During last year's chip shortage, from Samsung, TSMC to Intel and AMD, all paid close attention to a material, that is ABF (Ajinomoto Build-up Film). And the story of all this, it all starts with a company originally engaged in the production of monosodium glutamate, Ajinomoto.

According to the Ajinomoto Group's website, in the 1970s, the group began to explore the application of by-products of umami seasonings. We know that some of these substances have excellent material properties and may be used as resins and coatings in the electronics industry. Processors are becoming smaller and faster, and printed circuit board manufacturers need better insulating materials to maintain performance. By 1996, a CPU manufacturer contacted the group, hoping to use amino acid technology to develop a thin film type of insulator. This ultimately promoted the birth of ABF substrates.

As everyone can see, the advancement of circuit integration has made it possible for CPUs composed of nanoscale electronic circuits. These circuits must be connected to millimeter-scale electronic components in electronic devices and systems. Of course, this can be achieved by using the stacked substrate of the CPU composed of multi-layer microcircuits. And ABF promotes the formation of these millimeter-scale circuits because its surface can accept laser processing and direct copper plating. Today, ABF is the basic material for forming circuits that can guide electrons from nanoscale CPU terminals to millimeter-scale terminals on printed substrates.

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It can be said that the existence of ABF has made chip miniaturization possible. However, with the continuous development of chips, ABF has also ushered in new challenges.

ABF, as if facing a major enemy, plays the role of a bed inside the device package, connecting the multi-layer micro-circuits of the PCB and the nanoscale CPU.

And a key component of the ABF substrate built on it is the capacitor, which is mainly used for decoupling and occupies both sides of the substrate.

Anandtech also reported that modern chips are usually mounted on fine-pitch substrates (FPS: fine pitch substrates), and then placed on multi-layer high-density interconnect (HDI: high-density interconnect) substrates. And now the most advanced CPU/GPU HDI substrates all use Ajinomoto Build-up Film (ABF), which combines organic epoxy resins, hardeners, and inorganic micro-particle fillers. ABF is easy to use, can achieve high-density spacing (thus achieving high-density metal wiring), and has enough insulation performance, high rigidity, high durability, and low thermal expansion to meet the needs of modern chips.

Zhuang Guiyi from the Materials and Chemical Research Institute of Taiwan's Industrial Technology Research Institute also pointed out in his article that ABF substrate materials were materials led by Intel in the 1990s, used to introduce advanced substrate production such as flip-chip packaging process, which can produce finer lines, suitable for high pin count, high transmission IC packaging. The core structure of its substrate is still retained with glass fiber cloth pre-impregnated with resin (FR-5 or BT resin) as the core layer (Core Substrate), and then using the method of stacking build-up materials (Build up Materials) to increase the number of layers, based on the double-sided core, doing the upper and lower symmetrical layering, but the upper and lower build-up structures, abandon the original pre-impregnated glass fiber cloth laminated copper foil substrate, and replace it with electroplated copper on the ABF film layer, as shown in the figure. In this way, the overall thickness of the substrate can be reduced, breaking through the difficulties encountered by the original glass fiber resin substrate in laser drilling.

However, as the industry shifts to small chip design, the importance of packaging is increasing, which in turn puts forward new demands for packaging materials."Because these multi-chip designs will consume more power (and thus be hotter), and due to the expansion of memory and I/O interfaces, higher density metal writing is required. The increase in power requirements puts additional pressure on the peripheral substructures of the circuit. For many years, the search for new materials for the core composition of semiconductor industry chips has been a hot topic," AnandTech reported. In this case, glass has become a new target for many manufacturers to explore, as glass is considered to be more robust than organic resin-based substrates and has multiple advantages, but the adhesion between glass and copper (or other metal wires) remains a major challenge in bonding.

However, many manufacturers have already taken an important step forward.

Can glass take over?

Recently, Japan's Dai Nippon Printing (DNP) demonstrated a new development in semiconductor packaging - Glass Core Substrate (GCS) - which is said to solve many of the problems brought by ABF.

DNP claims that its HDI substrate with a glass core has superior performance compared to organic resin-based substrates. According to Dai Nippon, using a glass core substrate (GCS) can achieve finer pitch, thus achieving extremely dense wiring, because it is harder and less prone to expansion due to high temperatures. The schematic shown by DNP even completely omits the fine-pitch substrate from the package, implying that this part may no longer be needed.

