SMT (Surface Mount Technology) adhesive process is a key link in electronic assembly, mainly used to fix electronic components, ensure positional accuracy before welding and mechanical strength after welding. The following is a detailed introduction to the standard process of SMT adhesive, covering process overview, process steps, key parameters, and quality control.
Table of Contents
1、 Overview of SMT Adhesive Process
1. Application scenarios
-Component fixation: used for pre fixation of chip components, IC chips, shaped components, etc., to prevent component displacement due to high temperature during the soldering process.
-Auxiliary soldering: Combined with reflow soldering or wave soldering processes, especially suitable for preventing component dropping in double-sided mounting processes.
2. Common types of adhesives
| Type | Characteristics | Typical Applications |
| Epoxy resin | Epoxy resin has high strength and high temperature resistance after curing, but the curing time is relatively long and requires high-temperature baking (such as 150 ℃ for 5-10 minutes). | Fixed large IC and BGA components |
| Acrylics | Acrylic acid has fast curing speed (can be cured by UV or heat), high transparency, suitable for visual inspection, but poor moisture resistance. | Fixed small and precision components |
| Silicone types | Silicone materials are resistant to high and low temperatures, vibration, and have good flexibility, but their adhesive strength is low. They are commonly used in components that require buffering, such as sensors. | Fixed high-frequency components and sensitive components |
2、 SMT Adhesive Standard Process Flow
1. Process flow
Substrate pretreatment → Adhesive coating → Component mounting → Adhesive curing → Welding → Cleaning → Testing
2. Detailed explanation of key steps
(1) Substrate pretreatment
-Purpose: To remove grease, dust, and oxides from the surface of the substrate to ensure adhesive adhesion.
-Method:
-Wipe with alcohol or deionized water (suitable for lightly contaminated substrates).
-Plasma cleaning or ultrasonic cleaning (suitable for high demand scenarios such as medical electronics and automotive electronics).
(2) Adhesive coating
-Objective: Accurately apply adhesive to the designated position on the substrate, common methods include:
-Glue dispensing process (most commonly used):
-Equipment: Glue dispensing machine (pneumatic, screw, jet).
-Parameter control:
-Glue dispensing volume: Adjust by air pressure, needle diameter, and dispensing time, with an error controlled within ± 5%.
-Glue dispensing position accuracy: XY axis error ≤± 0.1mm, ensuring that the adhesive completely covers the edge of the solder pad after component mounting.
-Applicable scenarios: precise coating of single point or small quantity components (such as QFP, SOP packaged components).
-Printing process:
-Equipment: Screen printing machine or steel screen printing machine.
-Parameter control:
–Steel mesh thickness: usually 50-100 μ m, adjusted according to the viscosity of the adhesive.
-Printing pressure: 5-10kg/cm ², ensuring even filling of the mesh with the slurry.
-Applicable scenarios: Rapid coating of large areas or multiple components (such as LED light boards, Memory modules).
(3) Component mounting
-Equipment: SMT machine (high-speed machine, general-purpose machine).
-Key requirements:
-Mounting accuracy: The alignment error between component pads and substrate pads is ≤± 0.05mm (higher accuracy is required for ultra small components such as 01005).
-Mounting pressure: Moderate to avoid crushing the adhesive or causing component displacement, usually set to 1-3N.
(4) Adhesive curing
-Purpose: To crosslink and solidify the adhesive through heating or light exposure, forming a mechanical connection.
-Curing method:
-Thermal curing:
-Equipment: Reflux furnace or oven.
-Typical parameters:
-Epoxy resin: 150-180 ℃, insulation for 5-10 minutes (gradient heating is required to avoid substrate deformation).
-Acrylic acid: 120-150 ℃, keep warm for 3-5 minutes.
-UV curing:
-Equipment: UV curing machine (wavelength 365nm, light intensity ≥ 1000mW/cm ²).
-Typical parameters: irradiation time of 10-30 seconds, ensuring that the bottom of the component receives sufficient light.
(5) Welding (reflow soldering/wave soldering)
-Reflow soldering: suitable for single-sided or double-sided mounting, melting solder paste through high-temperature furnace airflow to achieve component soldering.
