PCBA Tech - SMT Factory Reflow Temperature Curve

The characteristics of the solder paste determine the basic characteristics of the reflow temperature profile. Due to the different chemical composition of alloy solder powder and flux, different solder pastes have different requirements for temperature and reflow temperature curve due to their chemical changes. Generally, solder paste suppliers can provide a reference reflow profile,and users can optimize based on their product characteristics on this basis. Take the lead-free solder paste Sn96.3 Ag3.2Cu0.5 with a melting point of 217 °C as an example to introduce two typical reflow temp profiles.

Conventional temperature curve

The conventional temperature curve is divided into four main stages: preheating zone,heat preservation zone, reflow zone and cooling zone. This kind of temperature profile has a heat preservation time during heating,so the surface temperature of SMA is relatively uniform,even if the PCB components are not uniform in size and assembled When the density is relatively large, the surface temperature of the SMA is still relatively uniform.Therefore,this temperature curve is required when the size of the components on the PCB is uneven and the assembly density is relatively high.

(1) Warm-up stage.

The pcb reflow temp is heated to 150 and the heating rate is less than 2 T/s, which is called preheat. The purpose of the preheating stage is to volatilize the lower melting point solvent in the solder paste.The main components of flux in solder paste include rosin, activator, viscosity improver and solvent. The role of the solvent is mainly to act as a carrier of rosin and to ensure the storage time of the solder paste. The preheating stage needs to volatilize too much solvent, but the heating rate must be controlled. Too high a heating rate will cause thermal stress impact of the component, damage the component or reduce the performance and life of the component, the latter brings greater harm. Another reason is that a too high heating rate will cause the solder paste to collapse and cause the danger of short circuits, and a too high heating rate will cause the solvent to evaporate too quickly, and it is easy to splash out metal components and cause tin beads.

(2) Insulation stage. 

Slowly heat the entire board to 170 so that the circuit board reaches a uniform temperature,which is called the soak or equilibrium phase.The time is generally 70 ~ 120 s. At this stage, the temperature rises slowly.The setting of the heat preservation stage should mainly refer to the recommendations of the solder paste supplier and the heat capacity of the PCB board. The heat preservation stage has three functions.One is to make the entire PCB substrate reach a uniform temperature,reducing the thermal stress impact that enters the reflow zone, and other soldering defects,such as component lifting,etc.;the other is that the flux in the solder paste begins to activate.The reaction increases the wettability of the surface of the weldment, so that the molten solder can well wet the surface of the weldment.The third is to further volatilize the solvent in the flux.Due to the importance of the heat preservation stage, the time and temperature of the heat preservation must be effectively controlled.It is necessary to ensure that the flux can clean the soldering surface well,but also to ensure that the flux is not completely consumed before reaching the reflow, which can prevent it during the reflow stage. The role of re-oxidation.

Mainboard PCBA processing

Reflow stage. Heat the board to the melting zone to melt the solder paste, and the board reaches the highest temperature, generally 230 ~ 245 none, called reflow stage (reflow) The time above 0 liquidus is generally 30 to 60 so the reflow stage temperature continues to rise and cross reflow Wire, the solder paste melts and a wetting reaction occurs, the intermetallic compound layer begins to form, and finally reaches the peak temperature. The peak temperature of the reflow stage is determined by the chemical composition of the solder paste, the characteristics of the components and the PCB material. If the peak temperature is too high during the reflow phase, the circuit board may be burned or scorched; if the peak temperature is too low, the solder joints will appear dark and grainy. Therefore, the peak temperature of this temperature zone should be high enough to make the flux fully effective and have good wettability, but it should not be high enough to cause damage, discoloration or scorching of components or circuit boards. In the reflow phase, the temperature rise slope should be considered, and the components should not be subjected to thermal shock. The reflow time should be as short as possible under the premise of ensuring good soldering of the components, generally 30~60 s is the best. Too long reflow time and higher temperature will damage the components that are susceptible to temperature, and will also cause the intermetallic compound layer to be too thick, which will make the solder joints very brittle and reduce the fatigue resistance of the solder joints.

Cooling stage. The process of temperature drop is called cooling stage, and the cooling rate is 3~5. The importance of the cooling phase is often overlooked. A good cooling process also plays a key role in the final result of welding.Faster cooling rate can refine the microstructure of solder joints, change the morphology and distribution of intermetallic compounds, and improve the mechanical properties of solder alloys.For lead-free soldering in actual production,without adversely affecting the components, an increase in the cooling rate can usually reduce defects and improve reliability. However, a too fast cooling rate will cause impact and stress concentration on the components and cause the product's solder joints to fail prematurely during use. Therefore, reflow soldering must provide a good cooling curve.

Pentium-like temperature curve

The tent-shaped reflow temperature curve is divided into the main stages of heating zone, preheating zone, rapid heating zone, recirculation zone, and cooling zone. When using this temperature curve, the heating rate of SMA from room temperature to peak temperature is basically the same, and the thermal stress on SMA Small; but when the components on the PCB to be welded are uneven, the surface temperature of the SMA is not uniform enough; the welding temperature of the components with large mass and high heat absorption cannot meet the requirements. Therefore, this temperature curve is mainly suitable for occasions where the size of the components on the PCB is relatively uniform.