Materials used in surface-mount assembly include SMT adhesive, flux, solder, solder paste, and cleaning agents. These materials have a significant effect on the quality and reliability of electronic products manufactured with surface-mount technology.
SMT adhesive is used to secure surface-mount devices (SMDs) to the underside of a PCB during wave soldering. It prevents components from shifting, misaligning, or falling off under the impact and vibration of the solder wave. As a result, adhesive application is an important step in wave-soldering processes.
Components commonly bonded to the underside of a PCB include rectangular chip resistors and capacitors, cylindrical MELF devices, and small-outline semiconductor packages. These components are often wave-soldered together with through-hole technology (THT) components.
The typical adhesive-dispensing and wave-soldering process is shown below.
Figure 1 | Typical SMT adhesive dispensing and wave-soldering process
1. Functions of SMT Adhesive
SMT adhesive is generally used when wave soldering is applied to a surface-mount assembly. Its primary function is to hold components in their designated positions. During subsequent processes, including wave soldering, components are exposed to mechanical forces such as solder impact and vibration. The adhesive prevents movement, misalignment, or component loss so that each device can be soldered accurately to the intended PCB location.
SMT adhesive is mainly associated with wave soldering. It can be used for single-sided assembly, mixed THT and SMT assembly, and double-sided assemblies that combine through-hole and surface-mount components. It is also applicable to double-sided surface-mount assemblies processed using combinations of wave soldering and reflow soldering.
Figure 2 | SMT adhesive used to retain components during downstream soldering
2. Required Properties of SMT Adhesive
SMT adhesive is typically a thermosetting or light-curable paste adhesive. A suitable material should provide the following properties.
2.1 Stable Single-Component System
Single-component SMT adhesives are widely used in surface-mount assembly. They eliminate the need to mix multiple components before use, reduce preparation time, and simplify process operation. A stable formulation also helps maintain consistent product quality.
2.2 Appropriate Viscosity
The adhesive viscosity must be compatible with needle transfer, pressure dispensing, screen printing, and other application methods. It must also meet the requirements of different dispensing machines and process temperatures.
If the viscosity is too high, stringing may occur during dispensing, and adhesive may contaminate solder pads and reduce solderability. If the viscosity is too low, the adhesive may not provide sufficient component retention. Components may shift, fall off, or be lost before soldering, reducing assembly quality.
2.3 Good Thixotropy
SMT adhesive should have good thixotropic behavior. A typical thixotropic index is approximately 3–6. After dispensing, the adhesive dot should retain its intended shape on the PCB rather than collapsing. It should also remain stable during component placement and curing without flowing into adjacent areas.
Good shape retention helps prevent insufficient solder joints and missed solder connections caused by adhesive contamination of the pads.
2.4 Easily Identifiable Color
The adhesive should have a distinctive color so that operators and inspection systems can identify missing adhesive, missing components, incomplete soldering, adhesive flow, or component contamination through visual inspection. Red, yellow, and white are commonly used because they provide good visual contrast.
2.5 Rapid Curing
Thermally cured adhesive should cure as quickly as possible at the lowest practical temperature. This helps reduce PCB warpage and component damage, limits pad oxidation, lowers energy consumption, shortens process time, and improves throughput. For example, some materials can cure at 150 °C in less than three minutes.
2.6 Storage Stability
A single-component adhesive must provide not only rapid curing but also stable storage performance and a sufficiently long shelf life. Under typical conditions, storage may be approximately one to one and a half months at room temperature and three to twelve months at 2–10 °C, depending on the material formulation and storage conditions.
2.7 Moderate Bond Strength
After curing, the adhesive bond must be neither excessively strong nor too weak. Before wave soldering, it must firmly hold the SMDs in place. After wave soldering, its bond strength should be low enough to permit replacement of defective components during repair.
A typical shear strength range for SMT adhesive is approximately 6–10 MPa. This value is a reference range rather than a universal requirement; the appropriate value depends on the material and application.
2.8 High-Temperature Resistance
Cured adhesive must withstand short periods of high temperature. During wave soldering, it may be exposed to approximately 250–270 °C for around 15 seconds. At these temperatures, the adhesive should not decompose or lose sufficient strength to allow components to detach.
2.9 Good Electrical Performance
Cured adhesive should have a low dielectric constant, high volume resistivity, and high surface insulation resistance. Typical requirements may include a dielectric loss factor of tan δ < 0.02 at 1 MHz, volume resistivity of at least 1 × 1013 Ω·cm, and surface insulation resistance of at least 1 × 1010 Ω.
