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Fluxless TCB vs TCB

Fluxless thermocompression bonding (fluxless TCB) is an advanced packaging technique that builds upon standard TCB. As interconnection pitches shrink below 10µm for advanced logic and memory applications such as high-end CPUs, GPUs and high-bandwidth memory (HBM), standard TCB encounters issues with the flux, a chemical substance used to remove oxidation from bonding surfaces. Fluxless TCB marks an advancement vs TCB, eliminating the need for flux and improving the reliability of the interconnections.

Before delving into fluxless TCB, let’s first understand what standard TCB does, and how it solves the problems that flip-chip presents when interconnection pitches shrink beyond 50µm (microns).

In flip-chip bonding a die is “flipped” so that its solder bumps (also known as C4 bumps) align with the bond pads on the semiconductor substrate. The entire assembly is placed in a reflow oven and uniformly heated at around 200ºC-250ºC, depending on the solder material. The solder bumps melt, forming an electrical interconnection between the bond and substrate.

Flip-chip presents some challenges as interconnection density increases and pitches shrink beyond 50µm. Because the entire chip package goes into an oven, the die and the substrate can expand at different rates due to heat (a.k.a different coefficient of thermal expansion, CTE), creating deformations that result in faulty interconnections. The molten solder can then spread beyond its designated area. This phenomenon, known as solder bridging, results in unwanted electrical connections between adjacent pads and can create short circuits leading to a defective chip. This is where TCB steps in, as it can solve the problems that flip-chip presents when pitches shrink beyond certain point.

Fluxless TCB: No Flux Means Higher Reliability, But at a Higher Cost

The advantage of TCB is that heat is applied locally to the interconnections with heated tool heads, rather than uniformly in a reflow oven (flip-chip). This reduces the heat transfer to the substrate, resulting in lower thermal stress and CTE challenges leading to stronger interconnects. Pressure is applied to the die to enhance bond quality and achieve better interconnection. Typical process temperatures range between 150ºC-300ºC, with pressure levels ranging 10-200MPa. TCB allows for higher contact densities than flip-chip, reaching up to 10,000 contact points per mm2 in some cases. The main caveat of higher accuracy is lower throughput. While a flip-chip machine can reach a throughput of more than 10,000 dies per hour, TCB is more in the 1,000-3,000 range.

The standard TCB process also requires applying flux. During the heating process, copper can oxidate and cause faulty interconnections. Flux is a coating used to remove the copper oxidation. But when interconnection pitches shrink beyond 10µm flux gets more difficult to clean and can leave a sticky residue that can cause tiny deformations in the interconnections, creating corrosion and short circuits. This is where fluxless TCB steps in.

To eliminate the challenges associated with flux, fluxless TCB operates in a vacuum or inert gas environment, such as nitrogen or argon, to prevent oxidation during the bonding process. The absence of flux improves the long-term reliability of the interconnection, as it removes any risk of contamination that can result in faulty chip performance. The “flip side” is that it requires stricter process control and has lower throughput due to the extra steps involved.

Fluxless TCB applications include high-end CPUs, GPUs or vertically stacked HBM among others. It’s important to remember that different packaging techniques do not compete with each other, but are complementary. A single chiplet package will use a combination of 2.5D/3D packaging techniques such as flip-chip, TCB or hybrid bonding. If flip-chip is good enough for a certain interconnection, it makes no sense to go for a more advanced assembly technique, given flip-chip is cheaper and has higher throughput.

Standard TCB

Fluxless TCB

Flux required to remove oxidation in the bonding process.

No flux required; bonding is done in a vacuum, clean environment.

Usually appropriate for interconnection pitches of 10µm or more (although it can vary by application).

Appropriate for interconnection pitches below 10 microns (although it can vary by application).

Typical temperature ranges between 150ºC-300ºC.

Slightly lower temperature, as no flux activation is required.

Throughput normally ranges 1,000-3,000 dies per hour.

Lower throughput than Standard TCB due to vacuum/inert gas conditions.

Lower cost, higher throughput, lower precision.

Higher cost due to vacuum requirements. Lower throughput, higher precision.

Adoption emerging for 2.5D and 3D applications in advanced logic and HBM.

Well established

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Fluxless TCB vs TCB

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