How to prevent solder balls after SMT Reflow soldering

Solder balls after SMT reflow soldering are almost a common defect that occurs in the printed circuit board (PCB) assembly process. Several tinny balls surrounded along with the peripheral edge of the flux residue after reflowing are known as solder balls. In this section, we will discuss how to prevent solder balls.

Solder Balls can be present in wave and reflow soldering and hand soldering as well. Solder balls can be the cause of the failure or affect adversely the reliability of PCB Assembly products.

Acceptance criteria of solder balls

The presence of solder balls on PCBA indicates that something going wrong in the fabrication process and need to take the required action to eliminate solder balls defect and need to implement inspection checkpoints so that they can be filtered.

Target Condition

how to prevent solder balls

Minimum acceptable

SOLDER

Defect

SOLDER BALL DEFECT

There are global standards to define the acceptance criteria of solder balling defects in surface mount assembly. According to the IPC inspection document IPC- A 610 the solder balling can be defined as 

“Solder balls can consider as a nonconforming defect for various class 1, 2, and 3 that violate min electrical design clearances or are not encapsulated in an everlasting coating or attached to a metal joint”.

Solder balls defect SMT reflow soldering can also consider as process indicators when they are within 0.13 mm of lands or tracks, and defective it exceeds diameter 0.13 mm, or solder balls are found in clusters of more than 5 solder balls of (<= 0.13mm) 600 mm square area.

Now question is that what causes solder ball formation in surface mount technology and how to prevent solder ball formation in the SMT process.

What solder balls causes & how to prevent solder balls during reflow

It was observed that causes of solder ball formation can be random or Non-random in various process steps. Solder ball defect can be a result of poor solder paste handling, improper paste printing or cleaning, poor reflow temperature settings, rough PCB Pad Designing, or usage of oxidized electronics components during an assembly process.

There are several factors with can be responsible for solder ball generation.  So, it becomes very important to analyze each step carefully in your process to find out the main cause of the defect and solutions to eliminate it from the process.

Based on data analysis and information gathering to prevent the solder balls formation recommended some sort of actions to prevent such Solder ball defect.

1. Solder ball defect due to poor Stencil solder paste Printing

During the Solder Paste, printing process paste spread under the under stencil will solder stuck onto the printed circuit board green mask masking.

Recommendations to avoid solder balls – 

  • Ensure that zero print gap set up between Stencil and PCB.
  • To use minimum Squeeze print pressure, need to Set.
  • Set and check for proper cleaning frequency of wiper paper cleaning and type such as wet/dry/vacuum need to set.
  • PCB Holding must be proper during the printing process.

2. Solder ball defect due to poor Reflow temperature Profile

Too High a ramp-up rate or preheat rate will not permit enough time for the flex solvent to evaporate off slowly.

Recommendations to avoid solder balls – 

  • Recommended preheat rate Slow is.
  • Preferred ramp-up <1.5°C/sec from normal room temperature to 150°C.

3. Solder ball defect due to Moisture Printed Circuit Board

Trapped moisture may result in quick-tempered flex evaporation.

Recommendations to avoid solder balls – 

  • Particularly for lesser grade PCBs such as FR2, CEM1, tends to fascinate moisture. Recommended to Bake 120°C for 4 hours if necessary prior to use.

4. Solder ball defect due to Oxidized Solder Paste

Particularly for water solvable solder paste which absorbed moisture, Moisture can source and increase powder oxidation and results from a stencil life inadequate because of moisture absorption.

Recommendations to avoid solder balls – 

  • Storage of solder paste in refrigerator and handling as per supplier guidelines.
  • Avoid re-usage of solder paste.
  • Do not allow to use of expiry date solder paste

5. Solder ball defect due to Poor Stencil Aperture Designing

Improper cleaning of misprinted PCB or realignment of poor under stencil cleaning and components. A flow of Solder paste on PCB Masks.

Recommendations to avoid solder balls – 

  • Follow the stencil design rules guidelines
  • Modify the apertures as per component requirements
  • Do not use tempered Stencil.

Tackling with Solder Ball Problem

To Tackling with Solder Ball Problem with solder balling it is required to map the whole process and take appropriate corrective actions to avoid solder balls and inspection to detect the defect.

More focus is required on solder paste printing quality, required proper Analysis of received inspection results from solder paste inspection machine. Few sample printed boards can be inspected on a magnifier as a visual inspection periodically.

There are other several solder balls causes that must be considered to make process solder ball free. A misprinted board must be properly cleaned and dried before use. Component placement will be done accurately, and bulky components must be placed at a slow speed.

The printed board must be reflowed quickly after component placement to avoid oxidation due to delay. Reflow profile must set precisely as reminded by the solder paste supplier. Monitoring and controlling of shop floor humidity and temperature and per electronic manufacturing requirements.

Electronics Engineer,Blogger and a Young Entrepreneur by passion.With more than 18 years of broad experience in New Product Introduction and Process Improvement.

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