|Ask the Experts|
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?
|Expert Panel Responses|
Yes, it is quitepossible to have voids form, grow and move, if the BGA balls are reflowed asecond time. Good techniques toavoid this possibility, is dependent on what solder and/or solder system youhave. If you are using apaste additive process, you may want to ensure you have full double drop of thesolder balls with first reflow, and ample soak time to exhume any and allvolatiles prior to solidification phase. If possible, youmight 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 secondpass. If this is case, you may want to change your BGA solder system toTSF only or change the metal content and particle size on added paste toimprove reflow at lower temperature and reduce flux content.
Capital Equipment Operations Manager
Specialty Coating Systems
BGA packages withback to back placement on double sided design are not recommended due toconsiderations of rework, testing, and reliability. It is difficult to placea 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-sidedarea which prevents the board from flexing during thermal cycling. The stresswould concentrate on the BGA solder joints and cause failures. If still need torun please inspect BGA void and should meet as per IPC 7095.
Supplier Quality Leader
It'snot uncommon for voids that may have been present at a size too small to detectto grow during the second reflow. Any non-gas material (such as flux) that maybe trapped within the void will outgas during the second reflow and the voidwill grow. The amount of void growth you will see is highly dependent on boththe paste formulation and the reflow profiles used.
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