Defect of the Month

Bob WillisInappropriately sized pads can result in excessive solder and, eventually, defects.

This month we show soldering of 01005 chip resistors from an early project with lead-free assembly. The microsection image in FIGURE 1 shows a few issues, but it’s the circles in both joints that caught my eye. Yes, they are voids before they are unmasked. During sectioning we stopped just before entering the void. Most would have continued a few more micros to remove the thin sliver of solder, which was the wall of the void, to show the void. But everyone has seen voids before!

Read more: 01005 Termination Lift

Bob WillisA lack of compression can be seen nondestructively.

This month we look at crimp connections.

FIGURES 1 and 2 show examples of simple compression connections. Figure 1 shows an excessive length of stripped wire within the crimp termination and a total lack of any compression, which should be easy to see on the wire bundle from the point of entry to the point of compression. Figure 2 lacks compression of the connector, and the stripped wire is barely within the barrel of the connector.

Read more: Incomplete Crimp Connections

Bob Willis

Tricks to eliminate exposed copper.

Read more: Solder Pad Coverage with NiAu and OSP Finishes

Bob WillisSolder paste escaping gaps in underfill can cause electrical failures.

Welcome to our latest Defect of the Month. This month we illustrate what can happen with area array packages that have been underfilled during rework.

During any rework, it is important to avoid overheating components on the opposite side of the board or adjacent to the parts being reworked. If solder joints surrounded by underfill undergo reflow, or are close to reflow temperatures, the solder will expand/extrude, then flow under pressure through any openings (FIGURE 1). (We have captured solder reflow in underfill during reflow simulation in x-ray investigations for a customer at Dage.) Voids in the underfill or gaps caused by expansion of the component can result in joints becoming intermittent. Such joints can fail at any time during the product’s life.

Read more: Post-Rework Underfill Expansion
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