What are solder beads? How are they formed?
Editorial Team - PCB Directory
Aug 17, 2020
Solder Beads are balls of solder that are created when soldering components on a PCB. They are created due to the use of excessive solder paste when soldering components on a board. Solder beads can cause a short circuit between the component terminals which can result in failure of the circuit. Also, in case there are vibrations, the solder ball can break loose and cause a short circuit somewhere else on the PCB. This is an undesirable phenomenon or defect in SMT (Surface Mount Technology).
Due to the solder beads, there may be a chance of occurrence of an electrical short circuit between terminals of chip resistors or capacitors or ICs where it was formed. Also, if any vibration in PCB due to any reason, there may be a chance of the solder bead to break loose and move across PCB and make short circuits anywhere on the board. It is an undesirable phenomenon or defect in SMT.
How does solder beading occur?
The pads of PCB contain the solder paste. The required components (Chip Resistor, Capacitors or other IC) of a circuit are placed on the corresponding pads, and the PCB is sent into the reflow oven. In the reflow oven due to heat, the solder paste starts to melt-down. If there is excessive solder paste, some solder paste separates from the main solder paste on the pads and ends up underneath the component.
Solder stuck underneath the component
The solder stuck underneath the component does not return to solder pads and tends to form a ball of solder also known as a solder bead. During the cooling process, the surface tension of the cooling solder pulls the component closer to the pads. Here, the component is drawn down, towards the board and as a result the solder squeezes out on the side of the component in the form of a bead or ball.
Solder bead squeezes out the side of the component and remains there
How to prevent solder bead formation?
Manual removal of solder removal is expensive and impractical. It is better to eliminate the solder beads before they occur. Since solder beading is usually caused by the use of excessive solder paste on the pads, the best solution would be to reduce the amount of solder paste being applied to the pads. This can be done by using the following ways:
1. Increasing Stencil Thickness
2. Modifying Aperture shape/size
3. Changing viscosity of the Solder paste
4. Adjusting the reflow profile
5. Modifying the Squeegee speed and pressure
What causes for Solder beading formation?
Solder beading is caused by the use of excessive solder paste on the solder pads of a PCB. Other factors include the closeness of pads on a PCB, the shape of the pads, the reflow or melting profile used, and environmental conditions.
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