Figure 3 | Typical electrical and thermal characteristics of cured SMT adhesive
2.10 Chemical Stability
The adhesive should not release gas or form voids during curing in a manner that affects electrical performance. It should not corrode components or the PCB, and it should not chemically react with flux, solder mask, or cleaning agents used in the assembly process.
2.11 Environmental and Handling Safety
SMT adhesive should be non-toxic, low-odor, and compliant with RoHS requirements. It should have low volatility and present minimal risk to personnel and the environment when handled correctly. Some applications may also require flame-retardant properties.
3. Main Performance Parameters
SMT adhesive performance parameters are used to evaluate material quality and process suitability.
3.1 Viscosity
Viscosity is one of the most important parameters because it directly affects dispensing behavior and flow. Different application methods and component types require different viscosity ranges. Viscosity is commonly measured with a rotational viscometer. Each specimen is measured three times, and the average value is reported, typically in Pa·s.
3.2 Thixotropic Index
Thixotropy describes the reduction in fluid shear stress over time when a material is subjected to a constant shear rate. SMT adhesive commonly uses the thixotropic index, Ti, to characterize this behavior.
Under standard room-temperature conditions, the thixotropic index is generally defined as the ratio of viscosity at low rotational speed to viscosity at high rotational speed. A common definition is:
Ti = V2 rpm / V20 rpm
The measurement is performed using a rotational viscometer.
3.3 Cure Time
Cure time is the period required for the adhesive to undergo the necessary chemical reaction and develop its specified bonding strength and related properties.
3.4 Tensile Shear Strength
For this test, a single-lap specimen is subjected to a longitudinal tensile shear load across the overlap area. The maximum load sustained by the specimen is used to calculate the average shear stress at the bonded interface.
The tensile shear strength is calculated as:
τ = P / (B × L)
- τ — tensile shear strength, MPa
- P — maximum load at shear failure, N
- B — width of the overlap area, mm
- L — length of the overlap area, mm
3.5 Bond Strength
Bond strength is the stress required to cause failure at the interface between the adhesive and the adherend, or in a region close to that interface.
3.6 Shelf Life
Shelf life is the period during which SMT adhesive can maintain its specified processing characteristics and bond strength under defined storage conditions.
3.7 Volume Resistivity
Volume resistivity is the ratio of the direct-current electric field strength inside an insulating material to the steady-state current density. It represents the resistance of a unit volume of the material and can be calculated as:
ρv = Rv × A / h
- ρv — volume resistivity, Ω·cm
- Rv — measured volume resistance of the specimen, Ω
- A — effective area of the measuring electrode, cm2
- h — average specimen thickness
Typical adhesive are shown in the following reference image.
Figure 4 | Reference performance parameters for typical SMT adhesive
4. Types of SMT Adhesive
SMT adhesive can be classified according to its bonding material, curing method, or application method.
4.1 Classification by Bonding Material
The two main material categories are epoxy-based and acrylic-based adhesives. Modified epoxy resins and polyurethane systems are also used.
Epoxy-based SMT adhesives provide good bond strength, chemical stability, and high-temperature resistance. They are among the most commonly used adhesives in SMT assembly.
Acrylic-based SMT adhesives generally cure faster, but their high-temperature resistance is relatively lower. They are suitable for applications with less demanding temperature requirements.
4.2 Classification by Curing Method
Based on curing method, SMT adhesives include intermittent oven-cured materials, continuous tunnel-oven-cured materials, light-curable materials, dual light-and-heat-curable materials, and ultrasonic-curable materials.
4.3 Classification by Application Method
Application methods include needle transfer, pressure injection or dispensing, screen printing, stencil printing, and high-speed jetting.
Figure 5 | Examples of SMT adhesive application methods
5. Composition of SMT Adhesive
5.1 Composition of Thermosetting Adhesive
Single-component thermosetting SMT adhesive commonly contains the following materials:
- Bonding resin: Bisphenol A epoxy resins such as E-55, E-51, and E-44, together with other modified epoxy resins.
- Curing agent: Single-component adhesives generally use latent curing agents.
Common curing-agent systems include:
- Boron trifluoride-amine complexes
- Imidazole salts and imidazole-metal salt complexes
- Complexes of aromatic amines and inorganic salts
- Dihydrazide compounds
- Dicyandiamide
- Encapsulated or microcapsule curing agents
3. Curing accelerator: Some curing agents, including dicyandiamide, require high temperatures and have relatively slow curing rates. Curing accelerators are therefore added to lower the reaction temperature and increase reaction activity. Common accelerators include organic urea compounds, imidazole and its derivatives, and phosphorus-containing compounds.
