An ounce of prevention is worth a pound of cure: having problems with solderability issues? Les guides two readers through possible solutions.

To begin, I'd like to share the following comment I received regarding my April column on reflow process anomalies. The reader offers good advice and the information is worth sharing. He writes, "Les, your recent column in Circuits Assembly listed typical reasons for paste drop through when using the intrusive reflow process (pin-in-paste) that were right on! One item you didn't mention, which is often overlooked, is the 0.020-in. gap between the bottom of surface-mount connectors and the top of the [printed circuit board (PCB)] that allows room for the paste to wet and reflow."

The reader continued, "In some organizations, the effects of that missing gap, when using through-hole connectors with the pin-in-paste approach, were discovered early and addressed in process development. With the pin-in-paste process and through-hole connectors, which don't have the gap, the solder paste drips through and smears--resulting in the occurrence of insufficient solder in some cases and solder balls and shorts in other locations. In addition, we experienced conveyor contamination from the solder paste drip. This [problem] required continuous cleanup. We added a standoff to the connectors giving us problems [using] the pin-in-paste approach. Adding the standoff successfully eliminated all problems."

Q: With newer, smaller components, the most difficult situation my operation faces is the component that does not tombstone completely. Instead, the component is lifted just enough to prevent a soldered connection---even though the termination rests on the pad. The condition is difficult to see, and, often, the component will work in the circuit for a short period of time before exhibiting an intermittent or complete open condition. What suggestions could you offer to prevent this problem?

A: The implementation of process control has been, and continues to be, the key to success when dealing with components and assemblies. In this particular case, which is commonly referred to as pillowing, control of component and board solderability, solder paste condition and deposition, component placement and soldering profile is critical. Use of an automated optical inspection (AOI) device, and possibly the use of an available sensitive thermal imaging technique, may help in detecting the situation--after the fact.

One specific cause could be rapid movement and a fast stop-and-go environment in the component placement operation. This situation has been shown to slide parts completely off pads. Another possibility could be driedout solder paste or paste that is deposited too thinly.

Q: Can you suggest a technique to use when reworking solder joints attached to heavy ground planes?

A: In the case of assemblies with component connections to heavy ground planes or other high heat sink features, the original connection should be addressed by using the best soldering practices and conditions. These conditions include ensuring optimum solderability of the surfaces to be joined, controlled heat application and the use of adequate flux and fluxing parameters. Attention to these factors should prevent the need for rework.

When solder joint rework is necessary, or removal and replacement of components attached to heavy ground planes is required, applying additional heat--by placing the assembly on a hotplate--can help. Good temperature control is required to avoid damaging other components, connections and features of the assembly. Other options for providing adequate heat input include: 1) applying a soldering iron to the lead on the component side and 2) using a hot air gun to raise the temperature of a large area on the component side of the assembly or the specific joint or component. The auxiliary heat should be added with care to either side of the assembly, and the joint(s) being reworked should be desoldered from the solder side of the assembly.

The best connection a joint will ever encounter is performed the first time the joint is soldered. Each additional thermal excursion can possibly damage the printed wiring board (PWB) or component. By recognizing thermal requirements at the outset, to provide an adequate joint on the first soldering attempt, the problem is solved.

Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; [phone]; e-mail: [e-mail]

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