-Wave soldering: mainly used for the second side of double-sided mounting, where components are soldered by melting solder waves (note that the adhesive has a temperature resistance of ≥ 230 ℃).
(6) Cleaning (optional)
-Purpose: To remove residual contaminants such as flux and adhesive spills during the welding process.
-Method:
-Solvent cleaning (alcohol, FC-72, etc.): suitable for precision components.
-Water based cleaning: It is necessary to ensure the water resistance of the substrate and components.
(7) Testing
-Appearance inspection: Confirm the position of the components, the curing status of the adhesive, and whether there are bubbles or overflow through AOI (automatic optical inspection) or manual visual inspection.
-Functional testing: Power on to test circuit performance and ensure that the adhesive does not affect electrical connections.
-Reliability testing (sampling):
-Vibration testing (simulating transportation or working environment vibrations).
-High and low temperature cycling test (-40 ℃~+125 ℃, to verify temperature resistance reliability).
3、 Key process parameters and control
1. Adhesive viscosity
-Impact: If the viscosity is too high, it can lead to poor dispensing or wire drawing, while if it is too low, it can easily flow and cause short circuits.
-Control:
-Use a viscometer (such as a rotary viscometer) to measure regularly, at least once per shift.
-Temperature control: The storage and use temperature of the adhesive should be constant (usually 23 ± 2 ℃) to avoid viscosity fluctuations with temperature.
2. Curing temperature and time
-Example of thermosetting curve:
| Stage | Temperature (℃) | Time (min) | Purpose |
| Preheating | 80-100 | 2-3 | Volatile solvent, avoid boiling and explosion |
| Constant temperature | 150-180 | 5-10 | Fully cross-linked curing |
| Cooling | Natural cooling | – | Reduced to room temperature to stabilize the structure |
3. Glue dispensing/printing accuracy
-Test tool: anime image measuring instrument, regularly spot check the dispensing diameter or printing thickness, and the deviation should be ≤± 10%.
4、 Common defects and solutions
| Defect type | Possible cause | Solution |
| Component offset | Insufficient adhesive coating amount or positional deviation | Adjust dispensing parameters and calibrate the coordinates of the SMT machine |
| Adhesive drawing | Low viscosity or high dispensing pressure | Replace with higher viscosity adhesive to reduce air pressure |
| Bubbles after curing | Rapid curing temperature rise or excessive solvent content in the adhesive | Prolong the preheating stage and use low solvent adhesive |
| Component detachment during welding | Insufficient adhesive strength or insufficient curing | Increase curing temperature/time and replace with high-strength adhesive |
| Short circuit | Adhesive overflow to solder pad | Reduce coating amount, adjust steel mesh opening size |
5、 Quality Control and Standards
1. Industry standard reference
-IPC-7351: Surface mount design and solder pad graphic standard (involving adhesive coating area design).
-J-STD-001: Welding process requirements (indirectly affecting the compatibility of adhesives and welding).
-ISO 9001: Quality management system requirements for process documents and records (such as adhesive batch traceability, process parameter recording).
2. Key points of process control
-First article inspection: Before each batch of production, process parameters need to be verified to confirm the effectiveness of component mounting and curing.
-Process inspection: 5-10 substrates are randomly inspected every 2 hours to check the quality of dispensing, curing status, and welding yield.
-File record: Save adhesive batch number, curing curve, testing data, etc. for at least 2 years for easy traceability.
6、 Process optimization direction
1. Automation upgrade: Introducing visual dispensing machines or high-speed jet dispensing technology to improve the coating accuracy of micro components such as 0201 and Flip Chip.
2. Environmentally friendly materials: Use halogen-free, low VOC (volatile organic compounds) environmentally friendly adhesives that comply with RoHS/REACH and other regulatory requirements.
3. Intelligent monitoring: Real time monitoring of curing furnace temperature, dispensing pressure and other parameters through IoT sensors, automatic alarm and shutdown in case of abnormalities.
By strictly following the above standard processes, the yield and reliability of SMT assembly can be effectively improved, especially in high reliability fields such as automotive electronics and communication equipment. Accurate adhesive processes are the foundation for long-term stable operation of products.
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