Why Does the Stencil Determine Whether a Solder Paste Is Good?

The stencil directly determines solder-paste printing quality and therefore affects soldering performance, making it a key factor in evaluating solder-paste quality. The specific reasons are as follows:

I. The Stencil Controls Solder-Paste Deposit Location and Volume

A stencil uses laser-cut or chemically etched apertures that correspond precisely to the PCB pads. Aperture dimensions are typically 5%–10% smaller than the pads, while stencil thicknesses commonly include 80 μm, 100 μm, and 150 μm, depending on joint pitch. During printing, a squeegee forces solder paste through the apertures and deposits it accurately on the PCB surface, forming a paste pattern corresponding to the pads. If the stencil is poorly designed—for example, if apertures are too large or too small—the deposits may shift or contain too little or too much paste, directly affecting soldering quality.

II. Stencil Testing Reveals Potential Solder-Paste Defects
Stencil testing simulates the actual printing process. By observing how the paste fills, releases from, and forms through micron-scale apertures, the following solder-paste characteristics can be assessed directly:

Viscosity compatibility: If viscosity is too high, the paste can clog apertures and leave pads with insufficient solder. If it is too low, the paste may spread and cause bridging or short circuits. An appropriate viscosity, typically 120–200 Pa·s, allows the paste to flow smoothly into the apertures under squeegee pressure and quickly recover its consistency when pressure is removed, preventing slump.

Particle-size compatibility: The solder-powder particle diameter should be less than one-third of the aperture dimension (for example, 5–15 μm for Type 6 powder); otherwise, the apertures may clog and cause skipped deposits.

Release integrity: The flux must prevent the paste from sticking to or clogging the aperture walls, enabling complete transfer to the pads without residue or tailing. Tailing indicates inadequate thixotropy, while incomplete deposits may result from excessive viscosity or rough aperture walls.

III. Stencil Quality Directly Affects Soldering Yield
Stencil aperture design, material selection, and surface treatment all affect solder-paste printing. Examples include:

Aperture design: Adjusting the aperture ratio according to component dimensions and pad geometry—for example, an aspect ratio >1.5 and an area ratio >0.66—helps prevent poor paste release.

Material selection: High-tension stainless steel, such as 304 stainless steel, reduces printing deformation, while nano-coated stencils reduce paste residue.

Surface treatment: Electropolishing improves aperture-wall smoothness and reduces the risk of paste residue.
If stencil quality is poor—for example, because of rough aperture walls or insufficient tension—the paste will print unevenly, leading to defects such as open joints and short circuits. Research indicates that approximately 70% of SMT soldering defects are related to the stencil-printing process.

IV. Stencil Testing Quantifies Solder-Paste Performance
Stencil testing can quantify several key solder-paste performance indicators, providing data for quality evaluation:

Aperture fill rate: A microscope is used to check whether each aperture is fully filled and whether the edges are well defined. Ideally, the paste completely fills the aperture without voids or depressions.

Paste-thickness deviation: The difference between the paste thickness on an individual pad and the target value should be <±5%, and the whole-board thickness standard deviation should be <3%. Otherwise, uneven joint heights may affect component-placement accuracy.

Area coverage: Paste coverage on the pad should be ≥95% of the pad area, with edge offset <50 μm, preventing open joints or bridging caused by misalignment.

Slump: After printing, allow the paste to stand for 10 minutes and observe whether the pattern edges deform. The height reduction should be <3%, ensuring that the paste remains stable while awaiting component placement.