Ensuring optimal performance of a printed circuit board (PCB) requires strong, reliable soldering techniques. Solder fuses PCB components, creating the needed electrical connection to perform essential tasks in the aerospace, entertainment, medical, and automotive industries.

But, slapping on PCB components haphazardly without learning about the different soldering techniques and tips to prevent errors can create several problems, none more prominent – or destructive – than solder balls.

Created by the capillary action of molten solder, solder balls are small spheres that form on the surface of a printed circuit board due to issues such as excessive heat or poor flux management. 

And though this particular PCB design error creates the potential for electrical shorts, product failure, and component damage, it is easy to prevent solder balls with trade secrets from leaders in the PCB industry.


Preheat Your PCB


Between multiple stages in the PCB manufacturing process, heat is necessary to ensure the proper form and function of components. But between subsequent rounds of heating, the materials that make up a PCB can cool down drastically, causing them to contract and expand at different rates.

It is this thermal shock that becomes a key catalyst in the formation of solder balls. The dramatic temperature change prematurely melts with the resulting moisture-rich evaporation creating tiny metallic orbs on the surface of a PCB.

To mitigate this issue, reputable PCB manufacturers recommend preheating a board before soldering components. Preheating allows the materials of a PCB to reach an equilibrium temperature, thus minimizing thermal shock and reducing the formation of solder balls.

While exact temperatures to avoid solder balls on PCB materials will vary on design metrics, most experts recommend gradually building the board temperature from 1.5 degrees Celsius above room temperature until you hit the maximum melting point threshold for solder paste, about 150 degrees Celsius.


Reduce PCB Moisture


Moisture saturation is a real threat to the PCB soldering process, capable of derailing the entire production process. Therefore, controlling moisture within a PCB environment is essential in preventing solder balls from forming during baking when water vapor interacts with molten solder.

Because moisture seeps into a PCB environment through the air, improper handling of boards, and random environmental factors, a multi-pronged approach is necessary to reduce the risk of solder balls forming:

  • Climate-controlled storerooms with the right temperature and humidity levels
  • Implementation and adherence to stringent protocols for handling and storage of PCBs
  • Regular physical inspections throughout the PCB manufacturing process 

Finally, experts recommend slow baking cycles that expose printed circuit boards to low temperatures over extended periods of time, usually about four hours. Slow baking mimics preheating protocols but with an extended time frame, allowing residual moisture to evaporate without causing the thermal shock that creates solder balls on PCB components.


Stencil Cleaning


PCB stencils facilitate a more reliable application of solder to a PCB compared to manual approaches, but without proper cleaning and maintenance, stencils become clogged with solder and flux.

In particular, the chemical properties of solder flux are especially problematic in solder ball formation. Too much and the solder paste collapses, creating a metallic sludge that can be transferred to boards during printing, resulting in solder balls on the surface of a PCB. On the other hand, solder flux with low activity is just as detrimental, creating poor metallurgic bonds prone to becoming solder balls.

Solder paste itself is also a culprit, with uneven ratios of metal particulates able to clump together to create solder balls on PCB parts. Furthermore, contaminated stencils risk depositing excess solder paste at other stages of PCB manufacturing, creating a prolonged risk of solder balls throughout the entire design process.

Cleaning PCB stencils is relatively straightforward, requiring wipe-downs with isopropyl alcohol pads before and after every printing cycle, in addition to more thorough scrubbing at the end of each production run.


Reduce PCB Mounting Stress


Physical stressors are just as much cause for concern as heat, moisture, and excess residue in the formation of solder balls on PCB components. Too much torque can split, warp, or elongate component pads, triggering one of two solder-ball-causing scenarios:

  • The pads are forced closed (inward pressure), creating a metallic connection that can solidify into a solder ball
  • The pads are stretched apart (outward pressure), resulting in solder paste seepage and the creation of a solder ball

The best solution is to avoid excessive mounting pressure and only tighten components to their recommended torque values. Additionally, avoid packing, storage, or handling practices that put undue physical strain on parts.


Use the Right Solder Paste


The right solder composition is necessary to mitigate the risk of solder balls. Generally, this means using a solder paste with higher metal content and lower oxidation rate, such as 50/50 lead-tin or 60/47 lead-tin. 

PCBs requiring more resilient joint strength without the risk of solder balls should consider 95/5 tin-antimony solder for its excellent wetting, corrosion, and heat resistance.

The selection of flux is just as critical as the solder paste selection. Fluxes with high activity levels are preferred as they help ensure an excellent metallurgic bond between solder paste and component pins, minimizing the risk of residual moisture in PCBs leading to solder balls.


Check Pad Spacing


Inadequate or excessive spacing between pads causes pooling and voids in solder joints, leading to a greater risk of solder balls. For this reason, carefully considering the PCB layout is critical to avoid solder balls.

Experts vary in their opinions on ideal pad spacing, as different PCB applications and designs require different tolerances. Generally, a pad-to-pad spacing of 0.2mm is considered optimal for most designs to avoid solder balls.

However, more complicated designs may require additional considerations, such as spacing between pads and the edges of the PCB, the distance between components themselves, and the size of the thermal reliefs connecting components to the board.


Work With an Experienced PCB Manufacturer


When faced with heat, moisture, composition, design, or physical stressors, PCBs are prone to solder balls. But, the all too common problem of solder balls is easily avoidable with the right know-how, tools, and techniques described above.

It is also important to partner with an experienced and reputable PCB manufacturer who can help prevent PCB solder balls during the design, fabrication, and assembly process. Learn more about different soldering techniques from reliable PCB manufacturers and prevent costly errors.