Surface wettability

In order for a joint to be made, the solder needs to wet the conductive surfaces on the board and the component within the process time available. There are two measures of surface wettability which impact on the ability of component and board metallisation to make a satisfactory joint:

  1. degree of wetting: how far the solder spreads
  2. rate of wetting: how fast the solder wets and spreads

Related to these, and often tested at the same time, is the capacity of the metallisation to withstand exposure to molten solder without dewetting. Such tests are related to the ‘process window’ shown in Figure 1. This indicates diagrammatically the way in which a surface is first wetted by solder, and then at a later stage dewetted.


Figure 1: A process window for soldering

Figure 1: A process window for soldering

Note that different components show different characteristics; some wet quickly, others take a long time to wet. Similarly, some components are resistant to dewetting (take a long time to do this) whereas others dewet quickly.

Typically those components which wet quickly take a long time to dewet, whereas those that wet poorly dewet first. The process window is between the time when a component must wet (given the soldering conditions obtaining) and the first occasion on which components may dewet.

Self Assessment Question

How would you expect the process window shown in Figure 1 to change if you were using a flux with insufficient activity?

compare your answer with this one

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