Appendix: QFN and BGA Assembly Guideline
It is recommended that the stencil aperture should be 1:1 to PCB pad sizes as both area and aspect ratio targets are
easily achieved by this aperture. The stencil should be laser cut and electro polished. The polishing helps in smoothing
the stencil walls which results in better paste release.
It is also recommended that the stencil aperture tolerances should be tightly controlled, as these tolerances can
effectively reduce the aperture size. It is recommended that smaller multiple openings in stencil should be used
instead of one big opening for printing solder paste on the center exposed pad region. See
for reference
solder mask design in the center of the package.
Figure 7-7 Exposed/Ground pad stencil design recommendation for QFN packages
7.2.2 Via Types and Solder Voiding
Voids within solder joints under the exposed grounding pad can have an adverse effect on high-speed and RF
applications. Voids within this ground plane can increase the current path of the circuit.
The maximum size for a void should be less than the via pitch within the plane. This recommendation would assure
that any one via would not be rendered ineffectual based on any one void increasing the current path beyond the
distance to the next available via.
7.2.2.1 Stencil Thickness and Solder Paste
The stencil thickness of 0.125 mm is recommended for 0.4 mm pitch parts. A laser-cut, stainless steel stencil is
recommended with electro-polished trapezoidal walls to improve the paste release. Because not enough space is
available underneath the part after reflow, it is recommended that “No Clean”, Type 3 paste (IPC standard J-STD-005)
be used for mounting QFN packages. Nitrogen purge is also recommended during reflow.
The most common surface finishes that are compatible with lead-free surface mount technology (SMT) process are:
•
Organic solderability preservatives (OSP)
•
Electroless nickel/Immersion gold (ENIG)
•
Immersion silver
•
Immersion gold
Selection of a suitable finish will depend on end users’ requirements for board design, assembly process, handling/
storage, and cost.
7.2.2.2 PCB Materials
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