Detailed explanation of reflow furnace temperature setting

In the PCBA processing process, reflow soldering is an important processing link with high process difficulty. It is a group welding process. All electronic components on the PCB circuit board are welded at one time through overall heating. This process requires experienced operators to control the furnace temperature curve of reflow soldering to ensure the welding quality and the quality and reliability of the final product. Introduce several temperature zones and furnace temperature setting skills of reflow soldering furnace.

Furnace temperature test curve

Reflow soldering furnace has several temperature zones and furnace temperature setting skills

The reflow furnace has four zones, which are divided into preheating zone, constant temperature zone, tin melting zone and cooling zone. Most solder pastes can operate in these zones. In order to deepen the understanding of the ideal temperature curve, the temperature, residence time and the change of solder paste in each zone are introduced as follows:

Preheating zone: the purpose is to heat the PCB board to achieve preheating effect and make it fuse with solder paste. However, at this time, the heating rate should be controlled within an appropriate range to avoid thermal shock and damage to circuit boards and components. The heating slope of the preheating zone shall be less than 3 ℃ / sec, and the set temperature shall be room temperature ~ 130 ℃. The residence time is calculated as follows: if the ambient temperature is 25 ℃, if the heating rate is 3 ℃ / sec, then (150-25) / 3 is 42s; if the heating rate is 1.5 ℃ / s, then (150-25) / 1.5 is 85s. Generally, it is best to adjust the time according to the difference of component size and adjust the heating rate below 2 ℃ / s.

Constant temperature zone: the main purpose is to stabilize the temperature of the components on the PCB circuit board and minimize the temperature difference. We hope that the temperature of large and small components can be balanced as much as possible in this area, and the flux in the solder paste can be fully volatilized. It is worth noting that in this section, the components on the circuit board should have the same temperature to ensure that there will be no poor welding when entering the reflow section. The set temperature of the constant temperature zone is 130 ℃ ~ 160 ℃, and the constant temperature time is 60 ~ 120s.

Reflux zone: the temperature in this zone is the highest, raising the temperature of the component to the peak temperature. In reflow soldering, the peak welding temperature varies depending on the solder paste used. Generally, we recommend to use the melting point temperature of the solder paste temperature plus 20 ~ 40 ℃. The peak temperature is 210 ℃ ~ 230 ℃, and the time should not be too long to prevent adverse effects on PCB. The heating rate of reflux zone shall be controlled at 2.5-3 ℃ / s, and the peak temperature shall generally be reached within 25s-30s. A trick here is that the tin melting temperature is above 183 ℃, and the tin melting time can be divided into two, one is 60 ~ 90s above 183 ℃, the other is 20 ~ 60s above 200 ℃, and the peak temperature is 210 ℃ ~ 230 ℃.

Cooling area: the lead tin powder in the solder paste in this area has melted and fully wetted the connected surface. It should be cooled as quickly as possible, which will help to obtain bright solder joints and good shape, and will not produce rough solder joints. The cooling rate of the cooling section is generally 3 ~ 4 ℃ / s. It can be cooled to 75 ℃, and the cooling slope is less than 4 ℃ / s.

Of course, in large-scale production, the actual working curve of each product should be repeatedly adjusted according to the size of SMA, the number of components and varieties. In terms of time, the whole reflux time is 175sec-295sec, i.e. about 3-5 minutes (excluding the time before entering the first temperature zone).