What are the characteristics of the change of
solder paste in the process of soldering heating?
First, the solvent used to achieve the desired viscosity and screen printing properties begins to evaporate and the temperature rise must be slow (about 3°C per second to limit boiling and splashing and prevent the formation of small tin beads, also, some components are sensitive to internal stress, If the temperature outside the component rises too quickly, it can cause breakage.
The flux is active, the chemical cleaning action begins, and the same cleaning action occurs for both water-soluble and no-clean fluxes, but at slightly different temperatures. Remove metal oxides and some contamination from the metal and solder particles to be bonded.
This stage is the most important. If the gap between the component pin and the PCB pad exceeds 4 mil, it is most likely due to surface tension. Separating the pins and pads creates an open tin point.
In the cooling stage, if the cooling is fast, the strength of the tin point will be slightly larger, but it cannot be too fast to cause temperature stress inside the component.
Summary of reflow soldering requirements:
It is important to have sufficient slow heating to safely evaporate the solvent, prevent tin bead formation and limit internal stress on the component due to temperature expansion, which can cause breakline reliability issues.
Second, the active phase of the flux must have an appropriate time and temperature to allow the cleaning phase to complete when the solder particles are just starting to melt.
The melting stage of the solder in the time temperature curve is the most important. It must be sufficient to allow the solder particles to completely melt, liquefy to form metallurgical welding, and evaporate the residual solvent and flux residue to form the surface of the solder leg.
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