Foldable Flex and Thinned Silicon Chips for Multichip Packaging

Citation

Liu, J. (2003), "Foldable Flex and Thinned Silicon Chips for Multichip Packaging", Soldering & Surface Mount Technology, Vol. 15 No. 2. https://doi.org/10.1108/ssmt.2003.21915bae.001

Foldable Flex and Thinned Silicon Chips for Multichip Packaging

Foldable Flex and Thinned Silicon Chips for Multichip Packaging

Keywords: Silicon chips, Packaging, IMAPS

John Balde (Ed.)Kluwer Academic Publishers, Boston, USAISBN 0-7923-7676-5Date of publication: December 2002340 p.; No. of figures and tables: 200 (hardbound)EURO 187.00/USD 185.00/GBP 120.00

Microelectronics packaging and integration technology continue to be driven by the needs of lighter weight, smaller size, lower cost, high performance and high reliability. The present book presents one of the latest developments in the microelectronics packaging methods using foldable flexible circuits and thinned silicon chips. The book is written by a number of the well-known individuals in the field and edited by John Balde. It is one of the first books from the emerging technology book series organized by the international microelectronic and packaging society (IMAPS). The book contains 13 chapters.

The first chapter deals with 3D assembles of stacked chips and other thin packages. Various stacked methods as well as advantages and disadvantages are described here. These technologies are mainly used for memory chips. However, for integration of passives, the chapter describes the possibility to use a folded flex construction using long flex circuit, testing the assembly, then folding the flex strip with either accordion folds or simpler rolled folds originated. These constructions can be used for both stacked memory chips and microprocessor chips or other devices. They are more like multi-chip modules that can combine digital and analog chips, and other circuit elements, including passives. This puts them in the category of system-in-a-package (SiP), a concept that is considered as an evolution from MCM.

The second chapter describes multi-chip carriers in a system-on-a-chip-world. Foldable flex solution enables the use of 3D MCMs. MCMs are more close to the concept of SiP rather than SoC, thus it brings up the debate of SoC vs SiP.

Moore's law predicts that billions of transistors per chip are possible in the near future, and it is the impetus of SoC. However, some of the main drawbacks of SoC are: mixed technologies which result in process complexity and low yield, too much processor cores on one chip, relative poor electrical performance and high price. The single chip SoC may exist in some products, but it is not the wave of the future for the majority of the products.

On the other hand, MCM SiPs come into play as a means to replace higher performance ASICs and microprocessor SoC, to reduce system cost. The reason is that MCMs coalesce the preponderance of the total chip I/O into the first level substrate with an intra-module wiring. MCM SiP is believed to be the future wave.

In chapter 3, the major packaging technologies for the flexible systems are described. To put the thin chips on thin flexible substrates, innovation in normal package technology is required. More knowledge is needed, for example, the new or modified bonding processes, understanding of mechanical stress as well as thermal ageing. In flip-chip technology for flexible systems, compared to conventional technology, it requires less solder volumes, fast heating ramps etc. And metallurgical interactions will be a major problem in reliability problems. In bumping technology for fine pitch structure, the wafer is immersed in liquid solder to get bumps. This method is proved to be good both for eutectic tin lead and some lead-free solder. As bonding technology, no-flow underfiller is a promising approach by using stencil printing. ACA and NCA can also be used for flexible substrate system.

There are three basically different approaches for 3D modules, as foldable modules, stackable modules, and direct integration of ICs in flex or PCB. The latter one is more or less like embedded technology plus thin film process. Photosensitive polyimide and BCB can be successfully used.

In chapter 4, the characteristics of ultra-thin silicon chips under mechanical stress are mentioned. Both thinning and mechanical stresses contribute to the change of the characteristics of these chips. However, the mechanical stresses are dominant. The handling, stacking and shipping techniques of ultra-thin wafers are also discussed in this chapter. The usage of adhesives/tapes in die-attach processes is very interesting. Then flip-chip assemblies of ultra-chips with low-profile bumps are proposed. The substrates as standard solder mask designs will not work. In the last part of the chapter, chip- in-board technology is used to embed ultra-thin chips in flexible substrates.

In chapter 5, the authors describe the application potential of ultra-thin ICs. The thinning, handling and dicing techniques of the ultra-thin silicon wafers are discussed. The current various thinning techniques are reviewed. In these thinning processes, stress relief is an important issue to be solved. A "dicing by thinning" technique is proposed to induce less mechanical stress in a dicing process. To prevent the risk of wafer breakage, carrier substrates are introduced in the handling of the thin wafers. Furthermore, the effects of different thinning, dicing techniques on the electrical and mechanical performance of the silicon are investigated. Finally in the chapter, the handling and assembly of ultra-thin single chips are discussed. It seems that the adhesives (NCA, ICA and ACA) can play important roles in the assembly processes.

In chapters 6-10, various applications examples are given to show the potential possibilities to use this interesting technology.

