Today, in the majority of high production soldering of electronic component applications, belt furnaces are used. The typical length of a belt furnace exceeds 20 feet. Some of these high volume production processes require <200 ppm oxygen level, a nitrogen flush (to achieve such a low ppm oxygen level) and constant “power on” that keeps the oven heated at all times. This process significantly increases the costs of operation as compared to a vacuum reflow oven system. In addition, belt furnaces are known to produce a significant amount of voids – as much as 40% – within the solder joint, which decreases long-term reliability of electronic components.
In contrast, vacuum reflow oven systems have shown to significantly reduce void levels of all types of electronic components, typically to <5%. This is a significant reduction in void levels. As a batch process system, a vacuum reflow oven system only operates when needed. No constant heating and nitrogen gas flow is needed, which decreases cost of operation.
Soldering under vacuum is a technology that provides excellent results for all soldering applications. Vacuum is used in several stages during the process execution:
- One or two evacuations of chamber followed by filling with pure nitrogen gas help to remove any residual water and oxygen from the chamber.
- A diluted atmosphere during cleaning step enables faster transport of the formic acid molecules enabling in-depth cleaning of surfaces.
- Vacuum can be used to remove the residues after a cleaning step. This takes less time compared to purging the chamber with nitrogen.
- Using vacuum while the solder is in molten phase helps removing the voids.
- In some applications when hermetic encapsulation is required we need to cool down in vacuum.
Flux less Reflow Soldering Using Formic Acid
The possibility to use formic acid (HCOOH) in combination with nitrogen results in a cost-efficient and stable soldering process, reducing oxide film formation while no fluxing agent is needed in the process. The formic acid surface activation provides good results with respect to void rate across the surface. Formic acid vapours remove the oxide films, which inhibit wetting.
At temperature over 150-200°C the metal oxide film on the surface is reduced to clean metal surface. The gaseous products CO2 and water are simply purged with dry nitrogen and exhausted to the atmosphere.
Metal Oxide + HCOOH -> Metal + CO2 +H2O
Advantage of this process is following
- Safety engineering is less complex compared to processes using hydrogen.
- It is cost efficient.
- No requirements of fluxing agent.
- No oxidizing Surface.
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