What is the SMT red glue process?

Full analysis of SMT red glue process technology: from principle to process optimization

1. Definition and naming traceability

The SMT red glue process is standardly expressed as "SMT Adhesive Process" in industry standards, and its common name "red glue" comes from the material characteristics in the early days of technology development - in the 90s of the 20th century, about 90% of the dispensing materials used epoxy resin systems with iron oxide red pigment (such as Loctite 3611 model), and this iconic red color became the visual symbol of the process. Similar industry names are not uncommon in electronics manufacturing, such as PCB solder masks are simply called "green paint" due to the mainstream green color.

With the development of material technology, modern dispensing materials have broken through the single color limit, and special varieties such as yellow colloids suitable for optical component assembly (such as Panasonic MP8060) and transparent colloids (dielectric constant ε<3.0) that meet the requirements of high-frequency signal transmission have emerged. However, the conventional term "red glue" is still widely used and has become the general term for this type of thermoset epoxy dispensing process in the electronics manufacturing industry.

2. Technical principles and core functional systems

The essence of the red glue process is to realize the temporary fixation of surface mount components (SMDs) and PCB substrates before wave soldering through the physical bonding and chemical curing of thermosetting epoxy resins, and its functional system can be decomposed into three levels:

2.1 Mechanical fixing function

During wave soldering, the impact force generated by the molten solder can reach 0.5N, and the red glue needs to provide sufficient bond strength (>5kgf/cm²) to resist this force. Shear strength data tested by the ASTM D1002 standard shows that the shear strength of qualified red glue stabilizes in the range of 5-8MPa after full curing at 150°C, which is sufficient to hold SMD components (such as SOP-8 packaged integrated circuits) within 10g.

In practical applications, the "3-4-6" principle is adopted: 3 dots for 0402-0603 package components, 4 dots for SOIC packages, and 6 dots for large components such as QFP, and the dots are distributed symmetrically in a center to ensure force balance. Tests at a consumer electronics foundry have shown that this layout keeps component displacement rates below 0.1% during wave soldering.

Process compatibility features

Red glue needs to be compatible with the core equipment of the SMT production line: Printing/dispensing equipment: suitable for both stencil printing (speed 20-50mm/s) and jet dispensing (accuracy ±0.02mm) processes, the viscosity needs to be stable at 150-250kcps (Brookfield DV-II + viscometer, 25°C, 60rpm), reflow oven: supports 120-150°C/90-120s Wave soldering equipment: The cured gel must withstand solder impact at 260°C/10s with a weight loss rate of < 0.5% (TGA thermogravimetric analysis).

Structural support function

Red glue forms a standoff height of 0.05-0.15mm (the gap between the component and the PCB) after curing, this subtle structure has a dual function: Buffer protection: during temperature cycling (-40°C~125°C), absorb thermal stress through the elastic deformation of the colloid (elongation at break 3-5%), reduce the risk of fatigue of the component solder joint, solder flow channel: provide flow space for molten solder during wave soldering, Ensure pad wettability (wetting angle < 30°).

3. Technological evolution and industrial driving factors

The development trajectory of the red glue process maps the transformation process of the electronics manufacturing industry from through-hole cartridge to surface mount, and its technological breakthroughs have distinctive characteristics of the times.

Birth background (early 1990s)

At that time, electronic components were in a hybrid era of "DIP to SMD", with 30-50% of components still in through-hole packaging (DIP), and the market demand for product miniaturization was becoming more and more urgent. Traditional processes face a double dilemma: single-sided boards cannot meet the need for increased I/O density, and the "two-wave soldering" process on both sides results in SMD component drop-off rates of up to 5%

The emergence of red glue technology provides a revolutionary solution, and its three major technological breakthroughs have reshaped the hybrid assembly process: Process compatibility: Achieve gel pre-curing using existing reflow soldering ovens without additional equipment investment, Strength controllability: Precise control of bond strength through curing parameter adjustment (temperature/time), double-sided assembly feasibility: First "SMD red glue fixation on one side + DIP on the other side" Cartridge" hybrid process increases PCB utilization by 100%.

