Solder balls after SMT reflow soldering is almost a common defect 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.
Solder Balls can be present in wave and reflow soldering and hand soldering as well. Solder balls can be the cause of failure or affect adversely reliability of PCB Assembly products.
There are global standards for to define the acceptance criteria of solder balling defect 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.
Presence of solder balls on PCBA indicates that something going wrong in fabrication Process and need to take required action to eliminate solder balls defect and need to implement inspection checkpoints so that it can be filtered.
Now question is that what causes of solder ball formation Solder balls formation is surface mount technology and how to prevent solder balls after SMT Reflow.
How to prevent solder balls after SMT Reflow soldering.
It was observed that causes of solder ball formation can be random or Non-random in various process steps. Solder balls 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 analyse each step carefully in your process to find out the main cause of 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 defects.
Stencil solder paste Printing
Description -During Solder Paste printing process paste spread under the under the stencil will solder stuck onto the printed circuit board green mask masking.
- 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 a printing process.
Reflow temperature Profile
Description – Too High ramp-up rate or preheat rate will not permit enough time for the flex solvent to evaporate off slowly.
- Recommended preheat rate Slow is.
- Preferred ramp-up <1.5°C/sec from normal room temperature to 150°C.
Moisture Printed Circuit Board
Description – Trapped moisture may result in quick-tempered flex evaporation.
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.
Oxidized Solder Paste
Description – 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.
- Storage of solder paste in refrigerator and handling as per supplier guidelines.
- Avoid re-usage of solder paste.
- Do not allow to use expiry date solder paste
Poor Stencil Aperture Designing
Description – Improper cleaning of misprinted PCB or realignment of poor under stencil cleaning and components. A flow of Solder paste on PCB Masks.
- Follow the stencil design rules guidelines
- Modify the apertures as per component requirements
- Do not use tempered Stencil.
Tackling with Solder Ball Problem
To dealing with solder balling it is required to map the whole process and take appropriate corrective actions to eliminate 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 magnifier as a visual inspection on periodically.
There are other several factors 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 with slow speed.
The printed board must be reflowed quickly after component placement to avoid oxidation due to delay.
Reflow profile must set precisely as per reminded by solder paste supplier. Monitoring and controlling of shop floor humidity and temperature and per electronic manufacturing requirements.