4. Solvent: Solvents help disperse the bonding resin, curing agent, and accelerator uniformly, while also allowing viscosity adjustment. Alcohols, ketones, and esters are commonly used.
5. Filler: Fillers can provide several specific properties:
- Improved electrical insulation and high-temperature performance
- Appropriate shear strength
- Viscosity adjustment and improved thixotropy
- Color adjustment
Common fillers include silica powder, fumed silica, titanium dioxide, talc, zinc oxide, ferric oxide, carbon black, and cadmium yellow.
5.2 Composition of Light-Curable Adhesive
Light-curable adhesives include visible-light-cured, ultraviolet-cured, and electron-beam-cured systems. Visible-light materials use fewer raw materials and generally require longer curing times. Electron-beam curing requires expensive irradiation equipment, which limits its use. Ultraviolet curing is therefore the principal light-curing method used for SMT adhesive.
UV-curable SMT adhesive commonly contains the following components:
- Photosensitive resin: Common resin families include polyether, epoxy, polyurethane, and acrylic resins. Acrylic resins are widely used because they cure rapidly and provide good chemical resistance and bond strength.
- Photopolymerizable monomer: Also called a reactive diluent, this component reduces viscosity, dissolves the photosensitive resin, and improves processing characteristics. Styrene and vinyltoluene are examples.
- Photoinitiator: Photoinitiators trigger polymerization under light exposure. Several chemical systems are available, including butadiene-, benzophenone-, and bromoacetophenone-based materials.
- Other additives: Depending on the required properties, additives may include plasticizers, tackifiers, antioxidants, thixotropic agents, and fillers.
Figure 6 | Representative composition of a light-curable SMT adhesive
6. SMT Adhesive Application Methods
The main application methods are needle transfer, pressure dispensing, screen printing, stencil printing, and high-speed jetting.
6.1 Needle Transfer
In needle transfer, the needle is first positioned over a container of adhesive and immersed in the material. When the needle is lifted, surface tension causes adhesive to remain on the needle. The needle is then moved above the PCB and aligned with the target location. It is lowered close to the board while maintaining a controlled gap. When the needle is raised, the adhesive affinity of the PCB surface is greater than the adhesive affinity of the metal needle, so part of the adhesive transfers to the PCB.
In practical applications, needle transfer often uses a matrix arrangement so that multiple adhesive dots can be applied simultaneously.
Needle transfer is widely used in manual dispensing and can also be performed with automated equipment. Its performance depends strongly on adhesive viscosity and PCB warpage. The method can apply adhesive to many component locations in one operation and requires relatively low equipment investment. However, the adhesive volume is difficult to control precisely, and contaminants can easily enter the open adhesive reservoir.
6.2 Pressure Dispensing
Pressure dispensing, also called injection dispensing or dispenser application, is one of the most widely used methods. Adhesive is loaded into a syringe, and compressed air or a mechanical rotary pump applies pressure to force the material through the needle onto the required PCB locations.
Air pressure, temperature, and dispensing time are key process parameters. In addition to controlling inlet pressure and dispensing duration, the adhesive reservoir may include temperature control to maintain stable viscosity. These parameters determine adhesive volume, dot size, and dot shape.
High viscosity can cause stringing, while low viscosity can produce excessive adhesive volume or leakage. Correct adjustment of pressure and dispensing time helps reduce both stringing and excess adhesive. Computer control can also be used to improve dispensing-volume and position accuracy through programmed operation.
Pressure dispensing is highly adaptable, particularly for mixed-product manufacturing. The adhesive volume can be adjusted easily for components of different sizes, while the sealed syringe environment helps maintain material stability.
6.3 Screen Printing
For screen printing, nylon or stainless-steel mesh is coated with photosensitive emulsion. A photographic exposure and development process creates the required openings in the mesh. Adhesive passes through these openings and is printed onto the corresponding PCB locations.
Screen printing is suitable for low-density adhesive patterns and small- to medium-volume production. The screen is relatively simple and inexpensive to manufacture, and the process is easy to operate. Its disadvantages include difficult alignment, a tendency to clog, the need for low-viscosity adhesive, limited screen life, difficult cleaning, and less consistent print quality.
6.4 Stencil Printing
Stencil printing is similar to screen printing, except that a metal or other solid stencil is used instead of mesh. Stencil printing avoids many of the limitations of screen printing. It is less prone to clogging, has a longer service life, is easier to clean, and provides better print quality. For these reasons, it has become a mainstream method for printing SMT adhesive.
