Solder paste is used to temporary attach to the anywhere to all the contact pads, after which the assembly is subjected to the controlled heat. Solder paste reflows in a molten state, creating permanent solder joints. Heating can be accomplished by passing the assembly, through a reflow oven.
Usually four stages: preheat,thermal soak, reflow, heating
Preheat zone: climb towards to a target soak, dwell temperature.
Why lead free?
1. Health care
2. Environment: To align with RoHS with the European Union
Tin frequently serves as a principle solder due to its low melting point and excellent wetting properties. Cu & Ag are sometimes added to enhance mechanical strength, and electrical properties of the solder.
-->Lead based solder: Tin-60%, Pb-40%, melting point:188oC
-->Lead free Solder: Sn-Ag-Cu, >95% Tin, melting point: 227oC, but less ductile than lead based solder, rendering it has susceptible to mechanical stress, tends to be oxidize more rapidly which can adversely affect its wetting properties and solder joint quality.
What is soldering
A process involves solder, to join together metal surfaces. The solder is heated until it melts and flows into the joint between the two surfaces. Upon cooling, it hardens and forms a reliable electrical and mechanical connection.
Before soldering, the surface of metal should be cleaned thoroughly to remove any oxidations or contaminations that could interfere the formation of a good solder joint.
Vaccum effectively reduce solder void in soldering process
