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Defect of the Month

Bob Willis

When BGAs move during reflow, intermittent shorts can result.

This month we look at ball grid array (BGA) opens and solder compression. Intermittent joints and shorts can be caused by package warpage at elevated temperatures. Hence the interest in lowering soldering temperatures commonly used for SAC alloys.

FIGURE 1 was part of an experiment to chart the movement of a BGA package during reflow soldering. Using our reflow simulation, we can see solder ball compression by the package laminate in the image. In many of our video experiments, we see package warpage causes solder shorts and open connections during second reflow. Intermittent open connections have been experienced on double-sided reflow and package rework of adjacent parts. This procedure has been helpful to demonstrate why and how this problem can exist, particularly with smaller packages.

 

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Bob Willis

AOI during fabrication will catch most pad shorts caused by etching.

This month we look at etching faults on PCBs. This is no ordinary set of pads. They are for an 01005 chip capacitor, the second-smallest chip component available. (Yes, there is an even-smaller size.) The 01005 component package is approximately 0.016" by 0.008" or 0.4mm by 0.2mm and small enough for most members of your staff.

Unfortunately, we found small copper shorts between the two mounting pads on one of our test boards that were not picked up during fabrication. The defective boards were spotted during printing trials. The nickel and gold plating are also present. As with many copper shorts, this is related to bare board etching and imaging, but they should have been picked up earlier.

 

 

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Bob Willis

Gold boards are susceptible to the defect.

This month we look at solder spotting, which is often seen after first- or double-sided reflow, most commonly on gold boards. The two examples below illustrate what happens. FIGURE 1a shows two spots on a nickel/gold pad, and FIGURE 1b shows one spot on a copper OSP pad finish.

 

 

 

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Bob Willis

As-supplied component residues are often the culprit.

This month we look at dewetting on the surface of solder mask or components in manufacture.

FIGURES 1 and 2 show the impact of dewetting on the surface of plastic components in conformal coating. Figure 2 illustrates dewetting on solder mask. In both cases if the coating does not cover all the critical areas of the assembly, it must be reworked. It is up to the quality and design departments to agree what level, if any, of dewetting is acceptable to the product and the customer, rather than just quoting a standard. In some cases, the position or level of the problem may not affect the product operation or reliability.

 

 

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Bob Willis

Should the toe be exposed?

This month we look at solder toe fillets. A solder toe fillet is part of the solder joint visible on most gullwing terminations. These are typically seen on SOIC, QFP and surface mount connectors. FIGURE 1 shows satisfactory joints with no or limited toe fillets.

 

 

 

 

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Bob Willis

Excessive temperature or moisture may be to blame.

Open connections on area array packages can be difficult to spot, particularly if they are intermittent electrical failures like the examples in FIGURE 1. One or more open joints can occur between the package and solder sphere or at the PCB pad interface. Reflow soldering with either convection or vapor phase can cause packages to move and separate. This can be caused by warping of the package or in some cases minor popcorning due to moisture. Both faults can be simulated and recorded with video for reference.

 

 

 

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