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May 16, 2012

Reflow of Double Sided Micro BGAs

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?


Experts Comments

Before we discuss in detail what you need to do, let's first list the risks you have to deal with:
  1. Risk of PWB warpage
  2. Risk of side 1 components falling off when reflowed inverted during side 2 reflow
  3. Risk of excessive oxidation of side 1 joints during side 2 reflow
  4. Risk of inadequate reflow temperature in areas of high thermal mass
  5. Reliability risks related to back-to-back mounting of BGAs
Risk item 1 may not be a concern if the board is small. If the board is too large, it may need support from a pallet. Such a pallet would need to provide support from printing through reflow, and as such must be designed with said processes in mind. Separate pallets may be required for first and second sides, and the pallet should not support by using the tops of the mounted first-side BGAs. The pallets must be designed to minimize their thermal mass so that they do not interfere with proper reflow.

Risk item 2 should not be a concern for small BGAs, unless the specific component has very few balls for its size, or unless the ball pattern is highly  asymmetrical.

Risk item 3 is a real concern. The solder joints are very small, and can oxidize during the second heat cycle. This oxidation can cause the ball surface to become rough, and the ball shape to be deformed. The best way of mitigating this is to reflow in nitrogen. Presence of a no-clean flux from initial reflow can also help protect the balls during the second reflow. Optimization of the reflow profile to avoid excessive peak temperature and/or liquidus time can also help.

Risk Item 4 should not be a problem, unless the profile is set based on the very thin PWB, and not taking into account the areas of highest thermal mass. A pallet can actually be a benefit here, since it will moderate the differences in thermal mass, and can even be specifically designed to minimize them.

Risk item 5 is a real risk. If the assembly contains back-to-back BGAs, the thermo-mechanical reliability will likely be lower than if the BGAs are not placed back-to-back. If the end use is in a high-reliability application, this can be a concern. You will need to assess whether the reliability is adequate in this case.
Fritz Byle
Process Engineer
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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