Kester - Solder Beads - What to do about them

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What are solder beads?


Solder beads are often confused with solder balls on the surface of a surface mount technology (SMT) board. Solder balls are small balls of solder that are randomly scattered across the surface of a SMT board. They can appear on the solder mask or on the board metallization and they are present around all different component types. Solder beads comprise a specific category or subset of solderballs. Solder beads are solder balls that occur in close proximity to chip resistors and chip capacitors. A typical example of a solder bead is shown in Photo 1.


How are solder beads formed?


There are several steps in the formation of solder beads. The first step is when the solder paste deposit starts to slump as the paste is heated in the reflow oven. If there is too much paste or the pads are too close together the paste can bridge between the adjacent component lands. When the solder paste reflows or melts the solder pulls back to the pads but in the process a small solder ball or solder bead can break off the main mass of the solder. This solder bead can be located closer to one land than the other but often times the solder bead appears equidistant between the two pads on one side of the component. The process of solder bead formation is shown schematically in Diagram A.


Why do solder beads form?


There are three primary problem areas that contribute to the formation of solder beads. These problem areas are correctable at the manufacturing level. This discussion will focus on the problem areas and what can be done in the manufacturing area to overcome them...


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