Solder balling.
Elevated temperatures and excessive
time at those temperatures during the warm-up and preheating stages
can result in inadequate fluxing activity or insufficient protection
of solder spheres in the paste, causing solder balling. In addition
to the quality of solder paste, the presence of solder balls may be
essentially related to the compatibility between the paste and the
reflow profile. On the other hand, inadequate preheating or heating
too fast may cause spattering, as evidenced by random solder balls.
The two heating stages preceding the spike/reflow zone are primarily
responsible for this phenomenon.
Solder beading.
Solder beading refers to the occurrence
of large solder balls (usually larger than 0.005 in [0.13 mm] in
diameter) that are always associated with small and low-clearance
passive components (capacitors and resistors).
This problem will occur even when the
paste may otherwise perform perfectly, i.e., free of solder balls at
all other locations (components) on the board and with good wetting.
The trouble with solder beading is that it may occur in most or all
board assemblies, rendering the first-time yield to nearly zero. The
current remedy on the production floor is to manually remove the
beads.
The formation of solder beads near or
under capacitors and resistors is largely attributed to paste flow
into the underside of the component body between two terminations
aided by capillary effect. As this portion of paste melts during
reflow, it becomes isolated away from the main solder on the wettable
solder pads, forming large discrete solder beads.
With other factors, reflow profile is a
contributor to this phenomenon The practice of adopting a slower
preheating rate and a lower reflow peak temperature can reduce solder
beading. However, if the reflow profile is at its optimum, and the
problem still persists, a new paste with a strengthened chemistry is
the solution.
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