Red adhesive is a polyolefin compound. Different from solder paste, it cures upon heating, with a solidification point of 150℃. At this temperature, the red adhesive starts to transform directly from a paste state to a solid state.
1. What is Red Adhesive
Red adhesive is a polyolefin compound. Different from solder paste, it cures upon heating, with a solidification point of 150℃. At this temperature, the red adhesive starts to transform directly from a paste state to a solid state.
The core component of SMT red adhesive is epoxy resin. Epoxy resin generally refers to organic polymer compounds containing two or more epoxy groups in the molecule, with a relatively low relative molecular mass. The molecular structure of epoxy resin is characterized by active epoxy groups in the molecular chain, which can be located at the end, in the middle, or in a cyclic structure of the molecular chain. Due to these active epoxy groups, epoxy resin can undergo cross-linking reactions with various types of curing agents to form insoluble and infusible high polymers with a three-dimensional network structure.
The formula of SMT red adhesive mainly consists of the following components:
- Epoxy resin: 40-60% by weight
- Curing agent: 10-18% by weight
- Thixotropic agent: 4-8% by weight
- Coupling agent: 1-2% by weight
- Diluent: 1-5% by weight
- Filler: 8-20% by weight
- Color powder: 0.5-1% by weight
The proportion of each component in the formula can be adjusted according to actual requirements to adapt to specific application scenarios and material specifications.
2. Properties of Red Adhesive
Red adhesive features viscosity fluidity, temperature characteristics, and wetting properties. Based on these characteristics, the core purpose of using red adhesive in production is to firmly attach components to the PCB surface and prevent them from falling off.
3. Usage Specifications of Red Adhesive
(1) To maintain the performance of the chip adhesive, please store it in a refrigerator for cold storage at (5±3)℃.
(2) Before use after being taken out of the refrigerator, it shall be thawed at room temperature for 2 to 3 hours.
(3) Toluene or ethyl acetate can be used to clean the rubber hose.
Dispensing
(1) Adding a back stopper in the dispensing syringe can achieve a more stable dispensing volume.
(2) The recommended dispensing temperature is 30-35℃.
(3) When subpackaging the dispensing syringe, please use a special adhesive dispensing machine to prevent air bubbles from being mixed into the adhesive.
Blade Coating
The recommended blade coating temperature is 30-35℃.
Note: After the red adhesive is removed from the refrigerated environment, it shall not be opened for use before reaching room temperature. To avoid contaminating the original product, never pour any used chip adhesive back into the original packaging.
4. Precautions for High-Quality Red Adhesive Printing
- Select an appropriate printing method according to the component type and substrate performance, such as stencil aperture printing, dispensing, or pin transfer method.
- Adjust printer parameters including pressure, red adhesive dosage, snap-off distance, printing speed, and automatic stencil wiping frequency, to ensure the red adhesive is accurately printed between two pads without offset.
- Select red adhesive with excellent heat resistance, superior electrical properties, and extremely low hygroscopicity, and ensure its batch stability.
- Maintain the ambient temperature and humidity within the appropriate range during the printing process, with a recommended temperature of 20-25℃.
- Clean the printing head and stencil in a timely manner to prevent aperture clogging and adhesive sticking.
- Strictly comply with the storage and usage specifications of red adhesive, avoid direct sunlight and high-temperature environments, and perform curing in strict accordance with the specified time and temperature.
By following the above precautions, high-quality red adhesive printing can be achieved, and the overall printing quality and production efficiency can be improved.
5. Process Methods of Red Adhesive
- Printing Method: The stencil aperture shall be designed according to the component type, and the recommended substrate thickness is 0.15mm.
- Dispensing Method: Dispensing uses compressed air to deposit red adhesive onto the substrate through the dispensing nozzle, with flexible and adjustable functions.
