The Power Electronic Soldering Process: An Evaluation of Soldering Materials and Basic Soldering Principles  
[PERSON]  
1 Semikron-Danfoss Slovakia, 922 O Vrbove, Slovakia; jan.sitar@semikron-danfoss.  
residues must be removed post-soldering due to their corrosive effects, which can lead to the long-term degradation of electronic components [6].  
Traditionally, rosin-based fluxes have been cleaned using halogenated or chlorofluocarbon solvents. While effective, these solvents pose environmental hazards, contributing to ozone depletion, and are classified as carcinogenic substances. Increasing regulatory restrictions on hazardous chemicals may lead to their phasing out in the near future. Consequently, researchers are exploring alternative flux formulations that minimize ecological impacts while maintaining soldering quality. Non-clean soldering pastes are designed to eliminate the need for post-soldering cleaning by allowing for flux residues to remain on the board. However, these formulations come with potential drawbacks, such as flux-sputtering and solder splashes, which can lead to unintended contamination and damage to sensitive components, including diodes and insulated-gate bipolar transistors (IGBTs). While non-clean fluxes reduce the risk of electrochemical migration, water-soluble fluxes tend to be more chemically active, necessitating thorough cleaning to ensure long-term reliability [7; 8; 9; 10; 11].  
Recent research has focused on the development of fluxless soldering technologies, an innovative approach with significant market potential. Fluxless soldering eliminates the need for a liquid flux by utilizing alternative oxide removal methods. Various reducing atmospheres, such as hydrogen gas, forming gas (a nitrogen-hydrogen mixture), and formic acid (HCOOH), are employed to achieve this aim [12; 13].  
While pure hydrogen is an effective reducing agent, its application at 300 \({}^{\circ}\)C is impractical for most soldering processes in the electronics industry. To circumvent this limitation, forming gas is used as an alternative. By diluting hydrogen with inert gases (primarily nitrogen), the hydrogen concentration remains below 5.7%--the threshold for spontaneous combustion. To ensure safe handling, gas suppliers typically provide forming gas with hydrogen concentrations of 4.0% or 5.0%. Another promising fluxless soldering technique involves the use of formic acid, a simple organic acid with efficient oxide removal capabilities. In its gaseous state, formic acid can adsorb onto metal surfaces and undergo decomposition into hydrogen (H) and formate ions (HCOO). At temperatures of approximately 200 \({}^{\circ}\)C, formate ions further decompose into hydrogen and carbon dioxide (CO\({}_{2}\)). The hydrogen generated from this reaction actively interacts with metal oxides, reducing them to metallic elements while producing CO\({}_{2}\) and water vapor (H\({}_{2}\)O) [14; 15; 16; 17; 18].  
Tin-based solders are the most commonly used alloys in the electronics industry. Their oxide films primarily consist of tin monoxide (SnO) and tin dioxide (SnO\({}_{2}\)). The chemical reaction between formic acid and tin oxides can be described as follows:  
\[\text{SnO}+2\text{HCOOH}\rightarrow\text{Sn}(\text{HCOO})_{2}+\text{H}_{2}\text {O} \tag{1}\]  
\[\text{SnO}_{2}+2\text{HCOOH}\rightarrow\text{Sn}(\text{HCOO})_{2}+2\text{CO} _{2}+\text{H}_{2} \tag{2}\]  
At temperature above 150 \({}^{\circ}\)C,  
\[\text{Sn}(\text{HCOO})_{2}\rightarrow\text{Sn}+2\text{CO}_{2}+\text{H}_{2} \tag{3}\]  
Formic acid becomes carbon dioxide and hydrogen in the thermal cleaning (reflow) process. By employing advanced fluxless soldering methods, manufacturers can enhance the efficiency, reduce the environmental hazards, and improve the reliability of electronic assemblies. This emerging technology holds great promise in the evolution of sustainable and high-performance soldering processes.