Typical Sn96.5Ag3.0Cu0.5 alloy traditional lead-free reflow soldering temperature curve. A is the heating area, B is the constant temperature area (wetting area), and C is the tin melting area. After 260S is the cooling zone.
Sn96.5Ag3.0Cu0.5 alloy traditional lead-free reflow soldering temperature curve
The purpose of heating zone A is to quickly heat up the PCB board to the flux activation temperature. The temperature rises from room temperature to about 150°C in about 45-60 seconds, and the slope should be between 1 and 3. If the temperature rises too fast, it may collapse and lead to defects such as solder beads and bridging.
Constant temperature zone B, the temperature rises gently from 150°C to 190°C. The time is based on specific product requirements and is controlled at about 60 to 120 seconds to give full play to the activity of the flux solvent and remove oxides from the welding surface. If the time is too long, excessive activation may occur, affecting the welding quality. At this stage, the active agent in the flux solvent begins to work, and the rosin resin begins to soften and flow. The active agent diffuses and infiltrates with the rosin resin on the PCB pad and the soldering end surface of the part, and interacts with the surface oxide of the pad and part soldering surface. Reaction, cleaning the surface to be welded and removing impurities. At the same time, the rosin resin rapidly expands to form a protective film on the outer layer of the welding surface and isolates it from contact with external gas, protecting the welding surface from oxidation. The purpose of setting sufficient constant temperature time is to allow the PCB pad and the parts to reach the same temperature before reflow soldering and reduce the temperature difference, because the heat absorption capabilities of different parts mounted on the PCB are very different. Prevent quality problems caused by temperature imbalance during reflow, such as tombstones, false soldering, etc. If the constant temperature zone heats up too quickly, the flux in the solder paste will rapidly expand and volatilize, causing various quality problems such as pores, blown tin, and tin beads. If the constant temperature time is too long, the flux solvent will evaporate excessively and lose its activity and protective function during reflow soldering, resulting in a series of adverse consequences such as virtual soldering, blackened solder joint residues, and dull solder joints. In actual production, the constant temperature time should be set according to the characteristics of the actual product and lead-free solder paste.
The appropriate time for soldering zone C is 30 to 60 seconds. Too short a tin melting time may cause defects such as weak soldering, while too long a time may cause excess dielectric metal or darken the solder joints. At this stage, the alloy powder in the solder paste melts and reacts with the metal on the soldered surface. The flux solvent boils at this time and accelerates volatilization and infiltration, and overcomes surface tension at high temperatures, allowing the liquid alloy solder to flow with the flux, spread on the surface of the pad and wrap the soldering end surface of the part to form a wetting effect. Theoretically, the higher the temperature, the better the wetting effect. However, in practical applications, the maximum temperature tolerance of the PCB board and parts must be considered. The adjustment of the temperature and time of the reflow soldering zone is to seek a balance between the peak temperature and the soldering effect, that is, to achieve the ideal soldering quality within an acceptable peak temperature and time.
After the welding zone is the cooling zone. In this stage, the solder cools down from liquid to solid to form solder joints, and crystal grains are formed inside the solder joints. Rapid cooling can produce reliable solder joints with bright gloss. This is because rapid cooling can make the solder joint form an alloy with a tight structure, while a slower cooling rate will produce a large amount of intermetal and form larger grains on the joint surface. The reliability of the mechanical strength of such a solder joint is low, and The surface of the solder joint will be dark and low in gloss.
Setting lead-free reflow soldering temperature
In the lead-free reflow soldering process, the furnace cavity should be processed from a whole piece of sheet metal. If the furnace cavity is made of small pieces of sheet metal, warping of the furnace cavity will easily occur under lead-free high temperatures. It is very necessary to test the track parallelism at low temperatures. If the track is deformed at high temperatures due to the materials and design, jamming and falling of the board will be unavoidable. In the past, Sn63Pb37 leaded solder was a common solder. Crystalline alloys have the same melting point and freezing point temperature, both 183°C. The lead-free solder joint of SnAgCu is not a eutectic alloy. Its melting point range is 217°C-221°C. The temperature is solid when the temperature is lower than 217°C, and the temperature is liquid when the temperature is higher than 221°C. When the temperature is between 217°C and 221°C The alloy exhibits an unstable state.
Post time: Nov-27-2023