Defect of the Month

Bob WillisSolvents in holes can heat and "pop."

This month we look at voids or missing conformal coating. Depending on the lack of coating and position, this condition may result in acceptance or rejection. Normally with conformal coatings, small voids not specifically associated with electrical termination or bridging connections are acceptable, depending on the level of inspection criteria.

FIGURE 1 shows voids or bubbles in conformal coating under UV dark light. I would suggest both these cases require rework, as the electrical termination points are exposed, and there is no protection. Most likely, solvent coating has run into the holes. As the coating starts its first transition from a liquid, the voids expand from the holes. Then they pop, leaving the surface or pads with no protection.

It is suggested too much coating was applied on one pass, and initial evaporation could not occur, resulting in a volcano-like reaction from the holes. Each of the holes associated with coating voids had limited solder fill or cavities. In the past, we have seen the same problem with selective coating around press-fit connectors.

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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.



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Bob WillisTry this test to determine paste problems.

This month we look at solder paste slump during preheating. It is important to know how much, if any, of the paste slumps like butter on a hot day during reflow. If solder paste does slump, it can lead to shorts, solder balls or solder beads, or cause variations in solder joint volume on selected joints.

Variation in joint volume occurs when one joint acquires more solder from an adjacent joint during reflow due to the paste being linked. Testing of solder paste is well covered in IPC specifications, and equipment is available to test paste and record the results.

Alternatively, if you think you have an issue, a simple shop floor test is to use the existing profile but change the temperatures of the final reflow zones. Setting the final zones to final preheat temperatures will slow the degree of slump. Normally the maximum slump is seen earlier during reflow. Typically, as solder paste is changed or the metal particle size is reduced, slumping can be seen more often.

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Bob Willis

Tricks to eliminate exposed copper.

This month we look at solder pad coverage. Some quality engineers still want to see solder coverage, regardless of the PCB surface finish. If they see the original surface coating, they become concerned, regardless of what is stated in IPC standards.

In the case of NiAu (FIGURE 1), it is most likely the solder paste will reflow and wet the pad toe area right up to the solder mask. In the case of an organic surface protectant (FIGURE 2), the solder paste will reflow successfully, wetting the termination and pad, forming a reliable joint. However, the solder may not reflow and wet any farther than the original print area on the OSP coating on the pad.


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Are parts falling incorrectly from the feeder?

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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.

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