SMT Red Adhesive Process and Mounter Belt Maintenance

The SMT red adhesive process uses chip adhesive to hold surface mount components on the PCB before wave soldering. The adhesive is often red, which makes it easy for the operator and AOI system to see where it was applied. Red adhesive is different from solder paste and requires a different stencil design.

Solder paste is printed over the pads so that it can form the solder joint. Red adhesive is placed between the pads, under the component body, so that it bonds the component to the board without interfering with the solder connection.

This guide explains the stencil and layout requirements for red adhesive, the wave soldering design rules, and the maintenance needed for SMT machine belts.

Red Adhesive Stencil Versus Solder Paste Stencil

The opening design is the main difference between a red adhesive stencil and a solder paste stencil. Solder paste openings are aligned with the original pad areas so that paste can be printed on every pad.

Red adhesive openings are placed in the center between two pads, where the component body will sit. The opening shape can be extended into a double-point or bone shape to create a stronger adhesive dot.

The stencil thickness and opening size must be matched to the adhesive and the component. A red adhesive stencil is usually 0.18, 0.20, or 0.25 millimeters thick, depending on the component size and required dot height.

For IC components, the adhesive opening width is often about half the width between the two pad rows, and several small round holes can be opened for a stronger bond.

Red Adhesive Component Layout

The layout of components on a board using red adhesive must follow the wave soldering process. The long axis of a chip assembly should be perpendicular to the transmission direction of the wave soldering machine.

The long axis of an IC should be parallel to the transmission direction. This orientation reduces the shadow effect that can prevent the solder wave from reaching the rear pads.

Components of the same size should be arranged so their ends are parallel with the direction of the solder wave. Components of different sizes should be staggered to avoid blocking the wave.

Small parts should be placed in front of large parts so that the larger body does not shadow the small component terminals. When this arrangement is not possible, the components should have a gap of 3 to 5 millimeters.

Component Polarity and Direction

The feature directions of components should be consistent across the board. Electrolytic capacitor polarity, diode anodes, and the single-pin end of transistors should be oriented in the same way.

IC pin one and other polarity markings should also follow a clear direction. Consistent orientation helps the placement machine, inspector, and repair operator identify an incorrect part quickly.

The layout drawing should include an arrow or direction mark for every polar component. This reduces the chance of a reversed component passing through inspection.

Adhesive Opening and Pad Design

Assembly holes and pads should be arranged on a basic grid, half-grid, or quarter-grid so that the design is compatible with standard manufacturing and placement equipment.

The clearance between a plated through-hole and the lead diameter of an inserted component should allow solder to wet the lead and fill the barrel. Too little clearance prevents the solder from reaching the joint, while too much clearance creates a weak connection.

For high-density component wiring, an oval grounding pad design can reduce solder bridging. The shape of the pad and adhesive opening should be reviewed together during DFM.

Component Heat Resistance

Components used in the wave soldering process must withstand repeated temperature exposure. A surface mount component body and its solder points should survive more than two soldering passes at a wave temperature near 260 degrees Celsius.

After soldering, the component body should not be damaged or deformed, and the ends of chip assemblies should not be coated by solder in an uncontrolled way.

The PCB substrate should also withstand the required heat resistance, commonly 260 degrees Celsius for about 50 seconds. The copper foil must have good peel strength.

The solder mask should keep enough adhesion at high temperature and should not wrinkle after soldering.

Adhesive Curing Before Wave Soldering

The red adhesive must be cured before the board passes over the solder wave. The curing process should be verified with a test board so that the adhesive reaches full bond strength.

If the adhesive is not fully cured, the wave can move the component or wash it off the board. The adhesive dot height should be controlled so that it holds the part without spreading onto the pads.

Dispensing and printing parameters should be recorded with the product program so the same setup can be used for the next production lot.

SMT Machine Belt Classification

SMT machines use many different drive belts. Common examples include the X-axis belt, Y-axis belt, Z-axis belt, R-axis synchronous belt, input board drive belt, and conveyor belts for various placement machines.

The belt type depends on the machine model and the axis it drives. Maintenance staff should identify the correct part number before replacing a belt so that the machine calibration is not affected.

Belts are available in different thicknesses, such as 0.6, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.5, and 2.0 millimeters. The wrong thickness can change the tension and movement accuracy.

How to Replace an SMT Machine Belt

The power should be turned off and the protective cover removed before belt maintenance. The operator should loosen the mounting bolts on the motor base so the motor can move forward or backward.

The motor is then moved to loosen the belt enough for removal. Tools should not be used to pry the belt from the pulley because they can damage the belt or the pulley groove.

After the old belt is removed, it should be checked for abnormal wear. Excessive wear may indicate a transmission design problem, a misaligned pulley, or an incorrect maintenance interval.

The new belt should be installed only after the pulleys and surrounding area have been cleaned.

Clean Belts and Pulleys

Belt and pulley surfaces should be cleaned with a cloth and a small amount of nonvolatile liquid. The belt should not be soaked in detergent or rubbed with strong cleaning agents.

Sandpaper and sharp tools should not be used on the belt because they can scratch the rubber or create particles that enter the drive system.

The belt must be completely dry before it is installed. A damp belt can slip or collect dust during the next production run.

The pulley groove should be checked for wear at the same time. A worn pulley should be replaced with the belt.

Check Other Drive Components

The remaining drive components, such as bearings and bushings, should be inspected during belt replacement. Bearings should turn smoothly without noise or excessive play.

The drive components should have the correct lubrication and should be replaced when they show wear. A worn bearing can create vibration that changes the placement accuracy of the machine.

After reassembly, the machine should be run slowly and checked for belt tracking, tension, and abnormal noise before full production is resumed.

ESD Requirements for Conveyor Belts

SMT machine belts have a high demand for antistatic properties. During electronic product manufacturing, electrostatic discharge can change the characteristics of semiconductor devices or damage them completely.

The conveyor belt should be made from or treated with an antistatic material so that boards and components do not accumulate static charge as they move through the machine.

Belt grounding should be checked regularly. A belt that has lost its antistatic property should be replaced before it creates a field failure in sensitive products.

A professional SMT PCB assembly service should maintain machine belts as part of its quality management program. Belt replacement should be recorded with the equipment history.

Red adhesive and wave soldering are commonly used with mixed technology PCB assembly, where SMT parts and through-hole parts are soldered together. The board layout should be reviewed by the PCB design and layout team before release.

The complete assembly should pass PCBA testing after wave soldering to confirm that all adhesive-bonded components and solder joints work correctly.

The maintenance schedule should also include belt tension measurement after installation. A belt that is too tight can overload the motor and bearing, while a belt that is too loose can slip and change the axis position. The machine manual should be followed when the final tension is adjusted.

Before a red adhesive product enters production, the first article should be checked for adhesive dot position, height, and curing condition. The wave soldering process should then be verified on a sample board. The first article record protects the production lot from a repeat layout or stencil error.

Conclusion

SMT red adhesive process control requires the correct stencil, component layout, heat-resistant parts, and adhesive cure. The adhesive must hold the part without blocking the solder connection.

Machine belt maintenance also protects SMT quality. Belts should be inspected, cleaned, and replaced according to the equipment specification, and ESD requirements should be followed.

With correct process design and equipment maintenance, red adhesive wave soldering can produce reliable mixed-technology PCBA boards.