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Defect of the Month

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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Bob Willis

When through-hole connectors move during soldering, damage to the nozzle ensues.

This month we we look at through-hole connectors and component pin float. All connector pins should be held in place by the body molding and not move during soldering. The connector should not be used outside of its specification. Suppliers typically define the temperature and time the pins and body of the part are exposed to a specific peak temperature. It is the designer’s job to ensure the correct parts are defined for the process. It is the purchasing department’s job to ensure the correct parts are ordered.

In the images shown, the pins in the connector have floated down. This happens easily during soldering or rework. In an automated process, if the pins drop down 1 to 2mm below the board in selective or wave soldering, they can cause damage. Pins can contact the solder nozzle or wave former, which will jam the machine. Using low-temperature solder with a lower specification connector will work fine, but consider the rework temperatures if parts must be removed.

 

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Bob Willis

Moisture is only one of the potential culprits.

PCB delamination can be subtle or obvious. It is caused by expansion of moisture in the PCB laminate, but that may not be the root cause. Eliminating moisture often prevents the energy buildup that forces apart different layers, but this is not the complete story. Poor bonding during manufacturing of the multilayer board or some form of contamination may result in poor adhesion on innerlayers, permitting moisture to accumulate on these surfaces.

FIGURE 1 shows solder mask cracking around a through via. The PCB expanded during reflow, then contracted during cooling. This resulted in lifting and cracking of the solder mask, plus an intermittent electrical connection. FIGURE 2 shows the innerlayer surface of the board after separation. The through vias are separated and there is no visible adhesion on this layer.

 

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