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April 10, 2013

Double Sided Reflow for Micro BGA Components

What is the best way to process a 30 mil FR406 circuit board assembly that has micro BGAs that need to be soldered on both sides? Do you recommend IR, Convection, Vapor Phase?

M.K.

Experts Comments

If the PCB permits I would use normal SAC305 on one side then a Sn,Bi,Ag alloy that's reflows at 170C on the other side guaranteeing the first side will not reflow again. You must make sure you use the Ag alloy for ductility with Sn, Bi.
Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
Both convection and vapor phase are potentially good processes. In either case, PWB warpage during reflow needs to be controlled. How this is best achieved really depends on the specifics of the assembly. If you use convection reflow, you will need to determine if an inert atmosphere (nitrogen) is needed. If you use vapor phase, then oxygen in excluded by design.

I would not recommend reflow processes with IR heating. As someone who has profiled, troubleshot and rebuilt both IR and convection conveyor ovens, I can tell you that the only advantage of IR is the very rapid heating that is possible. This is not normally needed, and is not always an advantage. IR heating will always be much less uniform than convection, since it is not an equilibrium process.
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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.
Forced convection reflow is by far the most prevalent method for manufacture of printed circuit assemblies whether for cell phones or massive servers.  There are relatively few IR reflow ovens available. Traditional IR ovens rely on either direct or indirect heat lamp irradiation for heating. The trouble with this approach is that some materials absorb more IR than others and even reflow is difficult to achieve. Vapor-Phase reflow is a low volume batch process.  

Accomplishing double-sided reflow of micro-BGAs on a 30mil board should not be a problem if certain rules are observed:
  1. Make sure paste is deposited evenly as the stencil apertures are small and paste release from the apertures may vary with process and material consistency.
  2. Ensure the forced convection does not blow the component off the paste deposits.
  3. Check to see that this thin board isn't warping/bowing as a result of the reflow.  If warped/bowed too much it may lift BGA balls off the paste deposits or otherwise abnormally stretch the solder joint.
  4. Voiding of the solder paste can be a problem with small joint configurations;  after all, there is very little solder volume  
The two-sidedness of the assembly shouldn't be a concern as surface tension of the solder should hold the bottom-side micro-BGAs on while the top-side micro-BGAs are being reflowed.
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Gary Freedman
President
Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
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