What is PCB Curing?
PCB Curing refers to the process where solder masks, adhesives, and resins materials used in the production of printed circuit boards solidifies. The materials usually undergo a chemical process that turns them from liquid or semi-solid to solid under particular circumstances, such heat, pressure, or exposure to ultraviolet light.
This helps to permanently attach the solder mask or silkscreen with the base material in PCB. PCB curing ensures the materials achieve the desired properties for durability, reliability and proper functionality in PCB.
Why Does PCB Curing Matter?
By fortifying the connection between PCB layers, maintaining structural integrity encourages robustness and resilience to mechanical stress.
1. Structural Integrity:
By fortifying the connection between PCB layers, maintaining structural integrity encourages robustness and resilience to mechanical stress.
2. Electrical Performance:
Proper curing can reduce electrical failures caused by incomplete adhesion or material breakdown.
3. Chemical Durability:
It increases the PCB’s capacity to withstand environmental elements like moisture, chemicals, and solvents.
4. Solidification:
Curing liquid or semi-solid solder masks and silkscreen inks produces a robust, long-lasting layer. It helps the copper traces and substrate behind the mask/screen layers to bond strongly.
5. Thermal Stability:
When used, cursed materials show improved resistance to high temperatures.
Temperature Ramp-Up Phase
Temperature profiling usually consists of three main stages: warming, holding and cooling.
I. Rising Temperature Stage:
The temperature usually rises at a rate of 1.5-2.5°C/min for material temperatures in the range of 80-140°C. This gradual warming helps to minimize thermal stress and prevent deformation of the material.
For thinner or thermally sensitive PCBs, a lower ramp-up rate, such as 0.5-1°C/min, can be used to further reduce the risk of thermal shock.
II. Holding phase:
The curing temperature usually needs to be at or above 175°C and held for at least 45 minutes. This time ensures that the resin is sufficiently cross-linked to form a stable three-dimensional network structure. The length of holding time needs to be adjusted according to the material properties and PCB complexity.
For example, for multilayer boards or high density PCBs, longer holding time may be required to ensure adequate curing.
III. Cooling phase:
Slow cooling helps to release internal stresses and prevent deformation caused by sudden shrinkage. The cooling rate is usually controlled at 1-2°C/min, but for easily deformed materials, the cooling rate can be further reduced.
Influencing Factors
Temperature profile design is influenced by a number of factors, mainly including:
- PCB size and complexity: Larger PCBs or high density PCBs may require longer holding times.
- Thermal conductivity: The thermal conductivity of the substrate and components affects the uniformity of temperature distribution.
- Environmental conditions: Humidity and air pressure may affect the curing process.
Advantages of PCB Curing
The following reasons highlight the importance of curing:
1. Enhanced Bonding
- Curing strengthens the adhesive and creates a strong bond between the two surfaces. The adhesive has the chance to penetrate the surface filaments during the curing process, creating a solid and robust bond.
2. Water Resistance
- After curing, adhesives acquire water resistance. For instance, PVA adhesives that have just been cured are water soluble, but after curing, they become waterproof, allowing it to be used externally.
3. Resistance to Heat & Chemicals
- Because of their ability to withstand high temperatures and chemical exposure, cured adhesives are ideal for use in electronic components, automobile parts, and various mechanical applications.
4. Prevents Movement
- Curing ensures that the strengthened objects stay in place and do not break. The surfaces may shift or constrain if the adhesive is not allowed to cure sufficiently.
5. Prevents Shrinkage
- Many types of adhesive tend to shrink as they dry which could lead to gaps and cracks between the bonded surfaces. The curing process ensures that shrinkage does not occur.
Key Steps in the PCB Curing Process
Step 1: Materials Application
The surface of the PCB is coated, sprayed, or screen printed with a curing material, like solder mask or resin.
The materials used in the PCB curing process mainly include resin systems and additives. The selection of these materials directly affects the performance and reliability of the PCB.
| Resin Type | Characteristics | Applications |
| Epoxy Resin | Good heat resistance, high mechanical strength | Multilayer boards, high performance boards |
| Polyimide | High temperature resistant, low dielectric constant | High frequency application boards |
| BT Resin | Heat resistant, low hygroscopicity | High density packaging |
Additives
In addition to the resin system, additives can not only improve the specific properties, the rational use of curing accelerator can significantly shorten the curing time, improve production efficiency. Common additives include:
- Fillers
- Flame retardants: Such as bromides to improve fire resistance.
- Curing accelerators: Such as imidazoles speed up the curing process.
Step 2: Initial Drying
A few materials require an early drying stage to expel solvents or get ready for the curing process.
