Editor | On 06, Aug 2007

By Gunter R. Ladewig and Robert Lyn

Genrich Altshuller wondered, “Could inventions be the result of systematic inventive thinking?” Over half a century, Altshuller and his associates investigated hundreds of thousands of patents. They found that exceptional patents improved the performance of a technological system through elimination of its fundamental constraint by resolving contradictory requirements. Altshuller discovered that as technological systems evolved to their next level of performance by resolving contradictory requirements, systems tended to progress along certain vectors, or trends, of evolution. Each vector of evolution had discrete phases, or performance levels, which define where a system was, where it is and where it will be in its evolutionary journey. Another revelation was the frequent occurrence of a windfall of benefits, or super effects, which occurred when a system progressed from one phase to the next by eliminating its fundamental contradiction.Not only were many costly add-ons or processes, and expensive tolerances were no longer required, many systems had inherited valuable, new, product differentiating, capabilities and features. The result, the Theory of Inventive Problem Solving (TRIZ), provides a methodology for systematic creativity.

Background

The increasing and never-ending demand for more function from electronic devices in smaller form factors continues to accelerate and force pressure upon electronics systems to meet these requirements. To satisfy these demands, designs must be optimized in order to remove the critical roadblocks to attaining maximum performance at minimal cost. An increasing amount of attention, which has traditionally been focused on semiconductor chip developments and systems design, is now being directed towards the most severe system performance constraint, i.e., the interconnection technologies used to join these components together. A new strategic interconnection technology, insulated wire bonding, is poised to remove the fundamental constraint that currently impedes the economic viability of next-generation, high performance designs. This paper describes the chain reaction of benefits, referred to by TRIZ as super effects, which occur when wire-bonding with with a micron-thin wire.

Current Wire Bonding Technology

Wire bonding is the process of welding a fine conductive wire (usually gold), approximately half the diameter (25 microns) of a human’s hair, from an integrated circuit (IC), chip interconnection pad to a substrate pad as shown in Figure 1. It is used in more than 90 percent of all IC packages.

Figure 1: Wire Bonding (Chip to Pad Interconnection) Source: Loctite

Since total system performance is most severely limited by the length of chip-to-chip interconnecting wire as shown in Figure2, requiring designers to minimize the number of chips used.

Figure 2: Chip-to-chip Interconnection Wire Length

This constraint forces the designer into a trade-off contradiction: wires should be close, ideally touch to maximize input/outputs (I/O) per chip and the wires should be far apart to prevent electrical shorts. The wire bonding industry’s answer to this dilemma was to space out wires by only bonding around the chip’s perimeter as shown in Figure 3, resulting in decreased performance (low I/O per chip) and wasted expensive “real estate” at the center of the chip.

Figure 3: Peripheral Bonding

Because the industry never solved this fundamental issue, and due to the ever-increasing demand for more performance (I/O per chip), other – but very expensive – new technologies with area array capability such as an area array flip chip were developed. (See Figure 4.)

Figure 4: Area Array Flip Chip (area array of solder balls for siliconchip to substrate inter-connection)

Although flip chip provides some performance improvement, the unresolved interconnect wiring problem was never eliminated and merely transferred to expensive, multi-layer substrates with long layer-to-layer connecting conductors, as shown in Figure 5.

Figure 5: Multi-layer Interconnect Substrates

TRIZ and the Trends of Evolution

As mentioned previously, Altshuller and his colleagues discovered that technological systems improve their performance, i.e., eliminate constraints, by evolving from one phase to the next, along predictable vectors of evolution. We can examine any technological system by determining which vectors are involved and what the system’s maturity is, subject to the phase locations along various vectors. In other words, we can perform an innovation potential assessment on any system to determine its opportunities for improvement. We can determine if the system has room for improvement (more phases to exploit) with current technology, or if it is bumping its head at the end of a vector and for this reason must change to a new technology (new vector). We can also determine if an evolutionary phase has been skipped, enabling us to go back to that phase, exploit it and hollow-out the competition that has committed itself to a more expensive process and or infrastructure.

