin is the bonding agent for glass fiber laminates like FR4s. It also joins circuit board layers. Other functions include component mounting, encapsulation, and rework needs.

Any glue for circuit boards is safe if it doesn’t severely off-gas. It shouldn’t damage the PCB or components, too.  It should not contract upon setting or take too long to cure.   

PCB Glue Selection Tips

As we saw earlier, PCB adhesives vary widely based on their materials, functions, and properties. So, how do you select a type to use?  The following tips will help you find what best suits your project.

• Curing Method: go for a technique you can safely and comfortably use. Choose room temperature and UV resins where heat can cause damage. Heat-curing is best for hidden spaces.
• Electrical Properties: the adhesive can be conductive or non-conductive. Choose what best suits your intended use.
• Bond Strength: determine how strong you want the bond to be. Different adhesives have varying shear and tensile strengths. They also have varying abilities to withstand impact forces.
• Heat Resistance: select a resin that fits your application requirements or conditions. Some can endure higher temperatures than others.
• Chemical Resistance: depending on the PCB’s usage environment, choose a resin that can resist solvents and other materials.
• Hardening or Flexible: decide if you need a flexible type for vibration dampening or shock resistance.
• Reworkability: you may want the resin easily removed for repair. If so, consider those that will not permanently bond to the PCB or part.

Tips for Optical PCBA

For optical PCBA, in addition to the conventional criteria, consider the following:

• Optical Transparency: Choose adhesives with high clarity and low yellowing to avoid any interference with optical signals.
• Low Volatility and Uniform Rheology: Ensure the adhesive does not generate bubbles or precipitates during curing so that the bond layer remains uniformly thin, meeting the precise thickness requirements of optical components.
• Fast and Stable Curing Performance: The adhesive should cure quickly without compromising long-term mechanical and optical stability.

Optical PCBA Bonding Process Requirements

For optical industry PCBA, the bonding process must adhere to strict standards:

Optical Transparency

The adhesive must exhibit high transparency and low yellowing properties to ensure that long-term use does not disrupt optical signal transmission.

Precise Curing Process Control

The curing process should be rigorously controlled in terms of temperature, humidity, and time. This ensures a uniform, bubble-free adhesive layer, which is critical for the precise alignment of optical components.

Accurate Control of Adhesive Thickness

Employ automated dispensing equipment and inline inspection technologies to ensure that the adhesive layer remains consistently even. Inconsistencies in thickness can lead to optical distortion.

Environmental Adaptability and Stability

The adhesive must possess excellent resistance to high temperatures, vibration, and chemical corrosion to endure harsh operating environments.

How to Properly Use PCB Glue for Electronics

Users must understand its application rules, as incorrect usage can lead to damage or bonding issues. The following are usage rules you should know about.

1. Choose a resin or glue that best fits your use and PCB type. You will protect the board and its components while ensuring proper bonding.

2. The application area must be free of any material that might prevent the glue from working. Coarsen it to improve adhesion.

3. Use the proper application method for the specific adhesive. Popular options include spraying, coating, and small controlled amounts.

4. Ensure adequate ventilation to avoid inhaling dangerous fumes. Some adhesives outgas more than others, requiring more care.

5. Avoid removing dried adhesive using force. That could damage the PCB or bonded component. Instead, heat it slightly to soften the glue. You could also use a laser.

6. Allow adhesives to cure before testing or other process. Use the correct curing method, too, for good results.

Conclusion

PCB glue plays a vital role in various manufacturing processes, from lamination to assembly and encapsulation. With the increasing incorporation of optical components in modern electronics, optical PCBA demands a higher standard in bonding processes.

By selecting adhesives that offer superior optical transparency, minimal yellowing, and high uniformity—and by strictly controlling the curing process and adhesive thickness—manufacturers can ensure precise alignment of optical elements and enhance the overall reliability and durability of the product.