Additional information
13 of 31
Revision 4.0
2021-10-11
Recommendations for board assembly of Infineon transistor outline
type packages
Mounting of surface-mount devices
3
Mounting of surface-mount devices
The following factors have to be taken into account to achieve the best assembly quality for a given
application:
PCB design
Footprint and stencil layout
Solder paste formulation
Solder paste application and inspection
Component placement
Reflow soldering process, especially the reflow profile
3.1
Solder paste stencil
In SMT the solder paste is applied onto the PCB metal pads by stencil printing. The volume of the printed solder
paste is determined by the stencil aperture and the stencil thickness. While an excessive solder paste volume
will cause solder bridging, an insufficient solder paste volume can lead to reduced solder spreading between all
contact surfaces. To ensure a uniform and sufficiently high solder paste transfer to the PCB, laser-cut (mostly
made from stainless steel) are preferred.
In most cases, the thickness of a stencil has to be matched to the needs of all components on the PCB. For
typical TO SMD packages, stencils with a thickness of 150 µm are recommended. Extending the solder pads
together with the print can provide a sufficiently high solder volume to overcome the typical single-ended
package stand-off.
A typical stencil aperture print reduction for TO leads is approx. 90% of the landing pad size. The solder paste
volume in apertures larger than approximately 5 mm may be scooped out depending on the specific squeegee
pressure and rigidity. Such apertures are necessary for many die pad prints and should be segmented into
smaller areas. When reducing the die pad print, potential vias in the pad can be considered as outlined in the
relevant section above (see
For individual design adaptations to reach the optimum amount of solder, the stencil thickness, the PCB pad
finish, quality and solder masking, the via layout, and the solder paste type should be considered. In every case,
application-specific experiments are recommended.
Further details and specific stencil aperture recommendations can be found in the package data base that is
available on the Infineon web page [1]. Please choose a specific package when searching the data base, which
will then show an example of the stencil aperture layout for each package.
For further information about solder stencil design, please refer to the
General Recommendations for Board
Assembly of Infineon Packages
document that is available on the Infineon web page [1]. Please also feel free to
contact your local sales, application, or quality engineer.
3.2
Solder paste
Pb-free solder pastes typically contain some type of SnAgCu alloy (SAC solder with typically 1-4% Ag and <1%
Cu). The most common alloy is SAC305 (3.0% Ag and 0.5% Cu). The average alloy particle size must be suitable
for printing the solder stencil aperture dimensions. Using Type 3 or Type 4 paste is recommended for the board
mounting of TO SMD, depending on the specific stencil aperture size and therefore solder paste transfer
efficiency.
