What are the characteristics of solder paste changes during soldering heating?

When in a heated environment,Solder pasteReflux is divided into five stages:

First, the solvent used to achieve the required viscosity and screen printing performance begins to evaporate, and the temperature rise must be slow, approximately per second3℃To limit boiling and splashing, prevent the formation of small tin beads. Also, some components are relatively sensitive to internal stress. If the external temperature of the component rises too fast, it will cause fracture.

When the flux is active, the chemical cleaning operation begins. Both water-soluble flux and no-clean flux will undergo the same cleaning operation, only at slightly different temperatures. Remove metal oxides and certain contaminants from the metal and solder particles that are about to be bonded. Good solder joints in metallurgy require a "clean" surface.

When the temperature continues to rise, the solder particles first melt individually and start the "lamp-like" process of liquefaction and surface tin absorption. In this way, it covers all possible surfaces and begins to form solder joints.

This stage is the most crucial. When all the individual solder particles have melted and combined to form liquid tin, the surface tension effect begins to form the surface of the solder joint. If the component pins are in contact withPCBThe gap of the solder pads exceeds4milIt is highly likely that the pins and pads will separate due to surface tension, resulting in an open circuit at the solder spots.

During the cooling stage, if the cooling is fast, the strength of the solder dots will be slightly greater, but it should not be too fast to cause temperature stress inside the components.

Summary of reflow soldering requirements:

It is important to have sufficient slow heating to safely evaporate the solvent, prevent the formation of solder beads and limit the internal stress of the component caused by temperature expansion, which may lead to reliability issues of cracking.

Secondly, the active stage of the flux must have an appropriate time and temperature, allowing the cleaning stage to be completed when the solder particles just start to melt.

The stage of solder melting in the time-temperature curve is the most crucial. It is essential to ensure that the solder particles are fully melted and liquefied to form a metallurgical weld. The evaporation of the remaining solvent and flux residue forms the surface of the solder joint. If this stage is too hot or lasts too long, it may affect the components andPCBCause harm.

The setting of the solder paste reflux temperature curve is best based on the data provided by the solder paste supplier. At the same time, the principle of internal temperature stress changes of the component should be grasped, that is, the heating temperature rise rate should be less than per second3℃And the cooling temperature drop rate is less than per second5℃.

PCBIf the size and weight of the assembly are very similar, the same temperature curve can be used.

It is important to check frequently or every day whether the temperature curve is correct.

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