Type 7 (T7) solder paste, with a powder diameter of only 2–11 μm (core range 4–9 μm), is one of the highest-precision solder paste grades used in advanced packaging today. As CPO (co-packaged optics) moves from R&D into volume production in this critical window, T7 solder paste is no longer an "option" — it is a hard requirement.

Role 1: Replacing "ball placement" with "printing" to reshape bump fabrication

 In CPO packaging, fabricating tiny bumps is a critical step, yet conventional ball placement or electroplating processes are costly and complex. The arrival of T7 solder paste has changed this.

Take Heraeus's Welco AP520 as an example: it prints bumps directly on the wafer in a single pass, with consistent height. This "subtractive" process not only simplifies the flow but also delivers a cost advantage of more than 15%, and it is already in volume application at RF customers.

 On the domestic side, Shenzhen Fitech can likewise supply ultra-fine solder pastes for flip-chip micro-bump applications, forming solder bumps by stencil printing; after reflow the joints are uniform in size and highly consistent in bump height. Its full T6–T9 ultra-fine solder paste series supports precision printing and is widely used in microelectronics and semiconductor packaging. The T7-grade SnCuX lead-free, silver-free ultra-fine solder in particular offers good powder sphericity and a narrow particle size distribution, making it suitable for high-precision bump fabrication. In addition, Fitech is the only company in the world with mass-production technology for ultra-fine solder powder across the full T2–T10 size range. Its proprietary liquid-phase powder-forming process yields powder that is almost 100% spherical, delivering excellent release and transfer efficiency in ultra-fine-pitch printing, which further consolidates its core competitiveness in high-precision bump fabrication.

Role 2: Precisely compensating warpage to overcome "open-circuit" failures

 In CPO packaging, when the μ-bumps of the photonic integrated circuit (PIC) and the electronic integrated circuit (EIC) are stacked, differences in materials and structures make warpage highly likely, which leads to "open" electrical connections after reflow.

T7 solder paste has high viscosity and a precisely controlled printed volume; like a "glue," it automatically fills the gaps created by warpage and achieves a reliable connection. This is difficult for coarser pastes such as T5/T6. Heraeus WS5112 T7 has been validated to solve this problem effectively and keep the signal path clear.

On the domestic side, Shenzhen Fitech also has the core technical capability to address this issue. Through in-house low-temperature soldering processes (such as the FL170/200 series low-temperature solder pastes, melting point 137–145 °C), Fitech can lower the peak soldering temperature by 60–80 °C, significantly reducing damage to heat-sensitive components and effectively bringing the warpage rate down. This approach mitigates warpage at its source and complements the "gap-filling" route offered by T7 solder paste.

 Role 3: Meeting zero-defect requirements for optical devices and pioneering water-wash processes

Optical devices in optical modules (TOSA/ROSA) are extremely sensitive to ionic residue; residue left by conventional no-clean solder pastes poses a corrosion risk.

T7 solder paste has given rise to water-wash processes. Shenzhen Fitech's ultra-fine water-soluble solder paste FWS3057 is the first to achieve a zero-halogen L0 formulation while maintaining soldering performance and process life. After soldering with T7 paste, DI water can be used for thorough cleaning, eliminating corrosion risk while meeting RoHS and halogen-free environmental requirements — a must-have solution for the optical communications field. Fitech presented this product series at the 2025 International Conference on Electronic Packaging Technology (ICEPT), targeting SiP (system-in-package) applications; with three hard-won capabilities — zero halogen, no spatter or solder balls, and cleanability with DI water alone — it resolves the residue-removal challenges of high-density assembly one by one. Fitech also offers ultra-fine solder pastes across the full T4–T8 range (including T7), all available in water-wash and no-clean formulations. Its lead-free, silver-free SnCuX and SnSb series solders carry no silver-migration risk and produce bright joints with good wetting, meeting the stringent requirements of optical devices for different temperature ranges, ultra-high cleanliness and long-term reliability.

Role 4: One-pass printing eliminates an entire process step

Conventional packaging flows involve multiple process steps, whereas the one-pass printing capability of T7 solder paste can simplify the system-in-package (SiP) flow.

With T7 solder paste (such as Fitech FTP3057), the pads of flip chips and surface-mount devices can be printed in a single integrated pass, consolidating several steps into one and greatly simplifying SiP processing.

On the strength of these technical advantages, T7 solder paste has become a core material for frontier applications such as CPO, Mini/Micro-LED, 5G RF modules and AI liquid-cooled servers. The technical barrier is extremely high: the finer the powder, the harder it is to make, with difficulty rising geometrically, and problems such as easy oxidation and spatter must also be solved. Today, fewer than five companies worldwide can reliably mass-produce T7-grade solder paste. Chinese companies, however, are catching up fast in the T7 field, with strong momentum:

Supply-chain self-reliance: Upstream, in materials, Shenzhen Fitech holds about a 50% domestic market share in microelectronics ultra-fine tin-based solder powder — No. 1 in China. Downstream, in finished products, Vital New Material's T7-grade solder paste has entered mass production and is under validation at leading optical module makers. Shenzhen Fitech also holds the world's only mass-production technology for ultra-fine solder powder across the full T2–T10 range, and its T7-and-above solder pastes have achieved high-precision printing in CPO packaging.

 Market share: As domestic substitution accelerates, Chinese companies are expected to keep winning share from foreign suppliers, starting from their current level of about 30%.

Conclusion

As optical module speeds advance from 1.6T to 3.2T, demand for T7 solder paste is expected to multiply. Its pricing differs from that of ordinary solder paste: the technical barrier is high and value added is high. T7 solder paste is therefore not merely a supporting material for CPO packaging — it is a core "engine" driving the technology toward maturity and commercialization.