Flexible Printed circuit board solution to explore the shift from component-based to system-based design and its impact on flexible printed circuit board technology and applications, it is useful to examine the recent development of a high-performance X-ray detector for GE Health Care’s LightSpeed VCT CT system, a state-of-the-art computed tomography system whose image quality is dependent on many subsystems, especially the X-ray detector.
The increasing use of direct chip attach results, in part, from the inability of flexible circuit board interconnect to meet the constant demand for higher pixel counts in displays. In this case, package interconnect by means of wirebonds with millimeters of length and flexible circuit board interconnect of tens to hundreds of millimeters in length has been replaced by stud bumps (of tens of microns in diameter, produced using the same wirebonding equipment and materials) and the complete elimination of the flexible circuit board interconnect. Electronic packaging technologists and applications engineers are currently developing similar system-based solutions and applications by revisions, enhancements, etc., to existing interconnect technologies in order to meet the never-ending demand for greater interconnect density and capability. However, even though its features continue to lag behind those of semiconductors, flexible circuit board has and always will play a significant role in the optimization of system design.
When create Flexible PCB Board, as witnessed in chip-sized and chip-scale packages, by adapting an existing technology to a novel configuration, the unmet needs for very fine pitch, high volumetric I/O packages effected the reduction in use or elimination of standard electronic packages. Plastic encapsulated lead frames and multilayer ceramic packages have been replaced by plastic encapsulated flex interconnected bare die or, in many cases, by bare die.
Moreover, even though feature sizes are decreasing, I/O capability in flexible circuit card is not increasing or, at a minimum, is not increasing at the same rate. Accordingly, second-level assembly continues to move closer and closer to the device.
The primary challenge in today’s flexible pcb card technology is its I/O capability, as determined by minimum feature sizes. For example, in single-metal-layer (1ML) flexible pcb card, trace and space widths as small as 10-microns have been produced using commercial processes, materials, etc. Even finer features have been produced, as shown in below figure, a transfer lithography process comprised of 4-micron-pitch (one-micron trace, three-micron space), flexible pcb card configuration. Similarly, feature sizes in two-metal-layer (2ML) flexible pcb card, the lion’s share of commercial flexible pcb card production, have been reduced to twenty-five microns. But minimum trace and space geometries do not tell the entire story of I/O capability for flexible pcb card.
As electronics miniaturization has continued, the use of flexible pcb circuit in electronics systems has flourished. Rigid substrates, common in the first multichip module technologies, have been replaced with flexible pcb circuit and either integrated or chip-on-flex.
Likewise, the use of flexible pcb circuit as a substrate for electronic systems utilizing chip-sized and chip-scale packages has increased dramatically. However, during the period that flexible pcb circuit has supplanted PCB as a substrate and increased its share as an interconnect technology, direct chip attach has been developed to circumvent the lack of high-density interconnect. For example, the earliest displays utilized in electronic systems included display electronics mounted on PCBs. These PCBs were attached to the display by means of a standard connector. Over time, PCBs with standard connectors were replaced by flexible cables, constructed using one to a few metal and dielectric layers, with standard and novel connectors. As the size of displays shrank to meet the demands of portable electronics, chip-on-glass technology was perfected.
Directly attaching the electronics to the display I/O reduced the system I/O demands to a quantity compatible with flexible pcb circuit capability. This is a key point to consider in the application of flexible pcb circuit.
As well, Flexible PWB is utilized in advanced packaging products such as the this memory module shown in below figure 1 and the 3D Multichip Module shown in 2nd figure the four-device memory module has the same.
Further, these products show that there exists a hierarchy of packaging and interconnect technologies to satisfy electronic system requirements. In particular, because there is no need for flexure and there is no significant disadvantage in weight, the major interconnect technology in today’s portable electronics systems is still PWB. Flexible PWB is relied upon to satisfy the need for thin, conformal, and Flexible PWB interconnect as well as to serve as the substrate of choice for attaching many (if not most) flip-chip and chip-scale packages. Flexible PWB and PWB have similar performance and reliability; cost can differ significantly, depending on the application. However, as every early technology adopter knows, higher costs are expected and well tolerated at the early stage in new and/or advanced product introduction.
Although the complexity of electronic systems has increased over the past ten years, the challenges in interconnect systems have been minimized due to the functional partitioning of electronic systems into key subsystems: encoding/decoding, processing, display, and power. Typically, flexible printed wiring board is utilized for interconnection within a subsystem as well as for system interconnect. As shown in below figure, laptop computers include examples of these types of interconnect.
As witnessed over the past ten years, the reduction in interconnect feature size has slowed. At the same time, device I/O counts have increased as semiconductor features have decreased, according to Moore’s Law. This disparity in interconnect and semiconductor feature size has created an interconnect “brick wall” that demonstrates the need for advancing the capabilities of interconnect systems. semiconductor device minimum feature size (and rate of decrease in feature size) far exceeds the equivalent in flexible printed circuitry and PCB technologies. The corresponding gap in interconnect capability has and will limit advances in semiconductor applications until advances in volumetric I/O density, form factor, weight, flexibility, etc., are implemented in flexible printed circuitry. In the meantime, flexible printed circuitry must be utilized as an integral part of the interconnect system.
Following a familiar path of technology development, the use of flexible printed circuit board has changed from first-level to second-level interconnect applications; more recently, the role of flexible printed circuit board has been expanded to include system-level interconnect. At each point in its changing role in applications, flexible printed circuit board technology has been modified to meet the application demands. For example, to meet the demands of die-to-die interconnect, the single-layer, via-less, point-to-point circuits found in 1970s TAB packaging sprouted additional layers. The transformation to multilayer interconnect required the development of new features such as vias for interconnecting layers; new materials such as adhesives to bond dielectric and conductor layers; and new processes and equipment for manufacturing the resulting incarnation of flexible printed circuit boards. As device I/O increased beyond the capabilities of existing multilayer technology, the trace and space features of flexible printed circuit board were reduced. Over the past twenty years, feature sizes in flexible printed circuit board such as conductor geometry, via diameter, dielectric thickness, etc., have been improved significantly. These improvements positioned flexible printed circuit board for its now-dominant position in portable consumer, display, and medical electronics interconnect applications.