Reflow soldering is a crucial step in the SMT process. The temperature profile associated with reflow is an essential parameter to control to ensure proper connection of parts. The parameters of certain components will also directly affect the temperature profile chosen for that step in the process.
On a dual-track conveyor, boards with newly placed components pass through the hot and cold zones of the reflow oven. These steps are designed to precisely control the melting and cooling of the solder to fill the solder joints. The main temperature changes associated with the reflow profile can be divided into four phases/regions (listed below and illustrated hereafter):
1. Warm up
2. Constant heating
3. High temperature
4. Cooling
1. Preheating zone
The purpose of the preheat zone is to volatilize the low melting point solvents in the solder paste. The main components of flux in solder paste include resins, activators, viscosity modifiers and solvents. The role of the solvent is mainly as a carrier for the resin, with the additional function of ensuring sufficient storage of the solder paste. The preheating zone needs to volatilize the solvent, but the temperature rising slope must be controlled. Excessive heating rates can thermally stress the component, which can damage the component or reduce its performance/lifetime. Another side effect of too high a heating rate is that the solder paste can collapse and cause short circuits. This is especially true for solder pastes with high flux content.
2. Constant temperature zone
The setting of the constant temperature zone is mainly controlled within the parameters of the solder paste supplier and the heat capacity of the PCB. This stage has two functions. The first is to achieve a uniform temperature for the entire PCB board. This helps reduce the effects of thermal stress in the reflow area and limits other soldering defects such as larger volume component lift. Another important effect of this stage is that the flux in the solder paste begins to react aggressively, increasing the wettability (and surface energy) of the weldment surface. This ensures that the molten solder wets the soldering surface well. Due to the importance of this part of the process, soak time and temperature must be well controlled to ensure that the flux completely cleans the soldering surfaces and that the flux is not completely consumed before it reaches the reflow soldering process. It is necessary to retain the flux during the reflow phase as it facilitates the solder wetting process and prevents re-oxidation of the soldered surface.
3. High temperature zone:
The high temperature zone is where the complete melting and wetting reaction takes place where the intermetallic layer begins to form. After reaching the maximum temperature (above 217°C), the temperature starts to drop and falls below the return line, after which the solder solidifies. This part of the process also needs to be carefully controlled so that the temperature ramp up and down ramps do not subject the part to thermal shock. The maximum temperature in the reflow area is determined by the temperature resistance of temperature-sensitive components on the PCB. The time in the high temperature zone should be as short as possible to ensure that the components weld well, but not so long that the intermetallic layer becomes thicker. The ideal time in this zone is usually 30-60 seconds.
4. Cooling zone:
As part of the overall reflow soldering process, the importance of cooling zones is often overlooked. A good cooling process also plays a key role in the end result of the weld. A good solder joint should be bright and flat. If the cooling effect is not good, many problems will occur, such as component elevation, dark solder joints, uneven solder joint surfaces and thickening of the intermetallic compound layer. Therefore, reflow soldering must provide a good cooling profile, neither too fast nor too slow. Too slow and you get some of the aforementioned poor cooling issues. Cooling too quickly can cause thermal shock to the components.
Overall, the importance of the SMT reflow step cannot be underestimated. The process must be managed well for good results.
Post time: May-30-2023