What are the characteristics of
solder paste changes during solder paste soldering heating? The following solder paste manufacturers will talk about it for you:
When the solder paste is always in a heated state, its reflow can be divided into five processes:
In the first step, apply up to the required viscosity and performance of the screen printing material. The solvent gradually begins to evaporate, and the surface temperature must be slow (about 3°C per second to limit boiling and splashing to prevent the formation of small tin beads. In addition, a part of the electrons The device is relatively sensitive to the structural stress, and if the external surface temperature of the electronic device is too fast, it will also cause fracture.
Flux activity, chemical cleaning uniform action gradually began, water-soluble flux and no-clean flux will have the same cleaning uniform action, but the operating temperature is relatively different. Removes metal oxides and some contamination from potentially tightly bound metal and solder particles. The metallurgy of the finish requires the integrity of the "clean" surface of the solder joint.
When the working temperature continues to rise, the first step of the granular solder melts alone, and gradually liquefies and absorbs tin on the surface of the "light grass" process. This allows for visible coverage over the entire workable surface and progressive solder joint formation.
It is very important at this stage. When the individual solder particles are all melted, they are tightly combined to form liquid tin. At this time, the surface pressure gradually begins to form the surface of the solder leg. If the gap between the electronic device pin and the PCB pad exceeds 4mil , it is very likely that the pin and the pad will be separated due to the surface pressure, that is, an open circuit of the tin point will be formed.
During the cooling process, if the cooling is fast, the strength of the tin point will be greater, but it must not be too fast to generate temperature stress on the electronic device itself.
Summary of Reflow Soldering Requirements:
The most important thing is to have better slow heating to safely evaporate the solvent to prevent the formation of tin beads and limit the structural stress of the electronic device due to the expansion of the operating temperature, forming fracture marks and reliability related issues.
Furthermore, the flux activity process should have an appropriate time operating temperature, and it is acceptable for the cleaning process to be successfully completed when the solder particles are just beginning to melt.
The stage of solder melting in the time-temperature curve is crucial. It is necessary to completely melt the solder particles in a reasonable manner, liquefy to form metallurgical welding, and evaporate the remaining solvent and flux residue to form the surface of the solder fillet. If this stage is too hot or too long, it may cause damage to electronic components and PCB.
The solder paste reflow temperature curve should be set according to the resources given by the solder paste manufacturer, and at the same time grasp the principle of temperature stress changes in the structure of electronic devices, that is, the heating temperature rise rate must be less than 3°C per second, and cooling The temperature drop rate must be less than 5°C.
If the PCB assembly is very similar in size and weight, you can choose the same temperature curve.
The most important thing is to check that the temperature profile is correct frequently or daily.
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