DNP also stated in the report that its glass core substrate can provide a high aspect ratio of high glass through-hole vias (TGV) density (compatible with FPS). In this case, the aspect ratio is the ratio between the thickness of the glass and the diameter of the via. As the number of through-holes increases and the ratio increases, the processing of the substrate becomes more difficult, and maintaining rigidity becomes more challenging.

From DNP's introduction, it can be seen that the glass substrate it has developed has an aspect ratio of 9 and ensures adhesion to achieve fine-pitch compatible wiring. The company said that due to the few thickness limitations of GCS, there is a great deal of freedom in maintaining a balance between thickness, warpage, rigidity, and smoothness. "We also have new proprietary manufacturing methods that enhance the adhesion between glass and metal, which is difficult to achieve with traditional technology, and this also helps them achieve fine pitch and high reliability," DNP also emphasized.

In addition to DNP, Absolics, a subsidiary of South Korea's SK Group, also sees opportunities brought by glass. Because they believe that glass has a high thermal resistance, they regard it as a reformer of semiconductor packaging. Absolics said that as the performance improvement of microprocessors has reached its limit, the semiconductor industry is actively using heterogeneous packaging, but existing semiconductor substrates must be connected to semiconductor chips through an intermediate substrate called a silicon interposer, and glass substrates with built-in passive components can integrate more chips at the same size, and power consumption is also reduced by half. It is worth mentioning that Absolics also received investment from the American equipment giant Applied Materials earlier.

In addition, the glass giant Corning also sees opportunities for glass in substrates.In a paper they wrote, new initiatives in semiconductor packaging have created a demand for new material solutions. Tremendous efforts have been made to extend the interposer technology used for 3D-IC stacking. A variety of solutions are being developed to meet some of these needs, including the use of traditional interposers made from various common materials and fan-out wafer-level packaging (FOWLP), which has become a widely considered factor in attempts to achieve low cost.

In addition, the surge in mobile devices and the Internet of Things (IoT) has led to increasingly demanding requirements for RF communication. These requirements include the introduction of more frequency bands, smaller/thinner packaging sizes, and the need to save power to extend battery life when introducing new features. Glass has proven to be an excellent solution to these challenges, as it has many properties that support the aforementioned initiatives, including high resistivity and low electrical loss, low or adjustable dielectric constant, and adjustable coefficient of thermal expansion (CTE).

Corning stated that one of the important challenges of 3D IC stacking is reliability due to CTE mismatch, and glass provides an excellent opportunity to manage warpage in 3D-IC stacking while optimizing CTE. The following figure illustrates the challenges faced by the substrate with multiple CTEs in the interposer application. The left image schematically shows the Si chip installed on the Si interposer, which is then installed on the organic substrate. When the substrate undergoes temperature cycling, CTE mismatch can lead to failure.

However, if a glass interposer with a CTE between glass and organic materials is used instead of the Si interposer, this warpage can be better managed and reliability can be improved, as demonstrated by the work of the Packaging Research Center (PRC) at Georgia Tech, as shown in the right image of the above figure.

In conclusion, we must acknowledge that the position of ABF substrates cannot be shaken in the short term, and we can also see this from the statistics and forecasts of QYR. According to their statistics, the global ABF substrate market sales in 2021 reached 4.368 billion US dollars, and it is expected to reach 6.529 billion US dollars in 2028, with a compound annual growth rate (CAGR) of 5.56% (2022-2028).

The substantial investment by manufacturers such as Intel and AMD can also be seen as a weathervane for ABF.

Taking Intel as an example, last year, the shortage of ABF caused trouble for Intel. As a result, Intel announced that its Vietnam Assembly and Test (VNAT) factory will now internally connect capacitors to both sides of the ABF substrate. This change will effectively reduce Intel's dependence on external suppliers in the ABF manufacturing process. According to Intel, the result is the ability to complete chip packaging at a speed of 80% faster; AMD has also secured long-term contracts with multiple manufacturers to ensure ABF supply.

However, as is always the case in this industry, nothing remains unchanged.