6. Typical Curing Conditions of SMT Red Adhesive
There are two main curing methods for SMT red adhesive:
- Thermal Curing: Place the PCB with applied red adhesive into a constant temperature oven, and perform curing according to the specified parameters of the red adhesive used. Thermal curing requires precise adjustment of oven temperature and curing time, and the required parameters vary for different brands and types of red adhesive.
- UV Curing: Curing is performed with ultraviolet light, which requires dedicated UV curing equipment. UV curing enables rapid curing of red adhesive, but the light intensity and irradiation time must be strictly controlled, which vary for different brands and types of red adhesive.
When selecting the curing method for red adhesive, the decision shall be made based on production equipment, process flow, and manufacturing requirements. Meanwhile, the stability, bonding performance, heat resistance and other core indicators of the red adhesive shall be taken into account to ensure reliable performance during application.
- After the surface temperature of the PCB reaches 150℃, the curing duration shall be no less than 120 seconds; or after the surface temperature reaches 120℃, the curing duration shall be no less than 150 seconds.
- Higher curing temperature and longer curing time can achieve higher bonding strength. The actual temperature applied to the adhesive will vary according to the material and size of the components mounted on the PCB. Therefore, the optimal curing conditions shall be determined based on the actual production situation.
7. Common Defects in SMT Red Adhesive Printing
- Missing Dot or Excessive Adhesive Volume: Caused by unstable adhesive dispensing and uneven adhesive application. Insufficient adhesive volume will lead to inadequate bonding strength, making components prone to falling off during wave soldering; on the contrary, excessive adhesive volume, especially for miniature components, will easily contaminate the pads and hinder electrical connection.
- Stringing: A phenomenon where the chip adhesive forms a filamentous connection along the moving direction of the dispensing head during dispensing. Severe stringing will cause the chip adhesive to contaminate the pads, resulting in poor soldering.
- Slumping: Caused by poor thixotropy of the chip adhesive, which leads to easy flowing of the adhesive, resulting in slumping. It may also contaminate the pads and cause poor electrical connection.
- Component Offset: A common defect that frequently occurs in high-speed mounter operations.
The above are the common defects in SMT red adhesive printing, which can be improved through quality control of the whole production process and targeted process parameter adjustment.
8. Storage of Red Adhesive
- To maximize the performance of the adhesive, it must be stored in a refrigerator at 2-5℃.
- Shelf life: 6 months.
9. Management of Red Adhesive
As the viscosity, fluidity, wetting and other core properties of red adhesive are significantly affected by temperature, standardized usage conditions and management specifications for red adhesive must be established.
- Red adhesive shall be stored in a refrigerator at 2-8℃ to prevent property changes caused by temperature fluctuations.
- The red adhesive must be thawed at room temperature for 4 hours before use, and shall be used in accordance with the first-in first-out principle.
- For dispensing operations, the red adhesive in the syringe must be degassed. For the red adhesive not used up at one time, it shall be sealed and put back into the refrigerator for storage, and used adhesive shall not be mixed with new adhesive.
- This product shall not be used in pure oxygen or oxygen-enriched systems, and shall not be used as a sealing material for chlorine gas or other strong oxidizing substances.
10. Safety Precautions
- Avoid contamination before use. Uncured adhesive stained on the circuit board can be wiped off with isopropyl alcohol. To avoid contaminating unused adhesive, never pour any adhesive back into the original packaging container.
- For people with allergic constitution, direct skin contact may cause skin allergies, so please take necessary protective measures.
- If the adhesive accidentally contacts the skin, please wash it immediately with soapy water.
- If it enters the eyes, please rinse it thoroughly with clean water as soon as possible and seek medical attention immediately.
- This product is non-flammable (flash point higher than 100℃). In case of fire, extinguish it with dry powder, foam or carbon dioxide fire extinguisher. Toxic gases such as carbon oxides will be generated during incineration, so avoid inhalation.
- For a small amount of leakage, wipe it clean with paper towels and place the contaminated waste in a dedicated container for disposal. Thoroughly clean the leakage area with soapy water or detergent, and prevent the material from entering drains or sewers.
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