Print Aperture and PCB Stackup Tradeoffs in High Density Packages
Fine-pitch package assembly requires pairing stencil foil reductions with thin outer-layer dielectrics to maintain solder transfer and controlled impedance.
Type 5 Solder Paste
Solder Material
Solder powder specifications for high density assembly define a mixture where the metal particles fall within a narrow range of 15 to 25 microns. This type 5 solder paste is defined by a particle size distribution where the majority of the powder is smaller than standard grades. Smaller spheres allow for consistent printing through the tiny apertures of a laser-cut stencil.
This material is typically used for components like 0.4 mm pitch bga packages and micro-passive devices.
Rheology Characteristic
The ratio of metal to flux determines the flow behavior and the stability of the printed deposit. When using type 5 solder paste, the increased surface area of the smaller particles requires a more robust flux chemistry to prevent premature oxidation. This high surface area to volume ratio also influences the shelf life and storage conditions of the paste.
Refrigeration is usually mandatory to maintain the chemical integrity of the flux and prevent the separation of the metal powder.
Printing Requirement
Stencil thickness and aperture design must be matched to the grain size of the solder to ensure a clean release. For type 5 solder paste, the five ball rule suggests that the aperture width should be at least five times the diameter of the largest solder sphere. Success of the paste transfer depends on the aperture filling completely and releasing without leaving residue on the stencil walls.
Fine grain pastes are more sensitive to the speed and pressure of the squeegee during the printing process.
Reflow Performance
Successful reflow performance depends on the thermal stability of the flux during the heating cycle. Because type 5 solder paste has more surface area exposed to the air, the reflow profile must be carefully controlled to avoid excessive oxidation before the alloy reaches its liquidus temperature.
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