Circuits Assembly Online Magazine - The Flexibility of Flex Circuits

The age-old technology is seeing new life in medical and other applications.

Old technologies never die; they just get new names. Flex circuits have been around for many years in the IC packaging business and have been called almost as many names. In the early days, it was tape automated bonding (TAB).

General Electric called it Mimi-Mod; Motorola called it Spider, and the first patent (despite an IP spat) was granted to a woman engineer, Frances Hugle, of Hugle Industries in 1969. TAB became famous with its use for driver ICs, connecting them to the ITO pattern on the glass LCD panel. The tape was Sn-plated, and issues with tin whiskers led Japanese companies such as Shindo Denshi to tackle the problem using an annealing process and nitrogen storage before bonding. Later the technology was called tape carrier package (TCP). Over time die were even mounted on the flex circuit without a bonding window, and the concept became chip-on-film (CoF).

In the 1980s, National Semiconductor licensed its TapePack technology to a several companies. This package also used TAB bonding. Kenzo Hatada at Matsushita developed a process called TB-TAB where a gold bump was placed on the TAB tape and die connected. Memory could be stacked this way. Other companies also worked on versions of stacked memory with flex circuits, including Thomson-CSF (now Thales). A 3-D package using flex is still in production today at 3D Plus, the spinoff from Thomson-CSP founded by Christian Val.

TCP was also used in Apple’s Newton. LSI’s ASIC was TAB-bonded in the package. The effort was the result of close cooperation among engineers with LSI, Sharp and Apple.

In the 1990s, flex became popular. A few ex-engineers (Tom DiStefano and Igor Khandros) from IBM got together and formed a company called IST. They came up with an idea to use flex circuit to make a really small chip-size package called a µBGA. They filed some patents, moved to California, and gave the company the name Tessera. Really, it was flex circuit. IC bond pads were connected to an array of bump connections on flex. Some DRAMs still ship in this type of package because this flex circuit structure provides excellent electrical performance. NEC developed a CSP using a flex circuit and called it a fine-pitch ball grid array (FPBGA). Nitto Denko used its ASMAT file with a z-axis conductive flex circuit material to form its Resin-Molded CSP. GE also applied its multichip module technology where flex was used for the redistribution layer to single-chip packages and called it a thin zero outline package.

Around 1995, Texas Instruments also came up with a CSP using flex circuit in order to make a low profile package called µSTAR BGA. This package is still in production. While these configurations were all single chip, flex circuit substrates were used for stacked die CSPs by Sharp, Fujitsu, NEC and many others because they provided an ultra-thin substrate that enabled a reduction in z-height. Tessera developed a folded, stacked package and Intel licensed it. This folder flex used a two-metal layer tape and was in production for many years until the business was sold to Marvell and the product reached end-of-life. Flex circuit substrate CSPs are being replaced by thin-core laminate substrates for new designs, but still accounted for approximately 1.5 billion packages in 2009.

Mainly driven by electrical performance, TAB tape was also used for large-size BGAs (TBGAs). Both TAB bonding and wire bonding have been used to interconnect pads on the die to flex circuit substrates. Semiconductor companies shipping TBGAs include Fujitsu, Freescale Semiconductor, NEC, Renesas and Toshiba. As rigid laminate materials have improved, TBGA volumes have declined.

While flex’s popularity seems waning in some IC packaging applications, it is increasingly popular in medical electronics. Hearing aids, catheters, imaging systems, some implantable devices, and other products depend on flex to meet space, performance and density requirements. The digital hearing aid is a good example. Flex is often a folded product, and depending on the number of folds, the overall length of the flex can be more than 1.5 cm with one or multiple arms and a width of 5 to 10 mm (Figure 1).

Flex is expected to play an important role in wearable electronics. One of a number of research programs underway in Europe, TIPS is targeted at medical and health monitoring both for implanted and non-implanted medical devices, sensors, and portable and wearable electronics systems. A folded thin flex module containing a hearing aid flip-chip set has been demonstrated.¹

Flex already is playing a role in the embedded component business for a multitude of applications. The Imbera technology, known as Integrated Module Board (IMB), which embeds active and passive die in laminate structures, has been extended to flex circuits. A number of companies have developed processes incorporating resistors in flex circuit material, including Asahi Chemical Research Laboratory, DuPont, Ohmega Technologies, Ticer Technologies, and Endicott Interconnect. Buried capacitors in flex are offered by Oak-Mitsui Technologies, 3M Electronics, DuPont and Hitachi Chemical.

The flexibility of flex circuits will enable it to find applications in a variety of future products. New and exciting possibilities are expected of a technology that has been around a long time.

Reference
1. G. Kunkel, “Ultra-flexible and Ultra-thin Embedded Medical Devices on Large Area Panels, “ European Semantic Technology Conference, September 2010.