Aqueous cleaning fluid is widely used for cleaning SMT solder paste stencils. However, if residue is not completely removed, it can affect stencil precision, cause printing defects, and even corrode the metal. Complete residue removal requires a multi-step, coordinated closed-loop control system, combining physical rinsing, chemical neutralization, drying, and process optimization.
Physical rinsing is a fundamental step in removing aqueous cleaning fluid residue. After cleaning, the stencil should be immediately sprayed with high-pressure deionized water, with the water flow perpendicular to the stencil openings to ensure complete drainage of the cleaning fluid from the pores. For fine-pitch stencils (pore size ≤0.3mm), a rotating nozzle or ultrasonic-assisted rinsing can be used. High-frequency vibrations disrupt the adhesion between the cleaning fluid and the stencil surface, preventing residue from crystallizing within the micropores. The water pressure should be maintained between 0.2-0.5 MPa during rinsing. Too low a pressure will not completely remove residue, while too high a pressure may damage the stencil surface coating.
Chemical neutralization eliminates the active ingredients in the aqueous cleaning fluid. Some aqueous cleaning fluids contain weakly alkaline or acidic additives. Direct rinsing may leave residual chemicals, requiring neutralization to reduce their corrosive properties. For example, alkaline cleaning fluid residue can be treated with a dilute acetic or citric acid solution for a short soak (≤2 minutes) and then immediately rinsed with deionized water. Acidic cleaning fluid residue can be treated with a sodium bicarbonate solution. The neutralization process requires strict control of the solution concentration to prevent excess neutralizer from forming secondary deposits on the stencil surface.
Drying is critical to prevent solidification of residues. After rinsing, the stencil should be dried immediately, preferably in a hot air circulation drying oven at a temperature between 60-80°C for no more than 15 minutes. If air drying is used, the stencil should be tilted to allow gravity to drain residual water droplets through the openings while preventing dust from adhering. For high-precision stencils, after drying, wipe them with a dust-free cloth dipped in a small amount of isopropyl alcohol to further remove fine residual particles. However, ensure that the isopropyl alcohol has completely evaporated before use.
Process optimization begins with adjusting cleaning parameters. The concentration, temperature, and cleaning time of the aqueous cleaning fluid directly affect the amount of carryover. Excessively high concentrations increase the viscosity of the cleaning fluid, making it difficult to thoroughly rinse; excessively low temperatures can reduce the activity of the cleaning agent, increasing the risk of carryover. It is recommended to dynamically adjust parameters based on the degree of stencil fouling. For example, for heavily fouled stencils, the cleaning fluid temperature can be appropriately increased (≤50°C) and the cleaning time can be extended (≤10 minutes). However, the optimal balance should be determined through experimentation to avoid excessive cleaning that may cause stencil deformation.
Equipment maintenance is crucial to reducing carryover. The nozzles, filters, and circulation piping of the cleaning equipment should be cleaned regularly to prevent aging or blockage that can lead to uneven distribution of the cleaning fluid. For example, a clogged nozzle can shift the water flow, resulting in incomplete rinsing of parts of the stencil. A damaged filter can allow large particles to re-enter the cleaning fluid and scratch the stencil surface. Weekly equipment inspections and replacement of worn parts are recommended to ensure the cleaning fluid circulation system is always in optimal condition.
Residue detection is the last line of defense in quality control. After cleaning, inspect the stencil under a magnifying glass to ensure smooth edges and no white or colored residue. Pay particular attention to the cleanliness of the pore walls of fine-pitch stencils. For demanding applications, a contact angle meter can be used to test the wettability of the stencil surface. A contact angle greater than 90° indicates the presence of hydrophobic residue and requires re-cleaning. Infrared spectroscopy can also quickly identify the composition of residues, providing a basis for process adjustments.
Environmental and safety requirements are driving the upgrading of aqueous cleaning fluids. With tightening VOC emission limits, new aqueous cleaning fluids must balance cleaning efficiency with low residue properties. For example, the use of biodegradable surfactants can reduce chemical residues and ease wastewater treatment. Companies should maintain cleaning fluid usage records, documenting the parameters and residue test results for each cleaning cycle. Through data analysis, they can continuously optimize processes to achieve near-zero residue levels.
