Relationship between solder paste characteristics and reflow temperature curve

The characteristics of solder paste determine the basic characteristics of reflow curve. Different solder pastes have different chemical compositions, so their chemical changes have different temperature requirements and different requirements for reflow temperature curve. Generally, solder paste suppliers can provide a reference reflow curve, which can be optimized by users according to their own product characteristics. It can be divided into four main stages:

1) Heat the PCB board to about 150 ℃, and the rising slope is 1-3 ℃ / s. Preheat stage;

2) Slowly heat the whole board to 183 ℃. It is called soaking or equilibrium stage. The time is generally 60-90 seconds.

3) Heat the board to the melting area (above 183 ℃) to melt the solder paste. It is called reflow spike stage. In the reflux stage, the board reaches a high temperature, generally 215 ℃ + / - 10 ℃. The reflux time should be 45-60 seconds, not more than 90 seconds.

4) The process by which the curve drops from a high temperature point. It is called cooling stage. Generally, the slope of cooling is required to be 2 - 4 ℃ / s.

Characteristic change of solder paste in reflow preheating stage:

The preheating stage of reflow soldering is to volatilize the solvent with lower melting point in the solder paste. The main components of flux in solder paste include rosin, activator, viscosity improver, and solvent. Solvent is mainly used as the carrier of rosin and to ensure the storage time of solder paste. In the preheating stage, too much solvent needs to be volatilized, but the temperature rise slope must be controlled. Too high temperature rise speed will cause thermal stress impact of components, damage components or reduce component performance and service life. The latter will bring greater harm, because the products have flowed into the hands of customers. Another reason is that too high heating rate will cause the collapse of solder paste and the risk of short circuit, especially for solder paste with high flux content (up to 10%).

Characteristic change of solder paste in reflow soaking stage:

The setting of reflow soaking stage shall mainly refer to the suggestions of solder paste supplier and the heat capacity of PCB. Because the soaking stage has two functions, one is to make the whole PCB reach a uniform temperature. The purpose of soaking is to reduce the thermal stress impact entering the reflow area and other welding defects, such as component warping, cold welding of some large components, etc. Another important role in the soaking stage is that the flux in the solder paste begins to have an active reaction to increase the wettability (and surface energy) of the weldment surface, so that the melted solder can well wet the weldment surface. Due to the importance of soaking section, the soaking time and temperature must be well controlled to ensure that the flux can clean the welding surface well and that the flux is not completely consumed before reflow. It is necessary to retain the flux until the reflow stage. It can promote the solder wetting process and prevent the reoxidation of the welding surface. In particular, with the increasing use of low residue and no clean solder paste technology, the activity of solder paste is not very strong, and the reflow soldering is mostly air reflow soldering. It should be noted that the flux cannot be consumed in the soaking stage.

Characteristic change of solder paste in reflow stage of reflow soldering:

As the temperature continues to rise, the solder paste melts and undergoes wetting reaction over the reflux line, and the intermetallic compound layer begins to form. Reach the high temperature, then start cooling, fall below the reflux line, and the solder solidifies. The rising and falling slope of temperature shall also be considered in the reflux area, and the element shall not be subjected to thermal shock. The high temperature of the reflow zone is determined by the temperature resistance of the temperature sensitive element on the PCB board. The time in the reflow zone should be as short as possible on the premise of ensuring that the components complete good welding, generally 30-60 seconds. Too long reflow time and high temperature, such as reflow time greater than 90 seconds, will cause the thickening of the intermetallic compound layer and affect the long-term reliability of the solder joint.

Characteristic change of solder paste in reflow cooling stage:

The importance of reflow cooling stage is often ignored. A good cooling process also plays a key role in the post welding results. Good solder joints should be bright and smooth. If the cooling effect is not good, there will be many problems, such as component warping, dark solder joint, unsmooth solder joint surface, thickening of intermetallic compound layer and so on. Therefore, reflow welding must provide a good cooling curve, neither too slow to cause poor cooling, nor too fast to cause thermal shock of components.