What Causes Solder Ball Formation During SMD Reflow?
PCB assembly still relies on soldering, and specifically reflow soldering for SMD components. Until an alternative process is developed, solder defects are still a risk during assembly, and there are design and manufacturing factors that can influence formation of solder defects.
One potential assembly defect is solder balling, or the formation of small balls of solder during assembly. This primarily occurs during reflow with SMD components, and it tends to arise in high volume production. When it occurs, the process needs to be inspected and the board needs to be cleaned. In the worst case, it can cause unintentional bridging across component leads when solder balls become very large.
The tips below outline some strategies for preventing solder ball formation during reflow. Some of these are the responsibility of the manufacturer and their quality control team, but there are some things a designer should do to prevent formation of solder balls during manufacturing.
Why Solder Balls Form
Solder balls are exactly as their name implies: small balls of solder that form on the PCB during soldering. These balls of solder could be unattached to a component or pad, essentially sitting on the board near an SMD pad or other piece of metal. In the worst case, solder balls will be large enough to cause a short across two pads or two component leads. Typically, solder balls could sit on the board and might be trapped in leftover flux residues on the surface layer.
Environmental testing chamber used for outgassing tests.
Paste expulsion - The main cause of solder ball formation is excessive solder paste being expelled around an SMD pad. If there is excessive paste expelled between two pads, there is a risk of ball formation that would create a short. Problems like misregistration and high pressure during placement can expel paste near pads and influence solder ball formation.
Excess moisture - Solder paste can migrate for another reason, where the solder paste contains excess moisture and some solder fails to coalesce into a solder joint as the board proceeds through reflow. This can happen when the initial preheat stage in the reflow profile is too low.
Flux activation - If the flux in the solder fails to activate within the preheat time window, this can lead to solder expulsion and solder ball formation. Flux activation requires some heat be applied during the initial stages of soldering, so this will also be related to the reflow profile.
What Manufacturers Can Do
The manufacturer will be on the front lines of ensuring quality control and inspecting assembled boards for a variety of defects, including solder balling on or near components. They will also be responsible for determining the root cause of solder balls should they occur. The assembler and tooling provider will also have to work together to ensure accuracy in tooling. Some of the steps manufacturers can take include:
- Adjust PNP machine pressure for depositing components
- Inspect stencils for damage
- Inspect solder mask dam size and gaps between pads
- Use a higher reflow temperature to quickly remove excess moisture from solder paste
- Verify the reflow profile for the chosen solder paste
- Verify solder mask dams have not flaked away from pads
According to the IPC-A-610 standard, an assembled board is considered defective if at least 5 solder balls are found within a 600 mm2 area of the board. Solder balling can be identified in visual inspection, but generally it will be identified in automated optical inspection.
What Designers Can Do
Designers can also take steps to ensure that their board will be less likely to experience solder balling. There are some points that can be checked both in component footprints and in design rules that can help ensure solder ball formation is prevented, such as:
- Ensure solder mask expansion on small package components will not allow solder mask to flake
- Reduce paste mask size if excessive paste is present on pads
- Reduce pad size while applying zero paste mask expansion
- Increase the gap between pads, specifically between SMD passives
In prototyping, stencils will (or should) be new and thus the tooling factors mentioned above should not contribute to solder balling. The other points above need attention by both the manufacturer and the designer to ensure solder balling does not occur.
When you need to specify pad size and footprint data to ensure accurate assembly, use the CAD tools in OrCAD from Cadence. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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