Rework methods of MLCCs (Multilayer Ceramic Capacitors) (4): What should I do if these settings for rework of MLCCs (Multilayer Ceramic Chip Capacitors) are not working?
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A. The key is to apply sufficient heat to the solder joint in order for the solder to wet. When the solder does not appear to wet when applying heat there are typically two reasons. One, other components such as EMI shields and large components are acting as heat sinks, drawing the heat away from the intended area. Two, the solder joint area is heating up too quickly. In this case, applying heat too rapidly will cause the solvents in the flux to burn off before the flux can activate.

For problem one, since various items are drawing the heat away from the targeted solder joint, turning up the heat on the hot air pencil will only deliver more heat to the heat sinks. In this case, it is best to select an indirect heat transfer method such as SMT tweezers.

Figure 1: Removal using SMT Tweezer

When choosing SMT tweezers make sure to select tips slightly larger than the width of the component being removed. As stated earlier in question 1, specialized tips are available for various case sizes. Tips should also be kept clean and smooth in order to minimize contamination. Though the iron tips are touching the component, thermal shock risk is reduced due to the fact that both termination ends are heated at the same time.

For problem two, either the flux is burning off too rapidly or there is not enough flux. In some cases, adding more flux will help but excessive flux can contaminate the PCB. In most cases, a gradual preheat will solve the problem of flux burning off.

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