When PCBA solder paste is in a heated environment, PCBA paste reflow is divided into five phases.
First, the solvent used to achieve the desired viscosity and screen printing performance starts to evaporate, the temperature rise must be slow (approximately 3°C per second) to limit boiling and splashing, prevent the formation of small tin beads, and some components are sensitive for internal stresses.if the temperature outside the device rises too fast, it will cause breakage.
The flux is active and the chemical cleaning action begins. Both the water-soluble flux and the no-clean flux will undergo the same cleaning action, except that the temperature is slightly different. Metal oxides and some contamination are removed from the soon-to-be-bonded metal and solder particles. Good metallurgy solder joints require a "clean" surface.
As the temperature continues to rise, the solder particles first melt individually and begin the process of liquefaction and surface absorption of tin. This covers all possible surfaces and begins to form solder joints.
This phase is the most important. When the individual solder particles are completely melted and combined together to form liquid tin, the surface tension begins to form the solder fillet surface. If the gap between the component leads and the PCB pad exceeds 4 mils.Detaching the pins and pads that caused by surface tension will cause the tin spot to open.
During the cooling phase, if the cooling is fast, the tin spot strength will be slightly larger, but it may not be too fast and cause temperature stress inside the component.
Reflow soldering requirements summary:
It is important that there is sufficient slow heating to safely evaporate the solvent, to prevent the formation of tin beads and to limit the internal stress of the element due to temperature expansion, resulting in fracture mark reliability problems.
Second, the active phase of the flux must have the proper time and temperature, allowing the cleaning phase to be completed just before the solder particles begin to melt.
The melting phase of the solder in the time temperature curve is the most important. The solder particles must be fully melted, liquefied to form a metallurgical solder, and residual solvent and flux residues evaporate to form the solder fillet surface. If this stage is too hot or too long, it may cause damage to the components and the PCB.
PCBA solder paste reflow temperature curve setting is best based on data provided by the PCBA solder paste supplier, while grasping the principle of internal component temperature stress change, that is, the heating temperature rise speed is less than 3°C per second, and cooling temperature drop rate Less than 5° C.
PCB assembly can use the same temperature curve if the size and weight are similar.
It is important to check the temperature profile regularly and even daily.
Contact: Lillian Zhu
Add: Hongtian industry,baoan area,shenzhen city,guangdong ,china