Formic Acid Soldering: Enabling the Next Generation of Power Devices—Part 1

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

As power devices evolve for applications in electric vehicles, renewable energy, AI servers, and advanced industrial systems, packaging material selection is a critical factor to achieve design targets for performance and reliability.

Soldering remains the most widely used interconnect method for die attach, top-side die connections, and large-area joints such as substrate-to-baseplate. A broad range of alloys and configurations makes soldering versatile, but the reflow method can significantly impact feasibility, cost of ownership, and performance.

Why Formic Acid Soldering?

The fundamental objective of soldering in power device packaging is to form a thermomechanical bond between metallized assembly components, which is achieved by wetting the solder alloy to the base metals. Traditional soldering relies on flux – delivered with a solder paste, pre-applied on a preform, or separately dispensed – to reduce surface oxides and promote wetting.

While traditional soldering is effective, the use of flux can create challenges, such as flux residue causing electromigration, especially considering power device design trends toward high voltage. While cleaning can mitigate risk, it increases process steps, equipment needs, and consumables in production.

Formic acid (HCOOH) soldering is a technique that introduces formic acid vapor in to the reflow oven/chamber, creating a reducing atmosphere that effectively removes surface oxides and promotes good solder wettability without a flux. When activated in nitrogenrich atmospheres at about 180–250°C, formic acid decomposes into reactive species that reduce surface oxides, leaving behind metal surfaces. Byproducts such as CO₂ and H₂O are easily removed via vacuum. The result is an oxide-free, clean joint that requires no post-reflow cleaning.

Reaction Mechanism:

a) 2HCOOH + Metal Oxide (MeO) → Me(COOH)₂ + H₂O

b) Me(COOH)₂ → “Clean” Metal + 2CO2 + H₂

Figure 1. Reaction mechanism when using formic acid.

Figure 2. Formic acid (HCOOH) reaction with metal oxide. Image used courtesy of Bodo’s Power Systems [PDF]

Formic acid soldering has seen growing adoption in power electronics applications where reliability, thermal efficiency, and high yields are critical. There are multiple advantages to eliminating flux in the material and process flow:

• Improved cleanliness: Eliminate residues that compromise reliability in high-voltage designs or interact poorly with encapsulants.
• Reduced voiding: With vacuum, <1% voiding is achieved—an order of magnitude lower than flux-based soldering—delivering superior thermal conductivity and lower resistance.
• Streamlined process: Removes the need for cleaning, reducing cycle time, equipment, and factory footprint.
• Environmental benefit: Cuts energy and waste associated with solvent cleaning.

For Design Engineers, this means fewer trade-offs: robust joint integrity, solid thermal performance, high reliability, and simplified process flow to scale and meet next generation power device requirements.

Adoption Across Industries

Formic acid soldering has already moved beyond the lab and into high-volume production in industries like semiconductor packaging, LED manufacturing, power electronics, and high-performance computing. Semiconductor packaging like wafer bumping, flip-chip, and SiC/GaN modules remains a stronghold due to cleanliness and fine-pitch requirements.

LED manufacturing benefits from voidfree bonds for efficient thermal dissipation, which is critical in highpower lighting. In power electronics and power modules, leading players in the industry employ formic acid soldering for substratebaseplate attach today, while adoption is growing for die-level and package-cooler applications.

High-performance computing and AI systems are emerging adopters, leveraging ultra-low voiding and cleanliness to improve thermal management in advanced packaging.

It’s clear that formic acid reflow is a mainstream soldering technique, and with continued adoption comes a growing knowledge base of best practices for materials selection and process design.

Key Design Considerations

On the materials side, fluxless solder preforms are the standard. Solder preforms are engineered parts consisting of a solder alloy and can be fabricated in a wide variety of sizes and shapes through precision manufacturing for consistent solder volume.

While a formic acid atmosphere is highly effective at removing surface oxidation to promote wetting, it does not react with organics or other potential impurities that can inhibit solderability. Therefore, residue-free materials, a high degree of surface cleanliness, and high-purity alloy compositions are must-haves for design engineers when adopting a solder preform approach.

Additional materials technologies are emerging, including tacking agents and solder pastes designed specifically for formic acid reflow processes. In this case, application of the material is done via stencil printing or dispensing, therefore material rheology and application suitability are critical attributes.

The elimination of post-reflow residues is the key innovation driving the design of these materials, where the typical residue-forming components in fluxes such as rosins, activators, and other additives are replaced by solvents that evaporate during reflow.

Metrics such as thermogravimetric analysis confirm that these emerging materials technologies leave virtually zero residue after reflow, giving engineers confidence in downstream processes like wire bonding and encapsulation.

Reflow Process Considerations

Not all reflow systems are capable of soldering with formic acid. Conventional convection ovens may be retrofitted in limited cases, but purpose-built formic acid/vacuum systems are the norm.

Figure 3. Typical solder joint quality with formic acid/vacuum soldering. Image used courtesy of Bodo’s Power Systems [PDF]

Figure 4. Thermogravimetric analysis (TGA) for fluxless tacking agent. Image used courtesy of Bodo’s Power Systems [PDF]

Figure 5. Typical formic acid soldering profile: temperature, gasexchange, and vacuum.

Phase 1: oxygen removal. Phase 2: preheat under low oxygen.

Phase 3: soak under formic acid for oxide removal at 180-250°C.

Phase 4: reflow and vacuum for void removal.

Phase 5: cooling/solidification.

Image used courtesy of Bodo’s Power Systems [PDF]

Vendors such as PINK, budatec, Heller Industries, Centrotherm, ATV Technology, STT Vacuum Reflow Systems (Palomar® Technologies), 3S Silicon Tech, HVT, and others, provide formic acid-capable systems. Heating methods vary between conduction, convection, induction, or IR. Oven designs can be set up for batch processing or in-line processing, therefore equipment capabilities and throughput requirements are key considerations.

While basic parameters including temperature, ramp, and time above liquidus are still relevant with formic acid soldering, precision control over the reducing environment is important for success. Parameters such as formic acid concentration (typically 3–15 % in nitrogen), flow rate, soak temperature, and exposure time must be optimized.

Key phases in a typical process are shown in figure 5; these key phases include oxygen removal, preheating under low oxygen, soaking under formic acid for oxide removal (180-250°C), reflow and vacuum for void removal, and cooling/solidification.

Materials compatibility must also be considered: Cu, Ni, Ag-plated, and ceramic substrates each behave differently and may require adjustments to the formic acid soak parameters. However, a balance is important - excessive formic acid exposure can lead to other side effects, such as “tin steaming,” a cosmetic surface phenomenon seen with heavily oxidized parts.

While precise process design is essential, the performance and cost-of-ownership advantages of eliminating flux make formic acid soldering highly compelling. In Part 2 of this article, we will explore case studies showing how material innovations expand process windows and enable its use in next-generation power device designs.

This article originally appeared in Bodo’s Power Systems [PDF] magazine.