BGA Fanout in the SBU structure further developed

Renate Kaufmann
March 11, 2026

In Part I of this series „What is important for the BGA fanout of SBU assemblies“, the basics for a robust fanout design with blind and buried vias were explained - including the typical layer distribution for L1, L2 and L3 as well as the influence on costs and reliability.

In this article, we would like to pick up exactly where we left off and present an extension that has proven its worth in our production:

Flipped microvias

With dense BGA fanouts, conventional SBU/HDI setups quickly reach their limits: additional press cycles, complex via filling and limited degrees of freedom in the layout. Our approach, which has proven itself in production, starts right here and streamlines the process. The result is more design freedom, robust electro-thermal performance and noticeable cost benefits, especially in series production.

The most important advantages at a glance

Design freedom

Optional: stacked or staggered - without additional effort
More degrees of freedom in the BGA fanout, even with tight pitches
Option to optimize impedance routing and pad/via geometry

Performance and reliability

Robust electrical connections
Good return current paths for signal integrity
Very good thermal connection
Suitable for power pins

Production and process

One less lamination/pressing cycle: shorter throughput time, lower thermal material load
No via-filling/plugging required for stacked transitions: less effort, less risk of cavities
Can be integrated into the existing process chain: no additional special steps

Economic efficiency

Direct design-to-cost effect: fewer process steps + no filling
Cost benefits, especially in series production
Reduced risk of downtime and rework due to elimination of filling-critical process steps

Conclusion

More fan-out freedom, better thermo-electrical properties and noticeably lower manufacturing costs - without an additional pressing process and without via-filling.