How To Prevent Solder Balls After SMT Reflow Soldering

Solder balls after SMT reflow soldering are almost a common defect 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. Solder balls can cause failure or adversely affect the reliability of PCB Assembly products.

Acceptance criteria of solder balls

The presence of solder balls on PCBA indicates that something is going wrong in the fabrication process and needs to take the necessary action to eliminate solder balls’ defects and implement inspection checkpoints to be filtered.

Global standards 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 a diameter of 0.13 mm, or solder balls are found in clusters of more than five solder balls (<= 0.13mm) 600 mm square area.

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

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

The causes of solder ball formation could be random or Non-random in various process steps. Solder ball defects can result from 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.

Several factors can be responsible for solder ball generation. So, it becomes crucial to analyze each step carefully to find out the leading cause of the defect and solutions to eliminate it from the process.

Data analysis and information gathering to prevent the solder balls formation recommended some actions to avoid such Solder ball defects.

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

During the Solder Paste, the 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 is 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.
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 average 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 fascinate moisture. It is recommended to Bake at 120°C for 4 hours if necessary before use.

4. Solder ball defect due to Oxidized Solder Paste

Particularly for water solvable solder paste, which absorbs moisture, Moisture can source and increase powder oxidation, resulting in 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 and 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 requires proper Analysis of received inspection results from the solder paste inspection machine. Few sample printed boards can be periodically inspected on a magnifier as a visual inspection.

Several other solder ball causes must be considered to make the process solder ball-free. A misprinted board must be adequately cleaned and dried before use. Component placement will be done accurately, and bulky components must be placed slowly.

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

Kiran lalachan

April 9, 2021 at 2:02 am

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How to Prevent Solder balls after Reflow