Solder pastes are a mixture of flux and fine solder powder. The particle size is classified into types (e.g., Type 3 to Type 7), standardized by IPC J-STD-005. The higher the type number, the smaller the particles:

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Type      Particle Size (µm)

Type 3  25–45 µm

Type 4  20–38 µm

Type 5  15–25 µm

Type 6  5–15 µm

Type 7  2–11 µm

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A widely used practical rule is the “5-particle rule”:

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At least five solder particles must fit side by side through the narrowest part of the stencil aperture. 

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Example:
A 100 µm stencil opening requires particles ≤ 20 µm → at least Type 6.

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Aperture Width (µm)  Max. Particle Size (µm)  Recommended Type

225                                    45                                           Type 3 or finer

200                                    40                                           Type 4 or finer

125                                    25                                           Type 5 or finer

75                                      15                                            Type 6 or finer

55                                      11                                            Type 7

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Why are particle sizes distinguished?
Fine particles are essential for modern SMT applications with small pads and fine pitch. They enable clean printing and reliable solder joints. Coarser types are suitable for standard components and offer better cost and storage stability.

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We recommend solder paste from FELDER GmbH, Oberhausen