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August 22, 2012

Voids with Back to Back BGA Components

With assemblies that have BGA components on both sides, is it common to have voids with BGA components on side one after reflow of side two?

V.S

Experts Comments

Yes, it is quite possible to have voids form, grow and move, if the BGA balls are reflowed a second time.

Good techniques to avoid this possibility, is dependent on what solder and/or solder system you have.

If you are using a paste additive process, you may want to ensure you have full double drop of the solder balls with first reflow, and ample soak time to exhume any and all volatiles prior to solidification phase.

If possible, you might see if voids are present and increase or move on second reflow cycle.  Most likely they exist with first pass and increase in size on second pass.  If this is case, you may want to change your BGA solder system to TSF only or change the metal content and particle size on added paste to improve reflow at lower temperature and reduce flux content.
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Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.
BGA packages with back to back placement on double sided design are not recommended due to considerations of rework, testing, and reliability. It is difficult to place a thermocouple inside the center BGA solder joints to create rework profile. Also, there may not be space for test points under the BGAs.

Additionally, reliability of the product may be reduced due to rigidity of the double-sided area which prevents the board from flexing during thermal cycling. The stress would concentrate on the BGA solder joints and cause failures. If still need to run please inspect BGA void and should meet as per IPC 7095.
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Subrat Prajapati
Supplier Quality Leader
Ge Healthcare
Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.
It's not uncommon for voids that may have been present at a size too small to detect to grow during the second reflow. Any non-gas material (such as flux) that may be trapped within the void will outgas during the second reflow and the void will grow. The amount of void growth you will see is highly dependent on both the paste formulation and the reflow profiles used.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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