Merely a novelty just a few years ago, flexible circuits have moved into mainstream use in many applications. Designers are deploying flex circuits for high-volume, surface-mount PCB applications, as well as for array and stacked IC packaging techniques, such as flip-chip, micro-ball grid array (BGA), tape BGA (TBGA), 3D, chip-scale packaging (CSP) and system-in-package (SiP).
Flex circuits are already the technology of choice for small, portable systems that require tiny, thin substrates, such as mobile flip-phones, laptop computers, watches and hearing aids, in addition to medical electronics and MEMS. More recently, flexible electronics are finding their way into RFID tags and photovoltaics, and being considered for lighting and displays.
There are, of course, design constraints when working with flex circuits. Generally, footprint and copper pad sizes must be larger than is the case with rigid PCBs. Unless special pad stacks are made, copper can become detached from the dielectric layer. Delamination can be a problem without sufficient extra clearance to board edges. Interfacing issues between pads and circuit traces can be avoided by using hold-down tabs or fillets at the ends of each pad. Routing traces at right angles to the curve can diminish stress in the copper when flexing occurs.
Flex Materials
Several different types of IC package and PCB substrate material are used for flexible circuits, depending on variables such as system size and application. These include polyimide films, liquid crystal polymer (LCP), adhesiveless polyimide laminates, and thermoplastic polyimides.
Traditionally, polyimide films have been used, manufactured with subtractive processes that give excellent mechanical strength and thermal stability. However, polyimide has several disadvantages. It is limited to speeds of about 10GBps in circuit lengths shorter than 12mm because of its moisture retention and humidity expansion characteristics.
Thermoplastic materials, such as LCP and thermoplastic polyimides, have become much more common in flex manufacturing, as well as in rigid PCB fabrication. For both, they offer higher frequency and lower moisture. In addition, the newer plastic substrates and manufacturing processes provide increased reliability, greater impedance control, and fewer mechanical connectors. They also offer excellent thermal performance and lower voltages, as well as lower assembly costs.
Organic polymers can be combined with various types of rigid substrate to create highly dense, multi-layer boards. LCP and its associated manufacturing processes meet the increasing demands of optoelectronic, medical, liquid crystal display (LCD), and MEMS applications.
In particular, conducting organic polymers manufactured with additive, not traditional subtractive, processes are at the forefront of flex circuit manufacturing and are being considered by manufacturers in the rigid PCB world, as well.
Bendable at 50µm Thick
One exciting recent announcement is of a new process flow for ultra-thin chip packages that results in bendable packaged chips only 50µm thick (see Fig). The technology, developed by IMEC and the University of Ghent, enables embedding packaged chips in flexible boards to create smart, highly-integrated, flexible electronic systems for a wide variety of applications, such as smart textiles and flexible displays.
The process has been demonstrated with silicon chips each thinned to 20-30µm. The base substrate is a 20µm-thick polyimide layer spin-coated on a rigid glass carrier. Bicyclobutane, which is resistant to the high curing temperature of the top polyimide, is used as an adhesive and proper chip placement results in void-free bonds. A spin-coated 20µm polyimide layer covers the die. A top metal layer of 1µm TiW/Cu is sputtered and photolithographically patterned, metallizing the contacts to the chip and providing a fanout to the chip contacts.
A more theoretical advance from Intel is the demonstration of high-speed, copper-based, chip-to-chip interconnects between IC packages via a flexible circuit, at speeds of up to 20Gbps. Recently, SiliconPipe demonstrated a similar concept with speeds of up to 10Gbps.
by Ann R. Thryft
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