Originally published in the Proceedings of the SMTA International Conference, Orlando,
Board Design, Sur
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evel Reliability Aspects of FusionQuad® Packages (Cont.)
voids primarily concentrate over and around the solder
a) 50% coverage, full
mask covering the vias. Notice also that the solder mask
didn’t wick down in the vias for “tented from bottom”
Aperture for bottom
scheme, leaving vias open – Figure 22c. The encroached
leads, 0.10mm
scheme, on the other hand, fills the vias with solder as
stencil, 32um
shown by darker circles in the x-ray picture – Figure 22d. standoff
b) 50% coverage,
reduced Aperture for
bottom leads,
0.13mm stencil,
43um standoff
a
b
c) 80% coverage,
reduced Aperture for
bottom leads,
0.10mm stencil,
57um standoff
Figure 23 – Effect of stencil parameters on standoff
height and fillet formation
c d
Figure 22 – Solder voids size and distribution for
Assembly Results Summary
different via treatments.
Some of the main findings from this assembly evaluation
are summarized below.
Solder Fillet formation for dual lead packages: As
1) This package exhibited good self-alignment capability
mentioned earlier, the saw isolation process creates a trench
during reflow (can tolerate up to 50% placement
between the leads for the dual row bottom lead design. This
offset).
trench exposes one side of the leads. Although the lead
2) Center THERMAL PAD solder paste coverage, thermal
plating is done before the isolation cut and exposed side of
via design and stencil thickness will all influence the
the leads are not plated, solder fillet can still form on the
standoff height of the bottom leads solder joints.
side if paste with highly active flux is used. The fillet
3) No significant difference in voiding was observed
formation as well as the shape and size of the fillet are also
between the different via and stencil design
dependent on the amount of solder paste deposited on the
combinations tested. Voiding was less than 30% for all
exposed pad and the board lands for the bottom leads. This
the design options considered in this study.
is shown in Figure 23 where experiments were conducted
4) 20-mil aperture gap between inner and outer row
by varying the solder paste amount on boards with thermal
bottom leads lands would help reduce the risk of solder
vias encroached with solder mask. The 50% coverage on
joint bridging between the two rows.
thermal pad results in lower standoff and large fillet if 1:1
5) For tented vias, 50% solder paste coverage on the
aperture is used for the bottom lead lands, Figure 23a.
thermal pad for 0.1mm thick stencil (or 35% coverage
Increasing the stencil thickness increases the standoff but
for 0.125mm thick stencil) results in target standoff
thicker paste – even with reduced aperture (75% aperture to
height of 50um for the bottom leads.
land ratio) – still pushes the solder on land up on the sides
6) For open or encroached vias, the paste coverage on
of the leads, Figure 23b. The last example, Figure 23c,
thermal pad would need to increase to >70% if 0.1mm
shows that increasing the paste coverage to 80% with
thick stencil is used.
0.1mm thick stencil raises the standoff to beyond 50um
target and the reduced aperture for the bottom lead lands
BOARD LEVEL RELIABILITY STUDIES
does not leave enough solder to cause fillet formation.
Based on the results from phase 1 assembly build, reliability
test boards were assembled for both 10x10mm-100
(3.8x3.8mm die) and 14x14mm-176 lead (5.0x5.0mm die)
packages. The Daisy chain boards were designed with a
Practical Components, Inc.  Tel: 1-714-252-0010  Fax: 1-714-252-0026 12
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