6.5 High-Speed Jetting
High-speed adhesive jetting provides fast and flexible dispensing and can apply several types of adhesive. Because the process does not require a positioning needle to contact the board, adhesive can be deposited at high speed.
A miniature screw delivers a controlled quantity of adhesive into a sealed pressure chamber. A plunger then expels a measured microdroplet, which is jetted at high speed between the solder pads. The jetting head operates according to a programmed path, producing uniform dots at rates of up to 200 dots per second.
7. Curing SMT Adhesive
After adhesive is applied and components are placed, curing should be performed as soon as practical. The most common curing methods are thermal curing and combined UV/thermal curing.
7.1 Thermal Curing
Thermal curing may use an intermittent batch oven or a continuous infrared tunnel oven.
For batch-oven curing, a specified number of adhesive-coated PCBs with placed components are loaded onto racks and placed together in a temperature-controlled oven. A setting of approximately 150 °C is generally used to limit damage to the PCB and components. Cure time may range from 20–30 minutes and can sometimes be reduced to approximately five minutes.
The oven should include forced-air circulation to prevent temperature differences between the upper and lower layers and to maintain a uniform temperature. Batch curing is simple to operate, but it consumes more thermal energy, requires more time, and is less suitable for continuous production lines.
Infrared curing, also called tunnel-oven curing, has become a common method for SMT adhesive. The equipment may be based on an infrared reflow system. Because SMT adhesive can strongly absorb certain infrared wavelengths, curing can be completed in a relatively short time. This method offers high thermal efficiency and is well suited to continuous production. The temperature profile must be selected according to both the adhesive properties and the infrared oven configuration.
7.2 UV and Thermal Dual Curing
UV/thermal dual-curing systems use ultraviolet exposure and heating together to cure adhesive rapidly in continuous production. When using this type of adhesive, the adhesive dot must extend beyond the underside of the component. UV light can cure the exposed adhesive around the outside of the component, while the central region beneath the component is cured thermally. This arrangement ensures that the entire adhesive volume is cured.
A typical UV wavelength is 365 nm, with a power density of 80 W/cm. The thermal curing temperature may reach 150–180 °C. UV exposure may require 10–30 seconds, while the total UV/thermal curing time can be less than three minutes.
The main advantages of UV/thermal curing are high speed and low energy consumption, making it suitable for high-volume production. However, high-intensity UV exposure can generate ozone from oxygen in the air. Adequate ventilation is therefore required to protect personnel.
8. Important Handling and Process Considerations
- Store SMT adhesive in a sealed container at 2–10 °C.
- After removing the adhesive from refrigeration, allow it to reach room temperature before opening the container. This helps prevent condensation, frosting, and moisture absorption.
- After use, reseal any remaining adhesive in its original container and return it to low-temperature storage.
- Control the adhesive volume carefully. Too little adhesive may produce insufficient bond strength and cause component loss during wave soldering. Too much adhesive may flow onto solder pads, interfere with soldering, and complicate repair.
- Operators should use appropriate personal protection and avoid contact with skin and eyes. If contact occurs, rinse immediately with clean water and seek medical attention.
9. Common Dispensing Defects and Corrective Actions
9.1 Stringing or Tailing
Stringing or tailing is a common defect in dispensing. When the needle moves away from the PCB, a thin filament or tail may remain on top of the adhesive dot. The tail can collapse onto a solder pad, contaminate it, and cause an insufficient solder joint.
Possible causes include an excessively small needle inner diameter, excessive dispensing pressure, or excessive distance between the needle and PCB. Other causes include incompatibility between the adhesive and dispensing process, inadequate material quality, viscosity change, or expired adhesive. Electrostatic discharge, PCB warpage, and insufficient board support can also contribute to the defect.
Corrective actions include adjusting process parameters, using a needle with a larger inner diameter, reducing dispensing pressure, and optimizing needle height. The adhesive manufacturing date, material properties, and process requirements should also be checked. If the adhesive has deteriorated, it should be replaced.
Local heating at or near the dispensing needle can also reduce viscosity and help the adhesive separate cleanly, thereby reducing stringing and tailing.
9.2 Satellite Dots
Satellite dots are small, unintended adhesive droplets generated during high-speed dispensing. In contact dispensing, they are commonly caused by tailing followed by separation at the needle tip. In noncontact jetting, an incorrect jetting height can produce the defect. Because satellite dots appear irregularly, they may contaminate solder pads or reduce local bond strength.
