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Reflow soldering uses solder paste to attach components to a circuit board and heat the entire unit, creating permanent welds. It is the primary method for surface mount devices. Solder paste is made from flux and powdered solder, and is applied through dispensing or printing with a stencil. The reflow soldering process involves four temperature profiles, including preheating, oxidation removal, solder reflow, and cooling.
The reflow soldering process involves attaching components to metal pads on a circuit board with solder paste and then heating the entire unit. When even heat is applied to components and the circuit board, temporary connections can become permanent welds. Reflow soldering can be used with traditional through-hole technology, although it is the primary method for connecting surface mount devices (SMDs). The purpose of the reflow soldering process is to subject the circuit board and components to a uniform level of heat that will melt the solder paste without damaging the electronics. Reflow soldering typically includes four distinct stages, each involving a different level of heat.
Traditional soldering typically involves through-hole technology, where component leads are passed through a circuit board and then individually heated as solder is applied. This type of welding can be time consuming and applying excessive heat to a single component can be harmful. It’s also difficult or impossible to use traditional methods with surface mount technology (SMT), where every component sits on top of the circuit board.
Solder paste is a compound made from flux and powdered solder. In addition to acting as an oxidizing agent, the flux in reflow soldering can also help bond an SMD in place until heat is applied. The paste is sometimes applied through traditional dispensing methods, although it is often printed onto the board using a stencil to ensure proper placement. Problems with initial solder paste application can lead to device failure later on.
After the solder paste and components have been applied to a board, it is typically placed in a reflow oven and then subjected to four distinct temperature profiles. The reflow soldering process usually begins with an initial preheat, where the temperature will be raised between 1.0 and 3.0 degrees Celsius (about 1.8-5.4 degrees Fahrenheit) every second. This preheating is usually the longest of the four stages and can be instrumental in allowing the volatiles in the stream to evaporate without damaging the components from thermal shock. The second thermal stage typically lasts one to two minutes and can allow the flux to remove any oxidation from the board or components.
Solder reflow typically occurs during the third part of the heating and cooling process. This period may be known as the time above liquidus (TAL), as the solder melts when the maximum process temperature is reached. By this time, the metal pads on the circuit board and the leads on each SMD will have reached the same temperature, allowing strong solder bonds to form. After a certain period of time, the final cooling phase can begin. Allowing components to cool in a well controlled manner can prevent thermal shock and ensure a successful reflow soldering process.
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