Technology Iteration (2000-2010)

With the wave of lead-free (implemented by the EU RoHS Directive in 2006) and the trend of component miniaturization, red glue technology has been upgraded: the development of silicon-modified epoxy resin has increased the short-term temperature resistance from 220°C to 260°C, adapting to the high-temperature requirements of lead-free solder, and improving the viscosity control accuracy from ±50kcps to ±20kcps, supporting 0.3mm dot printingNanoscale fillers such as SiO₂ are introduced to increase strength retention from 30% to 60% at 85°C/85% RH (500-hour test).

Current status and challenges (2010–present)

At present, red glue technology has formed a complete system, but it is facing the impact of SMD full surface mounting. Its living space focuses on two dimensions: in high-volume products such as consumer electronics, the red glue process can reduce costs by 15-20% compared with selective wave soldering, and is still irreplaceable in the fixation of oversized components (such as power modules above 50×50mm), according to Henkel's 2022 industry report, the global annual consumption of red glue is stable at about 1,500 tons. Among them, the Chinese market accounts for 45%, mainly used in set-top boxes, small household appliances and other fields.

4. Material performance indicators and testing system

The performance verification of red rubber materials requires the establishment of a multi-dimensional testing system, and the control of key indicators directly determines the process reliability.

4.1 Core Performance Parameters

The batch inspection data of a leading red rubber manufacturer shows that the high-end model can achieve a viscosity deviation of <±15kcps and a shear strength stability of < 5%, which is much higher than the industry average.

Reliability verification standards

The long-term performance of red glue needs to be evaluated by the accelerated aging test: damp heat aging: 85°C/85% RH storage for 500 hours, strength attenuation should be ≤ 40% (retention ≥3MPa), temperature cycle: -40°C~125°C, after 1000 cycles, the adhesive layer has no cracks, strength retention rate ≥ 60%, solder resistance: 260°C immersion in molten solder for 10s, After taking it out, it is cooled to room temperature, and there is no bubbling or peeling off in appearance, especially warning: the molecular structure of red rubber determines that it is not suitable as a permanent crystal bonding material. Comparative tests show that under the same aging conditions, the strength decay rate of red glue is 3 times that of conductive silver glue, so it is strictly prohibited to use it in places with high long-term reliability requirements, such as LEDs and sensors.

5. Application boundaries and process restrictions

The scope of application of the red glue process is restricted by both material properties and process principles, forming a clear application boundary.

Component type restrictions

Practical data shows that the following component types are not suitable for the red glue process: when the pad spacing is < 0.4mm, the glue dots are easy to contaminate the pad and cause bridging, the defect rate of 0.3mm pitch components can reach 8%, QFN, LGA, etc. have a large area of pad packaging at the bottom, red glue will hinder the wetting of the solder, the void rate of the solder joints is > 30%, the glue is easy to contaminate the pins/contacts, As a result, the contact resistance increases (>100mΩ), and MLCC ceramic capacitors have a 5-fold increased risk of cracking during temperature cycling when the amount of glue exceeds the component height by 30%

5.2 Package Size Limits

The adaptability of component size to the red glue process shows significant differences: the industry consensus is that the 0603 package (1.6×0.8mm) has a bridge defect rate of up to 12% after wave soldering, and the 0402 package (1.0×0.5mm) has a bridging defect rate of up to 12%, with 25g being the critical point 20mm×20mm QFP, the yield of the red glue process in this range can reach more than 99.5%

Circuit type restrictions

In specific circuit designs, red glue may affect electrical performance: the dielectric constant of ordinary red glue ε=4.0-5.0, which will lead to an increase in signal transmission loss of more than 1dB/cm in the frequency band above 5GHz, and when the glue point is close to sensitive nodes, it may introduce additional parasitic capacitance (>0.5pF), affecting circuit performance, and the thermal conductivity of red glue (0.3-0.5W/m). ・K) is only 1/100 of the solder, which can hinder the heat dissipation path

6. Modern alternative technologies and competition pattern

With the comprehensive development of SMT technology, the red glue process is gradually being replaced by more advanced processes, forming a diversified technology selection matrix.

