3.5D interposers: PCB replacement?


At the 15th Symposium on Polymers for Microelectronics (May 8-10 in Wilmington, DE), TSMC and Yole Developpement gave plenary presentations on the use of polymeric materials in wafer-level packaging (WLP) from foundry and overall industry perspectives.

The most controversial comment came from TSMC's Doug Yu, senior director of front-end and back-end technology development, who challenged the current nomenclature and pronounced that the versatile interposer technology should be called "3.5D" instead of 2.5D, since it is and will be capable of much more than the simple 3D packaging stack. Yu's presentation was title "Semiconductor Paradigm Shift and Increased Foundry Roles."

The term 2.5D is usually credited to ASE's Dr. Ho Ming Tong who, around 2009 (or even earlier), declared that we might need an intermediate step towards 3D packaging, since the infrastructure and standards were not yet ready. The silicon interposer, Tong felt, would get us a major part of the way there, and could be ready sooner than 3D technology, thus the term 2.5D, which immediately caught on with other practitioners.

Yu's new position is that interposer technology actually is more versatile and thus should be called 3.5D, since it not only offers a better thermal solution than 3D integration, but "can someday replace most of the high density PC boards [PCBs]." Yu's position is that this modular silicon technology will need minimal low-density PCB substrates to connect the functions that have been fabricated on silicon and will be, in essence, the perfect "fab-centric" solution. Future smartphones and tablets could be made up of such simple 3.5D silicon modules.

Figure 1. Global materials market for wafer-level packaging. SOURCE: Yole Developpement.

Representing Yole Developpement, I presented an overview of polymeric materials in wafer level packaging (WLP), considering the pros and cons of the major material choices ??? epoxies, PI, PBO, BCB, siloxanes ??? and the offerings of 25 major suppliers. There have only been 5 major categories of polymers developed for microelectronics in the last 50 years, or one per decade. "If you think you have the next great polymer chemistry for electronics, you better also have deep pockets and patience," I warned attendees. We also looked at the 6 key functions that these materials serve, and where they are used in the 7 key WLP applications.

According to Yole, the polymeric semiconductor material market reached a value of $274 million last year and will see 26%+ compound annual growth rate (CAGR) over the next 5 years, fueled by the expansion of key WLP applications.

Solid State Technology, Volume 55, Issue 5, June 2012

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