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November 5, 2007

Voids in BGA's

What are the causes of voids in BGA solder joints and what are the acceptable limits?



L. T.

Experts Comments

There are numerous process and materials-related variables that can contribute to void formation, but three primary physical differences between SAC and SnPb systems most often account for an increase in the number of voids in lead-free solder joints.

Because SAC alloys have a higher surface tension than SnPb alloys, any gasses that may become trapped do not escape as easily from the molten SAC alloy as they do from the molten SnPb alloy. Plus, the higher temperature processing that takes place with lead-free manufacturing means that there may be more trapped gasses that need to escape. The elevated melting temperature (approximately 217C for SAC alloys versus 183C for SnPb) of SAC alloys release more volatile compounds from the substrate and components, which means that more gasses are likely to be trapped in the lead-free solder joint.

In addition, SAC materials have decreased wetting characteristics which results in a larger wetting angle. The larger angle means that any voids that form must travel a further distance to escape.

As far as guidance on acceptable limits, the IPC is a good source. The industry association recently conducted a study which concluded that some voiding was OK and that the process-related voids formed using SAC alloys had "no statistically significant effect on interconnection reliability as tested by accepted thermal cycling methods." (IPC Solder Products Value Council, "The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper").

Our company's philosophy, however, is that minimizing voids is the best method by which to ensure the most reliable Pb-free solder joint. So, to that end, we have developed a series of low-voiding Pb-free solder pastes to address the issue through a unique materials-based solution that uses a modified flux system to minimize void formation.

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Doug Dixon
Global Marketing Director
Henkel Electronics
Mr. Dixon has been in the electronics field for over twenty years and is the Global Marketing Director with the electronics group of Henkel. Prior to joining Henkel, he worked for Raytheon, Camalot Systems, and Universal Instruments.

Voids in BGAs can occur within the bulk of the solder ball and/or in either of the interfacial joints of the of the solder ball (i.e. to the pad or to the device). Voids can be caused from a number of sources. These can include but are not limited to:

  • From the original solder ball manufacture
  • From flux volatiles being trapped as the solder joint solidifies, often caused by an insufficient reflow profile
  • From joint shrinkage
  • From moisture ingress to the solder material
  • From vias in pad expelling a trapped air volume during reflow

IPC-610D indicates that the total void percentage level within BGA solder balls should be less than 25%. This is the value to which many adhere. However, there is also the IPC-7095A which has tighter values. Both documents will provide further information.

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Dr. David Bernard
Product Manager
Dage Precision Industries
Mr. Bernard has been the X-ray Systems Product Manager at Dage for over 5 years and have been involved in all aspects of x-ray inspection and test for printed circuit board assembly applications. Prior to this, Dr. Bernard was working with radiation measurement instrumentation.

Voids are caused by trapped flux and flux vapors within the solder balls. They can be caused by improper thermal profiling. Typically as long as the volume of the void does not exceed 25% of the volume of the solder ball it is acceptable. The only exception to this rule would be if the void was located at either the ball pad interface or the ball component interface. This would cause weakening of the solder joint.

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Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
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