interconnection features (such as leads, bumps, or balls) of surface mount
devices (SMD's) are usually
soldered onto a printed circuit board (PCB)
consists of three basic steps: 1) application of solder paste on
specific locations on the PCB, or
solder paste printing; 2) positioning
of the components on the board; and 3) solder reflow.
which serves primarily as the
attachment medium between the device
interconnection features and the PCB itself,
is a specially blended paste that consists of a
The components of a solder paste are designed to give it excellent
printing and reflow characteristics. The term
refers to the
process of exposing the solder paste to elevated temperatures (e.g.,
230-260 deg C) to 'melt' the solder particles and allow the liquid
solder to form a good and reliable connection with the board-mounted
serving as the source of solder material that forms the mechanical and
electrical connection between the SMD's and the board, the solder paste
also accomplishes the following: 1)
on the board
prior to the reflow process; 2)
the solder landing sites on the PCB as well as the external
interconnections of the components; and 3)
PCB solder lands and device interconnections from
until the soldering process is completed.
experts, there are several things to consider when choosing the right
solder paste. These solder paste
include: 1) the size of the solder alloy particles in the solder paste;
2) the properties of the flux medium; 3) the design of the stencil to be
used; 4) the paste printing parameters to be used; 5) the tendency to
form voids and other defects; and 6) in sensitive devices, alpha
particle emission rate. Of course, reliability tests must show
that the solder paste forms reliable
of the suitable solder paste for a given application is limited by the
minimum size of the
aperture openings of
the stencil to be used in printing the solder paste over the board or
substrate. Excessively large particles can easily clog the stencil
apertures, resulting in poor printing quality and requiring frequent
cleaning that slows down production. Particle size becomes more critical
as the amount of solder to be deposited becomes smaller. According to
experts, the particle size of the solder paste should be no more than
12% the size of the smallest aperture opening of the stencil, i.e., at
least 8 particles should be able to pass through the narrowest aperture
gap at the same time.
of the solder
paste must have rheological properties that allow high-yield printing at
very fine pitches. Of course, the flux must also exhibit excellent
chemical activity for removing the thin oxide films and other
contaminants from the surfaces of the metals being soldered. The
flux must be easy to activate thermally, but should not decompose
must also form benign residues that are quickly removed by washing.
also impacts the effectiveness of the solder paste. As mentioned,
very fine aperture openings require solder pastes with small particles.
The stencil's aperture size-to-spacing ratio affects the printability of
the solder paste. The shape of the aperture can also affect the size of
the deposited solder for the same pitch. The stencil must be thin but
rigid enough to resist deformation.
must also be
optimized with respect to the solder paste. For instance, paste
viscosity affects the speed at which printing can be done. Adequate
fluidity is required to allow a good roll that fills up apertures
properly. However, the paste also needs to exhibit enough
stiffness to form a well-defined deposit when the stencil is separated
from the board or substrate.
the tendency to form excessive
must be avoided. If total void elimination is impossible, voids must not
be more than 5% of the solder. X-ray inspection can be used to
check for voids. Lastly, experts warn about the possibility of the
solder paste emitting alpha particles that can cause
memory devices, so this must be looked into if the process involves
high-density memory devices.
performed for selecting solder pastes include the following: 1) solder
balling test; 2) wetting test; 3) solder void potential test; 4)
shelf-life test; 5) tack life test; 6) stencil life and abandon time
tests; and 7) slump tests. In-process evaluations must also look
at the printability of the paste (relax/recovery properties, print
speed, print durability), its component placement characteristics, and
the quality of its solder joint/fillet formation.
used for qualifying solder pastes include the temperature cycle test,
the thermal shock test, the impact resistance test, the pressure cooker
test, and the temperature-humidity-bias test.
solder pastes are also used for
PCB Solder Printing;
Solder Reflow; Solder Joint Reliability;
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