The soldering process involves the connection of two metal parts by means of solder (an alloy containing an active flux).
Soldering provides a mechanical connection of the terminals of the components to the contact pad, and also provides electrical contact.
The process of obtaining high-quality soldered joints involves the following steps:
1. Getting clean and shiny metal surfaces from the parts to be joined (removal of dirt and oxides) using a special flux;
2. Heating the solder above the melting point for a specific soldering time;
3. Activation of the flux by heating;
4. Uniform spreading of the molten solder on the surfaces of the parts to be joined;
5. Diffusion of atoms from the solid metal phase of the joined parts into the molten solder.
The soldering quality of the SMD component leads to the contact pads is characterized by a wetting angle
Active substances - fluxes
that are part of solder pastes, are activated on the surface of the parts to be brazed when heated in the preheating zone of the furnace and improve the solder wetting due to the surface tension force.
With insufficient surface tension force, the wetting angle increases, which leads to non-penetration and as a result - poor-quality solder joint. Therefore, only a properly selected thermoprofile
in a convection reflow oven for a specific flux can guarantee the correct activation of the flux and complete wetting and, as a result, the quality of the solder joint.
It is necessary to most accurately maintain this profile in the furnace according to the recommendations of the manufacturer of the solder paste, as well as depending on the size and heat capacity of the printed circuit board and the components on it.
Usually the temperature profile is selected by the technologist for each serial product.
The main stages
of soldering in convection reflow ovens:
: prevents thermal shock of SMD components and boards. Preheating speed 2-4 ºС / sec
stage activates flux
It is characterized by a slow rise in temperature. The flux is fully active when the temperature reaches 150 ºС
. The temperature above the melting point of the paste alloy is 30-40 ºС. This is necessary for reliable formation of the solder joint.
The profile peak is 205-225 ºС (on the board). Time above the melting point is 30-90 seconds
4. Cooling. Optimum cooling rate 3-4 ºС / sec to a temperature below 130 ºС