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Shorter leads will prevent solder from being trapped in 'oxide envelopes.'

After dip or wave soldering, solder spikes or solder flags can often form on longer leads (e.g., those that protrude more then 2 mm on the solder side of the PCB). If the protruding leads have a relatively large surface area, such as with shielding frames, this effect will be more prominent. Typically, playing with the process settings will not eliminate this phenomenon.

During soldering, the flux on the parts to be joined will be flushed off as solder wets and covers the metal parts. After wetting with solder, the only flux that might be left on the PCB surface is that which has been trapped between the PCB and the solder source. When a PCB separates from the solder source (wave or nozzle surface), the flux that remains on the PCB surface must create an atmosphere that will prevent oxidation of the solder. If the space between the joints is limited, not much flux will be present at this stage of the process, so almost no oxide-reducing activity remains during this process phase. As a result, solder will begin to oxidize as it separates from the joints and an oxide film will cover the solder that is draining.

At the final separation, the part of the breaking solder column that remains on the lead will wick up to the lead by the surface tension of the solder, provided the solder is molten. If that part is covered with an oxide layer, however, the solder will remain in this oxide envelope, forming a spike or flag. This effect will be more pronounced if a relatively large area is covered with solder, while on the other hand almost no flux is left to assist the process. It is therefore understood that longer leads will be more vulnerable, because the flux that should be active is the flux that remains on the PCB surface. During the separation, this surface and therefore the flux moves to a greater distance from the solder source, thus the flux's influence lessens as the distance increases.

It should be noted that the same underlying conditions are the reason that peaks often remain on large metal areas on the PCB that are soldered. There is simply not enough flux surrounding such an area to assist in proper drainage. Solder joints on shielding frames may also cause solder spikes due to another reason related to a heat sink effect. If the heat that the solder brings to the joint is drawn off rapidly to the mass of the frame, the solder begins to solidify almost immediately upon separation from the wave. As a result, solidifying solder separates during the drainage stage and the non-liquid solder cannot flow back to the joint.

Keep Leads Short

Keep protruding leads short, so that the flux on the PCB surface can still be effective. More flux generally will not help, since this extra flux will in most cases be washed from the PCB surface as soon as the PCB contacts the solder source. This extra flux might assist in better wetting of the parts being soldered. A flux that has a better adhesion to the PCB surface and has a better "tail activity" might facilitate drainage. (Per Klein Wassink, tail activity, or the protective capability of a flux, can be tested.¹)

Provide an inert or an oxide-reducing environment at the point where the solder separates between the PCB and the solder source. Only then can spikes and flags can be avoided. If the spikes are forming due to a strong heat sinking effect of the mass connected to the joint, then attempt to optimize joint design. Klein Wassink shows examples of such optimizations.²

References

1. R.J. Klein Wassink, Soldering in Electronics, second edition, chapter 7.2.5, figure 7.31.

2. Klein Wassink, chapter 3.4.1., figure 3.29.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. His column appears monthly.

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