Board Design, Surface Mount Assembly & Board Level Reliability Aspects of FusionQuad® Packages (Cont.)
voids primarily concentrate over and around the solder mask covering the vias. Notice also that the solder mask didn’t wick down in the vias for “tented from bottom” scheme, leaving vias open – Figure 22c. The encroached scheme, on the other hand, fills the vias with solder as shown by darker circles in the x-ray picture – Figure 22d.
Originally published in the Proceedings of the SMTA International Conference, Orlando, Florida, August 17 – 21, 2008.
a) 50% coverage, full Aperture for bottom leads, 0.10mm stencil, 32um 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
c d
Figure 22 – Solder voids size and distribution for different via treatments.
Solder Fillet formation for dual lead packages: As mentioned earlier, the saw isolation process creates a trench between the leads for the dual row bottom lead design. This trench exposes one side of the leads. Although the lead plating is done before the isolation cut and exposed side of the leads are not plated, solder fillet can still form on the side if paste with highly active flux is used. The fillet formation as well as the shape and size of the fillet are also dependent on the amount of solder paste deposited on the exposed pad and the board lands for the bottom leads. This is shown in Figure 23 where experiments were conducted by varying the solder paste amount on boards with thermal vias encroached with solder mask. The 50% coverage on thermal pad results in lower standoff and large fillet if 1:1 aperture is used for the bottom lead lands, Figure 23a. Increasing the stencil thickness increases the standoff but thicker paste – even with reduced aperture (75% aperture to land ratio) – still pushes the solder on land up on the sides of the leads, Figure 23b. The last example, Figure 23c, shows that increasing the paste coverage to 80% with 0.1mm thick stencil raises the standoff to beyond 50um target and the reduced aperture for the bottom lead lands does not leave enough solder to cause fillet formation.
Figure 23 – Effect of stencil parameters on standoff height and fillet formation
Assembly Results Summary Some of the main findings from this assembly evaluation are summarized below. 1) This package exhibited good self-alignment capability during reflow (can tolerate up to 50% placement offset).
2) Center THERMAL PAD solder paste coverage, thermal via design and stencil thickness will all influence the standoff height of the bottom leads solder joints.
3) No significant difference in voiding was observed between the different via and stencil design combinations tested. Voiding was less than 30% for all the design options considered in this study.
4) 20-mil aperture gap between inner and outer row bottom leads lands would help reduce the risk of solder joint bridging between the two rows.
5) For tented vias, 50% solder paste coverage on the thermal pad for 0.1mm thick stencil (or 35% coverage for 0.125mm thick stencil) results in target standoff height of 50um for the bottom leads.
6) For open or encroached vias, the paste coverage on thermal pad would need to increase to >70% if 0.1mm thick stencil is used.
BOARD LEVEL RELIABILITY STUDIES 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
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