A medical application is given in chapter 6. To meet the special requirements for active medical implants, a flex-tape based interconnection technology, microflex interconnect (MFI) has been developed. Ultra-thin, highly flexible polyimide (PI 2611) was chosen as the substrate due to its good physical properties and biocompatibility. The MFI utilizes simple ball studs from wire bonding as rivets to interconnect the discrete components directly. The procedure of screen-printing bumps onto the chip or onto the substrate is avoided. Furthermore, the thermal and mechanical reliability issues of MFI structures were discussed. It has been demonstrated that MFI is a rather robust and reliable interconnection technology. In the last section of chapter 6, several successful biomedical applications of MFI are shown.

Chapter 7 describes CSP techniques used in folded flex structures, which can be adopted in SIP design. As 3D CSP examples, Tessera's μBGA^® packages, including folded, stacked and hybrids, are discussed in detail. The stack-up height, electrical, thermal and interconnect issues are analyzed. The cost consideration is also studied. Furthermore, the authors show a demonstration of the use of flex interposer for complex SIP design. It is shown that the folded package provides a means to achieve high-density 3D configuration. It has several advantages on electrical, thermal and reliable issues though there still exist a few risk factors.

Chapter 8 describes a patented 3D packaging solution based foldable flex from Valtronic. The technology is based on flip-chip or CSP module mounted on the flexible substrate. The whole module can then be soldered or glued on to a motherboard.

Chapter 9 provides a succinct assessment of the current state of 3D IC packaging technology. The author regards the Tessera version of fold stack IC packaging as 2.5D solution. This solution could not cost-effectively compete the standard extensions of stacked die packaging technology on thin rigid substrates. The vertical integration, die stacking and package stacking, is promising for 3D packaging. When the number of the stacked dies is more than two, package stacking is more advanced than die stacking. The author proposes two approaches to stackable packaging. One is Amkor etCSP^TM (extra thin CSP). The other is to use a fold-over flex arm.

In chapter 10, the authors give an introduction of the high density interconnect flexible circuits. HDI flex is defined as circuits with less than 200μm pitch and/or via diameters of less than 250μm. The substrate materials and fabrication techniques of HDI flexible circuits are presented. It also provides application examples of HDI flex circuits on IC packages, LCD modules, computer and medical products. In the last section, the flexible circuit suppliers are listed.

Chapter 11 describes the recent advancements in flex circuit technology using liquid crystal polymer substrates. LCP is a thermoplastic, aromatic polyester with very unique properties. The improvement on LCP film manufacture process lower the LCP price, and make it a promising substrate for the flex circuit. Traditionally, flex circuits have been made with polyimide films, and now LCP is more competitive. The comparison of LCP and polyimide film in the chapter is interesting. The main advantages of LCP are its low dielectric constant for high frequency application, less water absorption, high solvent and chemical resistance and adjustable CTE. The chapter also describes various manufacturing methods for LCP and reliability testing data. The author points out that LCP should become a complementary flex circuit offering to polyimide for many emerging application areas.

The interconnected mesh power system (IMPS) topology using for double-sided flex wiring is introduced in chapter 12. With a set of X and Y conductors and vias forming an interconnected mesh plan, it can be extended to multiple metal layers, and to the distribution of several voltages. Obviously, using IMPS in flex applications has several benefits on the design, manufacturing, test and repair processes. Moreover, the Sheldahl's ViaGrid process is presented to demonstrate an IMPS design and fabrication in flex. The author also gave the detailed electrical evaluation of IMPS. A four-layer IMPS test vehicle built in co-fired ceramic was fabricated. Its electrical performances, including impedance of the bare substrate, impedance of IMPS lines and cross talk, etc., were measured. It is shown that IMPS topology is attractive for the construction of real packages both for performance and cost reasons.

Several advanced packaging materials, including monolithic carbonaceous materials, metal matrix composites (MMCs), polymer matrix composites (PMCs), carbon/carbon composites (CCCs) and advanced metallic alloys, are described in chapter 13. The thermal properties of these materials are discussed. They can offer order-of-magnitude improvements in specific thermal conductivity over the traditionally low- CTE packaging materials. Furthermore, the author presents the approaches for using these advanced materials in flex packages. Some passive methods are described in detail.

To conclude, the foldable flex solution is probably very interesting as a platform and concept for future microsystem applications. This includes photonics, RF, medical as well as biosensor field. However, in my opinion, it can hardly be used conceptually for products, which are produced with extremely high volume such as mobile phones as in this case robotics are needed to handle all the components and the assembly. And the foldable flexible circuits solution will not allow that with high speed.

The book is very much based on the research work from the various Fraunhofer institutes in Germany. For that matter, I miss the pioneering research work from the Helsinki University of Technology, Finland on the integrated passives and active devices into the thinned and flexible FR-4 substrates. By adding a chapter on that, it probably further enhances the completeness of the topic.

As the book was written by many different individuals, there is overlap to some extent, but I found it was still quite easy to follow the logical thinking behind the composition of this book. The book is also written in a way more like a scientific report rather than a traditional archival book form. If you want to know the latest developments in this field of microelectronics packaging, I strongly recommend it to you.

P.S. If you order through IMAPS, you will get an extra CD copy of the book free of charge.

Johan LiuChalmers University of Technology and Division of Electronics Production, 431 53 Mölndal, SwedenE-mail: johan.liu@me.chalmers.seWeb site: http://www.elprod.chalmers.se