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Flexible Thinking: A Once and Future Idea

Electronic assemblies are typically comprised of a mix of discrete resistors, capacitors, inductors, and the like, along with numerous integrated circuit chips, each chip having a certain function or range of different functions. In such assemblies, there are also several different connectors and/or sockets that allow for the assembly to be connected to other assemblies. Making interconnections between and among these many and various active and passive devices is the job of the circuit designer. To date, a broad range of IC packaging and electrical interconnection techniques have been used in such assemblies, especially at the higher end.

As folks try to extend the runway of Moore’s Law, these include various multichip module (MCM) structures along with system-in-package (SiP) offerings, both precursors to the currently much talked about range of heterogeneously integrated packaging solutions found in virtually every electronics engineering magazine. These package assembly devices are still in need of and typically mounted onto an interconnection substrate, most commonly a printed circuit board. Actual interconnection continues to be carried out using a soldering process, though efforts have been underway for some time (by myself and others) to eliminate soldering in favor of more environmentally friendly and less costly methods.

That aside, with ongoing advances in semiconductor technology design, packaging and interconnection innovations, and improved manufacturing technologies, the feature size of integrated circuits has continually decreased. Correspondingly, the operating speed of integrated circuits has increased with each new generation. Many in the electronics industry hope and expect that this trend will continue into the foreseeable future, though most acknowledge that there are limits to the materials and physics of semiconductor technology. One thing that is well known and understood is that for the highest operating speed, it is desirable to have short interconnection paths between electronic elements, both on an individual integrated circuit and between the terminals of interconnected chips, whether in an MCM, SiP, or discrete package.

The challenge of design is that there are often conflicting requirements. What is an advantage to one area of production can be a detriment to another, or even negatively impact the performance of the design itself. One such example is pad pitch. As termination pitches continue to shrink, from an assembler’s perspective it remains desirable to have a large pad pitch, for a larger pad pitch will improve yield and reduce the overall cost of assembly at the board level. Unfortunately, the large pads typically mean a larger package and a greater capacitive loading at the termination, thus a larger PCB and greater cost for the elements to be interconnected.

There are, however, ways to reduce the I/O requirement at the board level; by using a flexible circuit to make interconnections to terminations on the upper surfaces of the chip packages, high-speed signals can be lifted out of the PCB into the space above the board. I described this approach at my previous start up, Silicon Pipe, in the early 2000s, calling it “OTT” technology, meaning off (or over) the top.

OTT technology was invented to circumvent the problem of dealing with noise and crosstalk in the PCB. As the march into the third dimension of electronic interconnection using heterogeneous interconnection solutions continues, the OTT solution was among the first proposed 3D interconnection solutions. Figure 1 offers a graphical representation of that radical approach.

The basic concept of OTT is now being conceptually exploited as “bridge” technology and employs through-silicon via (TSV) technology that allows chips to be interconnected—not just vertically (stacked) but also laterally between chips. This is illustrated in Figure 2. The advantages are obvious: This layout ensures the shortest path routing of critical signals and clocking between chips.

In summary, the OTT concept proposed by Silicon Pipe some 20 years ago is finally finding practical application in today’s heterogeneous integration tool chest, offering an alternative means to getting higher performance while staying within the boundaries marked by the limits of physical science and package assembly technology. Some ideas are just born too early and need the future to catch up to them.

This column originally appeared in the February 2023 issue of Design007 Magazine.

Joe Fjelstad is founder and CEO of Verdant Electronics and an international authority and innovator in the field of electronic interconnection and packaging technologies with more than 185 patents issued or pending. Download your copy of Fjelstad’s book Flexible Circuit Technology, 4th Edition, and watch his in-depth workshop series, “Flexible Circuit Technology.”