The primary challenge in today’s flexible printed circuit board technology is its I/O capability, as determined by minimum feature sizes. For example, in single-metal-layer (1ML) flexible printed circuit, trace and space widths as small as 10-microns have been produced using commercial processes, materials, etc. Even finer features have been produced, a transfer lithography process comprised of 4-micron-pitch (one-micron trace, three-micron space), flexible circuit board configuration. Similarly, feature sizes in two-metal-layer (2ML) flexible circuit board, the lion’s share of commercial flexible circuit board 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 circuit board.
There are many questions to pose when assessing the capabilities of potential flexible circuit board suppliers; perhaps the most critical is, what are the supplier’s minimum via land pad dimensions? The dimensions of the via land pad highlight the alignment capability of the flexible circuit board supplier’s processes and dictate the minimum pitch of the flexible circuit board interconnect. Consequently, not all 2ML flexes with twenty-five micron features have equal interconnect density. Moreover, even though feature sizes are decreasing, I/O capability in flexible circuit board 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. 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 Plastic encapsulated lead frames and multilayer ceramic packages have been replaced by plastic encapsulated flex interconnected bare die or, in many cases, by bare die. Similarly, the increasing use of direct chip attach results, in part, from the inability of flex 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by means of wirebonds with millimeters of length and flex interconnect of tens to hundreds of millimeters in length has been replaced by stud bumps (of tens of microns in diameter, produced using the same wirebonding equipment and materials) and the complete elimination of the flex interconnect.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.