Problems when soldering fine parts

Since the amount of solder printed is small and the flux used is small, most of the soldering of fine parts deteriorates in the preheat stage.
At the same time, the solder particles also oxidize, causing insufficient melting and unmelting of the solder.
There is a method to deal with it by increasing the amount of flux or flux with high heat resistance, but it still increases the risk of scattering, voids and potato solder.
 
Wetness experiment with copper oxide plate
 

 
Even with a copper plate that has been reflowed and oxidized about 4 to 5 times, wettability can be ensured unless the flux is deteriorated by preheating.
Flux deterioration is mainly affected by the hot air from the upper heater in the preheat stage.
 
This tendency is especially strong in large air reflows, where the solder tends to be simply melted and the flux effect is lost.
 
When the wettability of the solder becomes a problem, it is often said that the preheat is lengthened and the preheating is sufficient, but this is counterproductive.
 

 
It is not the waveform of the temperature profile that matters. Residual heat (preheat) is from room temperature to the melting point of the solder.
Wetness can be ensured if the flux is not deteriorated at the preheat stage. The length of the preheat part changes depending on the heat capacity of the board, but the reflow part is basically the same.
* In the flow, the substrate is immersed at 250 degrees and gets wet in less than 5 seconds. Even in reflow, it is sufficient if the solder is completely melted for 5 seconds.
* The waveform of the temperature profile is the temperature captured by the sensor fixed with high temperature solder or bond, not the actual solder temperature.
 
The heat of vaporization of the flux and the latent heat of melting are not displayed.
Rather than simply controlling the time and temperature of the temperature profile, it is necessary to create a pass / fail criterion by also observing the fillet part.
 
In large air reflow furnaces, it is necessary to keep the fan speed as low as possible to suppress hot air. Suppressing the air in the air reflow furnace goes against the characteristics of the equipment, but simply, equipment that melts solder is not suitable for fine soldering, and it is necessary to melt the solder without degrading the flux.
 
Solder makers use highly heat-resistant flux, but then the solvent does not vaporize and the residue causes voids.
 

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The technical information posted here was written by Mr. Kazuo Kawai, an implementation advisor, at the request of our company.
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