defect of the month
Spotting Solder Contamination
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.

Solder spots are basically the result from one or more particles of solder paste in random positions on gold pads. When the board is reflowed, these also reflow and wet the gold. In some cases, they are a cause for rejection, if the gold area is a contact point, bond pad or other functional point. If a spot is random and will not affect the product function, it should be considered cosmetic.

Solder spots on a NiAu pad
a single solder spot on a copper OSP pad finish
Figure 1. Solder spots on a NiAu pad (left) and a single spot on a copper OSP pad finish (right).
The most common reasons for these faults are board rinses or paste spitting. In the case of rinsing, it starts with poorly printed boards that are sent for cleaning. If the cleaning step is not done correctly, however, paste particles may remain. It’s not uncommon with water-soluble solder paste, or paste that has been reflowed with an aggressive profile: Small particles of flux and solder can be ejected from the bulk of the solder during reflow. This Defect of the Month video explains some of the dos and don’ts (

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Bob Willis Caricature
Bob Willis
is a process engineering consultant; His column appears monthly.