Traditionally, reflow soldering has been used with liquid flux additives to further increase the solder’s wettability to metals with high oxide layers. However, using flux during soldering can cause defects or problems.
Voids
Since all fluxes contain liquid components, they can easily degas and evaporate at the high temperatures of the soldering process. This outgassing is the cause of voids (trapped gas) between the two soldering surfaces. A fact that can be seen when soldering high-power semiconductors is that heat transfer during the chip mounting process is critical to the performance of the chip and products. Voids can cause localized hot spots on the chip surface, leading to stress and fatigue cracks. While adding soldering under vacuum further reduces voiding, it’s still not ideal.
Flux Residue
Soldering with flux naturally leaves a residue that can then be removed and cleaned of the part. Subsequent processes such as wire bonding require clean parts to be free of contamination, so cleanliness is critical. Flux residue is known to react with water vapor to form an acidic solution on the surface of the part, which can affect the long-term reliability of the equipment.
Fluxless Reflow
The ideal solution is to perform the soldering process in a flux-free environment. Soldering in a 100% hydrogen environment is a fluxless soldering method used to remove surface oxides, which increases the risk of explosions and is dangerous; the equipment required needs to be ATEX certified. Molding gases (a mixture of nitrogen and hydrogen, 90% and 10% respectively) are safer, but effective temperatures at or above 350°C and are not compatible with low-melting solder.
Formic Acid Reflow
A suitable alternative to fluxless soldering at lower temperatures is solder reflow under formic acid (HCOOH) vapor. The vapor chemically reacts with metal oxides at lower temperatures (150–160°C) to form format; increasing the temperature even further breaks the format into hydrogen, water, and carbon dioxide. When used in conjunction with a vacuum reflow soldering system, these gases and vapors can be removed by the vacuum system.
Typical formic acid vacuum solder reflow, after two vacuum stages with nitrogen refill, leaves the chamber free of air and oxygen. The temperature increases with the introduction of formic acid vapor, (nitrogen serves as a carrier for the formic acid vapor) and stops at 160°C. The temperature is further increased to 220°C and held to provide time for solder reflow and oxide removal, then the chamber is purged with nitrogen and evacuated to remove all voids.
Formic acid reflow is a proven method of fluxless soldering and is also a very flexible process because the oxide removal properties of formic acid vapor are effective at lower temperatures. It eliminates the need for pre-reflow flux and post-reflow flux removal. And because of the corrosive nature of formic acid, it leaves exposed metal surfaces suitable for further diffusion processes, such as wire bonding.