Subject to how the trends of evolution are grouped, there are more than 30 trends. The trend that applies to our most fundamental IC constraint, chip-to-chip interconnect wire length, chip wire-ability, is the trend of geometric evolution for a line. Generally speaking, geometric structures tend to evolve from a single point toward complex three dimensional structures as follows:

Point»» Line»» 2-D lines (planar) or curves»» 3-D Lines (cubic) or curves»» 3-D complex curves

Using sewing as an example, the first phase would be a single stitch, then a straight line of stitched thread, then a plane of woven or stitched fabric, next three dimensionally, interconnected layers of fabric, then three dimensionally interconnect complex structures.

Figures 1 to 3 reveal that phase 3, 2-D lines, in a planar arrangement was only partially completed. Because the contradiction of having wires touch to maximize the number of wires and at the same time have wires that are far apart to prevent electrical shorting was never solved, only peripheral chip bonding was used to maximize wire spacing, thus leaving the whole center of the chip completely empty. Or, if performance demands couldn’t be satisfied by wire bonding, then expensive new technologies (like flip chip) were adopted.

Touching Without Shorting

This contradiction was solved by developing a one micron-thin, bond wire insulating material that has the ability to be used on standard wire bonding assembly equipment. With this technology the whole area of the chip, including its center, could now be wire bonded as seen in Figure 6, requiring fewer chips and resulting in improved total system performance due to fewer and shorter inter-chip wire connections.

Figure 6: Wire Bonded

Numerous obstacles had to be overcome. To name a few, the coating 1) had to be thin, yet have high dielectric strength, 2) must have high flexural strength but crack resistant (see Figure 7), 3) must not inhibit bonding, 4) must be non-contaminating (leaving no residue on equipment), 4) be solvent resistant, 5) adhere to gold and 6) be temperature stable up to 250°.

Figure 7: Crack Resistant Strength (left – .5 ml radius, right – 2.5 ml radius)

Chain Reaction of Super Effects

A chain reaction of super effects is poised to have a global impact on the electronics industry due to the implementation of this wire. To name a few, these are:

1. Increased total system performance due to fine pitch, high I/O per chip capability,
2. Relaxed design specifications due to increased process robustness and improved performance,
3. Lower substrate cost due to increased connectivity,
4. Reduced die size resulting in potential billion dollar savings due to increased wafer yields (see Figure 8),
5. The wireis a plug-and-play technology that enables next generation products with the current worldwide wire bonding infrastructure,
6. Quick response engineering changes can be implemented to correct design errors by wiring directly from the bond pad to the substrate pad without worrying about wire shorting – regardless of location, routing complexity or length of wire,
7. It’s an enabling technology for system in a chip (SiP) and stacked die assembly (Because the new wires don’t short, they can be tightly spaced for high I/O, input/output, systems in a chip or for stacked die assemblies.),
8. Increased manufacturing quality and product reliability due to immunity from wires shorting during the bonding process or during molding when the chip is encapsulated with plastic, and most importantly
9. Savings resulting from all of the above.

Figure 8: Increased Wafer Yields

Summary

A windfall of benefits were achieved in this situation when a technological system’s fundamental constraint or contradiction was solved. Just one trend of evolution (out of more than thirty) can be used as a competitive weapon. Trends not only tell us where our products are on their evolutionary journey versus a competitor’s product, for example, but also where products must evolve to. Perhaps most importantly, they show opportunities for improvement or, as in this case, how to hollow-out the competition when an evolutionary phase has been skipped.

TRIZ has many more applications and tools than what was shown in this example with the use of the trends of geometric evolution of a line. TRIZ provides a methodology, distilled from the real world of the worldwide patent base, for the creation of world-class products and processes that are based on the best tools used by the best inventors for the creation of their best ideas.

References

1. Garasomov, V.M., Litvin, S.S., “Basic Statements of the Technique for Performing VEA. Convolution and Super Effects,” Journal of TRIZ, 3, 2/92, pp. 7-45 (in Russian).
2. Savransky, S.D., Engineering of Creativity, Introduction to TRIZ Methodology of Inventive Problem Solving, CRC Press, 2000.
3. Tummala, R.R., Fundamentals of Micro System Packaging, McGraw-Hill, 2001.
4. Tummala, R.R., Microelectronics Packaging Handbook, Van Nostrand Reinhold, 1989.
5. Harman, G.H., Wire Bonding in Microelectronics, The International Society for Hybrid Microelectronics, 1989.