For contact dispensing, inspect the needle regularly, prevent needle damage, and adjust equipment parameters to reduce tailing. For noncontact jetting, optimize the height between the jetting head and PCB. The result also depends on the positioning accuracy of the dispensing equipment, software control, and height-sensor design.
9.3 Popcorning and Voids
Voids or popcorning after curing occur when air or moisture enters the adhesive and is suddenly released during curing. These defects reduce bond strength and may create a path for molten solder to enter beneath the component, potentially causing bridging or short circuits.
Adhesive stored in a syringe generally contains little moisture, but prolonged exposure to the production environment, especially under humid conditions, can lead to moisture absorption. A long delay between dispensing and curing increases this risk. Needle-transfer dispensing exposes a relatively large adhesive area, making moisture absorption more likely. Air may also be introduced by incorrect adhesive loading, particularly when syringes are filled manually.
Possible corrective actions include low-temperature, slow curing; extending the heating period so moisture can escape before the adhesive gels; shortening the time between placement and curing; using a low-moisture-absorption adhesive; and pre-drying the material. Storage, handling, and use procedures must be followed strictly. Manually filled adhesive should be degassed before use.
9.4 Empty Shots or Insufficient Adhesive Volume
If the dispenser actuates but produces no adhesive, or produces too little adhesive, the usual causes are air bubbles in the material, a blocked needle, or insufficient line pressure.
Adhesive in the syringe should be degassed, especially when the syringe has been filled manually. Clean needles should be installed regularly, and machine pressure should be adjusted appropriately. If clogging occurs frequently, another adhesive material may need to be evaluated.
9.5 Discontinuous Adhesive Dots
Inconsistent dot size causes variation in the bond strength between components and the PCB. Possible causes include the needle tip landing on a solder pad, insufficient recovery time, or insufficient pressure and dispensing time as the adhesive level in the reservoir decreases.
Changing the needle can correct interference from the needle tip. Increasing the delay time can provide sufficient recovery. Adjusting the ratio between pressure and cycle time can compensate for changes in adhesive volume and restore dot-size consistency.
9.6 Component Displacement
Component displacement after curing can move leads away from their pads and cause open circuits. Causes include insufficient adhesive volume, low initial tack, and excessive waiting time between dispensing and component placement. Excessive adhesive volume can also displace components.
First check whether the adhesive dots are uniform. Then adjust the placement-machine operating condition and optimize placement height. The adhesive may also need to be changed. The process documentation should define a maximum waiting time between dispensing and placement; a typical limit is no more than four hours.
9.7 Component Loss After Wave Soldering
Components may detach after curing or after wave soldering, and in some cases may fall off when touched manually. A common cause is an inadequate curing profile, particularly insufficient temperature. For thermally cured adhesive, failure to reach the required peak temperature can result in component loss. For light-curable adhesive, UV lamp aging can reduce the available curing intensity. Insufficient adhesive volume and contamination on the component or PCB can also cause detachment.
Recheck the PCB curing profile, with particular attention to curing temperature. Adjust the profile as required. For UV curing, inspect the lamp for aging and check whether the tube has darkened. The adhesive dot diameter and height should also be verified, along with the cleanliness of the components and PCB.
9.8 Floating Leads or Component Shift After Curing
If component leads lift after curing, solder may enter unintended areas during wave soldering. Severe cases can result in shorts or opens. The primary cause is excessive adhesive volume, which can shift the component during placement.
Adjust dispensing parameters to control adhesive volume, and optimize placement parameters so that components remain accurately aligned.
10. Selecting SMT Adhesive
The appropriate adhesive type depends on the factory equipment, component geometry, curing capability, and production requirements.
Epoxy-based thermally cured adhesive generally requires only an infrared reflow oven for curing. The same oven can be used for solder-paste reflow and adhesive curing, so a dedicated UV lamp is not required. A low-temperature storage cabinet is normally sufficient, and its storage requirements are compatible with those of solder paste. Because epoxy adhesive is thermally cured, it does not suffer from UV shadowing and can be used with components of different shapes without requiring a special adhesive-dot location.
Acrylic-based light-curable adhesive requires a UV lamp but may not require a refrigerated storage cabinet. It generally offers stable performance and fast curing. However, adhesive-dot placement is more restrictive. The dots should be located around the outer perimeter of the component; otherwise, the adhesive may not receive sufficient UV exposure and the final bond strength may be reduced. Acrylic adhesive also generally provides lower bond strength than epoxy adhesive.