6.1 Comparison of mainstream alternatives

Process switching data from an automotive electronics manufacturer shows that after switching from red glue process to selective wave soldering, the product defect rate decreased from 1500ppm to 300ppm despite increased equipment investment, reducing overall quality cost by 40%.

The living space of the red glue process

In the following scenarios, the red glue process is still irreplaceable: such as 100 yuan smart speakers, set-top boxes, etc., the red glue process can reduce the manufacturing cost of a single unit by 0.5-1 yuan, there is no need to modify the wave soldering program when switching products, and the changeover time is shortened by 50%

7. Process optimization and quality control

The quality of the red glue process depends on the collaborative optimization of materials, equipment and parameters, and the establishment of a systematic process control system is the key.

Glue design optimization

The scientific design of the dot parameters can significantly improve reliability: the diameter of the dot should be 60-80% of the width of the component, for example, a 1.6mm wide 0603 component corresponds to a 1.0-1.3mm dot, and the height of the dot after curing should be 20-30% of the height of the component, which not only ensures strength but also avoids interference, following the "weight - dot" matching principle - 2 dots for components under 1g, 4 dots for 1-5g components, 6-8 dots for 5-25g components, and the recommended dot layout for a PCB design guide: diagonal distribution for rectangular components, square components with four corners, and equilateral triangles for round components.

7.2 Curing process optimization

Curing is the core link that determines the performance of red glue, which needs to be precisely controlled by the temperature curve:

a. Heating section: room temperature to 120°C, rate 3-5°C/s (avoid glue dripping).

b. Constant temperature section: 120-150°C for 90-120s (ensure complete curing).

c. Cooling section: natural cooling below 60°C (reducing internal stress).

Verification of curing effect: Quickly evaluated by thrust test (1.5kgf / component), the qualification criteria are that the component falls off rather than the adhesive layer breaks, and there is ≥80% of the adhesive layer residue on the fracture surface.

Defect analysis and prevention

Root causes and countermeasures of common red glue defects: Due to low viscosity or too fast dispensing speed, the solution includes increasing the viscosity to 200-250kcps and reducing the dispensing speed to less than 20mm/s, which is manifested as stickiness of the adhesive layer in the thrust test, and the temperature zone of the reflow soldering oven (the deviation between the actual temperature and the set value should be <±5°C), and the deviation of the glue position is > 0.1mm It can be effectively prevented by the visual positioning system (accuracy ±0.01mm), due to the large glue dots, and the risk of contamination can be reduced by more than 90% through the optimization of the mesh plate opening (5-10μm smaller than the theoretical value), and the regular SEM (scanning electron microscopy) inspection of the colloidal section structure can detect potential defects such as microscopic cracks in advance, increasing the early troubleshooting rate by 40%.

epilogue

As a landmark technology in the transition period of the electronics manufacturing industry, the red rubber process still plays an irreplaceable role in specific fields despite the impact of modern technology. Its technological development process reveals an important law: the value of manufacturing technology depends not only on technological advancement, but also on its adaptability to the industrial stage. For enterprises still using the red glue process, it is recommended to adopt the strategy of "precise application + continuous optimization": by strictly defining the scope of application and establishing a refined process control system, it can give full play to its cost advantages under the premise of ensuring quality. With the continuous advancement of electronic manufacturing technology, the red glue process may gradually withdraw from the mainstream stage, but its historical value and technical enlightenment in the upgrading of the electronics industry will exist for a long time.