Step 3: Curing Strategy
The PCB is uncovered by the chosen curing technique like pressure, UV, or thermal under carefully observed conditions. The properties of the material manage the desired density, temperature, and duration.
Tip-Time Management
Pre-treatment Stage
| Cleaning Method | Time Range |
| Soaking Cleaning | 30-60s |
| Ultrasonic Cleaning | 1-3min |
| Drying | 5-15min |
Curing Phase
| Curing Method | Time Range |
| Heating up | 15-30min |
| Temperature maintaining | 30-60min |
| Cooling | 20-40min |
Post-processing Phase
| Curing Method | Time Range |
| Natural Cooling | 30-60min |
| Forced Cooling | 10-20min |
| Preliminary Inspection | 5-15min |
Step 4: The Stage of Cooling
After the curing handle, the PCB is permitted to cool to stabilize the materials and prevent deformation.
Step 5: Quality Inspection
PCBs that have been cured are inspected (either visual or automated) thoroughly for defects such as bubbles, breaks, or insufficient curing.
The following factors are key inspection items:
- Scratches: may affect circuit integrity
- Oxidization: affects solder quality
- Discoloration: May indicate material deterioration
- Solder pad abnormalities: such as irregular shapes or defects
- Coating defects: such as bubbles, cracks, or flaking
Different PCB Curing Process
The types of PCB curing process are categorized from three perspectives: steps, curing method and speed.
1 PCB Ink Curing
A layer of ink is applied to the PCB surface during the ink curing process to create a protective coating. This layer protects the circuits’ parts from wear, oxidation, and damage from the outside.
How It Operates
- Application of Ink: Screen printing or spraying are two methods used to apply ink to the PCB surface.
Curing Procedure
The applied ink is cured, usually by:
- UV, or ultraviolet Irradiation: UV light causes a photochemical reaction that rapidly and uniformly hardens the ink.
- Thermal Heating: A solid ink coating is left behind after solvents are evaporated at high temperatures.
Benefits of Ink Curing
- Enhanced Protection
- High Precision
- Rapid Processing
2 PCB Glue Curing Process
Applying PCB glue is just as important as choosing the right type. Appropriate application ensures strong bonds, durable execution, and reliable device functionality.
Clean the Surface
The surface must be prepared before you apply any adhesive. Pollutants such as oil, clean, or other contaminants can weaken the connection.
Consider these tasks:
- Clean the surfaces completely with the suggested solvents, such as isopropyl liquor.
- Let the board dry completely by using suggested glue.
- To improve adhesion, gently roughen problematic regions using fine sandpaper.
Glue Curing Process
Since improper curing can reduce toughness, this process should not be rushed.
- UV-Curing Adhesives: When exposed to UV light, you can cure the glue in a few seconds. Make sure every area is exposed to light for consistent curing.
- Heat-Curing Adhesives: To activate the glue, apply regulated heat, usually using a stove or heated weapon.
- Room-Temperature Curing Adhesives: Let the glue set according to room temperature in order to give it enough time to cure.
Benefits of Glue Curing
- Strong Component Stability
- Compatibility
- Precision Bonding
3 PCB Solder Mask Curing Process
The PCB’s copper traces are held in place by a protective material known as the solder mask.
How It Operates
- Solder Mask Application:
The solder mask is applied on the PCB via splashing, screen printing, or curtain coating techniques.
- Methods of PCB Curing:
- UV Irradiation: The solder mask layer is rapidly and reliably solidified by UV light.
- Heat Curing: Heat is used to provide certain materials to get the required level of toughness and hardness.
- Advice of PCB Solder Mask Curing:
- You should use a tool to remove any resin that might be obstructing your PCB‘s mounting holes or through-hole components.
- You can use a needle, a pair of fine-tipped tweezers, or a similar instrument will be used.
Benefits of Solder Mask Curing
- Electrical Insulation
- Environmental Protection
- Improved Aesthetics
4 Air Curing in PCB
Applicability in PCB Processes
- Air curing is a simple and cost-effective method in PCB manufacturing. It is suitable for certain types of coatings and adhesives used in PCB components.
- For example, some low viscosity solder resistant ink can be air-cured. This process allows for the solvents in the ink to evaporate gradually in the presence of air, leaving behind a solid protective layer on the PCB surface.
- In the case of some PCB components like small resistors or capacitors that are attached using air curable adhesives, air curing provides a convenient way to achieve a reliable bond without the need for complex curing equipment.
Considerations for
Air-cured PCB Components
- Environmental factors play a crucial role. The humidity and temperature of the air can significantly affect the curing process. High humidity may slow down the evaporation of solvents, leading to longer curing times or even incomplete curing. For instance, if the relative humidity is above 80%, the curing time for a standard air-curable solder resist may increase by 50% compared to normal conditions.
