Before we discuss in detailwhat you need to do, let's first list the risks you have to deal with:
Risk item 1 may not be aconcern if the board is small. If the board is too large, it may need supportfrom a pallet. Such a pallet would need to provide support from printingthrough reflow, and as such must be designed with said processes in mind.Separate pallets may be required for first and second sides, and the palletshould not support by using the tops of the mounted first-side BGAs. Thepallets must be designed to minimize their thermal mass so that they do notinterfere with proper reflow.
Risk item 2 should not be aconcern for small BGAs, unless the specific component has very few balls forits size, or unless the ball pattern is highly asymmetrical.
Risk item 3 is a realconcern. The solder joints are very small, and can oxidize during the secondheat cycle. This oxidation can cause the ball surface to become rough, and theball shape to be deformed. The best way of mitigating this is to reflow innitrogen. Presence of a no-clean flux from initial reflow can also help protectthe balls during the second reflow. Optimization of the reflow profile to avoidexcessive peak temperature and/or liquidus time can also help.
Risk Item 4 should not be aproblem, unless the profile is set based on the very thin PWB, and not takinginto account the areas of highest thermal mass. A pallet can actually be abenefit here, since it will moderate the differences in thermal mass, and caneven be specifically designed to minimize them.
Risk item 5 is a realrisk. If the assembly contains back-to-back BGAs, the thermo-mechanicalreliability 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. Youwill need to assess whether the reliability is adequate in this case.
- Risk of PWB warpage
- Risk of side 1 components falling off when reflowed invertedduring side 2 reflow
- Risk of excessive oxidation of side 1 joints during side 2reflow
- Risk of inadequate reflow temperature in areas of high thermalmass
- Reliability risks related to back-to-back mounting of BGAs
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.