Solder Balls and their Negative Effect on the PCB Assembly Process

If you are a PCB manufacturer, the word ‘solder balls’ must be extremely familiar to you. In fact, you have likely marked the phenomenon of solder ball formation as one of the greatest challenges of your SMT reflow process. The issue is fairly common and can be caused by a host of different factors, mainly poor process conditions, and can genuinely compromise the electrical reliability of the assembled printed circuit board. This article presents a comprehensive overview of the solder ball defect along with an insight into possible causes of the same and also lists out recommended corrective actions that you can take to prevent the problem from recurring.

What are solder balls?

Solder balls are the most common defect that can occur in an SMT reflow process. They are created as a result of uneven melting of solder particles during reflow or wave soldering. The occurrence of solder balls can be random, deliberate or a result of material splash back. Balls greater than 0.13mm in diameter can violate the minimum electrical clearance principle, thereby having a devastatingly adverse effect on the reliability of your PCB assembly. You can imagine it as trapped air or water vapor in the solder paste. When this trapped air/moisture escapes too fast, it can appear as a ball on the soldering joint as the alloy cools down.

What causes solder balls?

The uneven melting of solder particles that causes solder balls to occur generally results from process defects such as oxide layer formation or unwettability of surfaces. Following are some common reasons behind solder balls formation:

• The PCB surface was stored in a humid environment and hasn’t been dried properly before assembly.
• The PCB surface is new and hasn’t been dried enough.
• The quantity of flux applied to the surface is more than optimal
• The solder flux has failed to effectively vaporize out of the assembly because the pre-heat temperature wasn’t high enough.
• The assembly stencil isn’t clean, causing the solder paste to stick to unexpected places.
• There is residual paste stuck under the stencil, causing it to get transferred on the solder mask of the next circuit board.
• The solder paste is unsuitable and has limited stencil life due to water absorption.

How to prevent solder balls?

Once the root cause of solder ball formation has been established through careful investigation of your PCB assembly process, you can go ahead and implement the following corrective measures to prevent the recurrence of the problem:

• Baking the PCB surface before assembly can help prevent unnecessary moisture build-up. Recommended parameters are 120°C for 4 hours.
• Tweak the pre-heat temperature and bring down its rate so to provide more time for effective vaporization of flux residue. Recommended parameters are 1.5°C/sec from room temperature to 150°C.
• Check your assembly process for print gaps, pressure used, wipe frequency and cleaning efficiency. Recommended parameters depend on your specific PCB assembly process.
• Verify the integrity of your solder paste. Recommended parameters include minimum exposure time, controlled humidity and temperature of the printer and use of a coarser powder size.

Use the above information to streamline your SMT assembly process and minimize the occurrence of solder balls.

Alpha Assembly Solutions Inc. is the global leader in the development, manufacturing and sales of innovative specialty materials used in a wide range of industry segments, including electronics assembly, power electronics, die attach, LED lighting.