- The ventilation in the curing area is also important. Poor ventilation can cause the build-up of solvent vapors, which not only affects the curing quality but also poses a safety hazard. Adequate air flow should be maintained to ensure that the solvent vapors are continuously removed from the curing environment.
5 Electric Curing in PCB
Working Principle
- Electric curing involves the use of an electric field to accelerate the curing process of certain materials in PCB manufacturing. When an electric field is applied to a curable material (such as an epoxy resin), the charged particles in the material are forced to move. This movement causes the molecules to align and react more quickly, leading to a faster curing process.
- In some cases, the electric field can also generate heat within the material due to the resistance of the material to the electric current. This heat further promotes the curing reaction.
- For example, in the curing of an electrically conductive adhesive used for bonding PCB layers, the electric field not only aligns the conductive particles but also heats the adhesive, enabling it to cure more rapidly.
Equipment and Parameters
for Electric Curing
- The equipment for electric curing typically consists of a power supply, electrodes, and a curing chamber. The power supply provides the necessary electric voltage and current.
| Layer | Voltage | Current | Time |
| Thin Layer | 50 – 100 V | 0.1 – 0.5 A | 1 – 2 min |
| Thick PCB Laminate | 200 – 500 V | 0.5 – 1 A | 5 – 10 min |
6 Rapid PCB Curing Process
Benefits of Rapid Curing in PCB
- Short curing time allows for a higher production throughput in PCB manufacturing. For example, some rapid curing adhesives can cure within seconds to minutes, compared to hours or days for traditional curing methods.
- A more accurate control of the curing reaction is also made possible by the quick curing process. By using advanced curing techniques such as UV curing, the curing process can be initiated and stopped precisely, which is beneficial for applications where high-precision bonding or coating is required in PCB production.
Technologies and materials enabling rapid curing
UV curing
- UV curing method is commonly utilized in rapid PCB curing. UV-curable materials contain photo-initiators that, when exposed to ultraviolet light, initiate a polymerization reaction. The UV light source can be a high-intensity mercury lamp or a more modern LED-based UV lamp.
- For example, UV-curable solder masks are commonly used in PCB manufacturing.
Microwave curing
- Microwave curing is another technology for rapid curing. Microwave energy can penetrate the PCB material and heat it directly from the inside, leading to a very fast curing reaction. Some epoxy-based PCB laminates can be cured using microwave energy in a matter of minutes.
7 Slower PCB Curing Process
Epoxy in PCB fabrication
Epoxy is a key material in PCB fabrication. It is used as an adhesive for bonding different layers of the PCB, such as the copper foil to the substrate.
Specific Steps & Conditions for Slower Curing in PCB Production
- First, the epoxy resin and its hardener are mixed in the correct ratio. For a typical slow curing epoxy used in PCB production, the ratio of epoxy resin to hardener may be around 2:1.
- Then, the mixed epoxy is applied to the desired area on the PCB, such as the bonding surface between layers or the coating area.
- Next, the PCB with the applied epoxy is placed in a curing environment. The temperature and humidity in the curing environment are closely monitored. For slow curing epoxy, the curing temperature is usually relatively low, around 25 – 50°C. The humidity should be maintained at a stable level, typically around 40 – 60% relative humidity.
- The curing time can range from several hours to days depending on the type of epoxy and the thickness of the application. For a thin layer epoxy coating, the curing time may be around 8-12 hours, while for a thick layer epoxy used for bonding PCB layers, the curing time may be 24-48 hours or even longer.
| Aspect | Slower Curing | Rapid Curing |
| Efficiency | Takes hours/days | Minutes/seconds, Boosts production by 50 – 100% in PCB line |
| Quality | Prone to over/under – curing | Less likely; More consistent |
| Limitations | Not stated | UV may not penetrate thick PCB, may need extra steps |
To learn more, check out these 2 blog posts from ELEPCB detailing its full range of knowledge.
Conclusion
The PCB curing process is a crucial step in ensuring that your circuit boards function properly in various scenarios. The quality of this process can be increased by being aware of the best practices and potential issues. Whether you decide on thermal, UV, or curing techniques, achieving excellent results requires precision and close attention to detail.
Do you want to know how to make your PCB fabrication better? Comment below or contact ELEPCB for professional assistance！
References
[1] F. Liu and K. Kirane, “Type I size effect and failure behavior of woven composites under biaxial flexure,” Composites Part B: Engineering, vol. 254, pp. 110580, 2023. doi: 10.1016/j.compositesb.2023.110580.
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