This effort is part of the IMAPS Emerging Technologies Book thrust. In this goal the editor has done well to gather the cutting edge experts in all aspects of the emerging technology of folded flex. If you are responsible for deciding on the next generation of packaging that will be used for your companies application or are steering university research and want to find high impact focus, this book is a must read. The various chapter authors each see the future differently with many "pros and cons" surfacing and allowing the reader to decide in their circumstances what the best route forward is.
Balde sets the tone for the book by outlining the options to
packaging chips in 3D structures and the progress of the wafer
thinning and flex substrates. His introduction puts the reader
in the perspective of comparing the many options opening up for
the next generation of high density packaging. Several of the
authors, including Evan Davidson (IBM), discuss the wall that
System-on-Chip leaders have reached and how they were often forced
to re-inventing multi-chip approaches (such as Intel's BBUL and
foldable flex).
Many of the chapters deal with the many details of techniques
for thinning wafers, handling thinned wafers, fixing the wafer
while singulating, bumping and placing the chip, thermal management,
testing and reliability. Leonard Schaper (U of Arkansas) shows
that the limitation of using just 2 sided conductor patterns will
not prevent good power distribution and control of impedances
using is IMPS approach.
The chapters clearly show many different trade-offs with the authors often in opposing positions. For example the chapter by Mike Warner and Bill Carlson (Tessera) clearly shows the advantages of starting with CSP components; whereas the chapters by Catherine Kallmayer, Thomas Harder, and Karlheinz Bock use the flex substrate for most of the evaluation of the individual thinned chips rather than the normal CSP process. Most chapters assume polyimide is the flex substrate of the future but the work of Rui Yang and Terry Hayden (3M) show the promise of liquid crystal polymer particularly for low loss RF work. Both Davidson and Ted Tessier take "devil's advocate" positions comparing folding flex with other established 3D approaches. Ted points out that packaging roads often loop back in history and that wire bonding keeps reinventing itself. Jan Vardaman and Dominique Numakura show the many players in the different materials/processes/ equipment infrastructure for folding flex systems. They also make the point that the decades long efforts in TAB should really boost the folded flex progress. Perhaps missing from the discussion was a more explicit discussion of high frequency behavior of the folded flex -thinned chip approach since this is the future direction of many applications.
After reading the advanced copy over a weekend it is easy to conclude that this book is a must reference for any microelectronics company needing high density and trying to position itself with the constant evolution of the many packaging technologies. By placing all this information between two covers and having the various perspectives of the many authors clash and add to each other (and providing access to the authors), the reader is placed in a knowledgeable position from which to make their own technology investment decisions in this 3D world.
David W Palmer
Manager microsystems applications, Sandia National Laboratories