|Ask the Experts|
April 3, 2020
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?
|Expert Panel Responses|
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.
Technical Sales Manager
BLT Circuit Services Ltd
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.
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:
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