Adjustment and control of welding temperature
(1) The optimal welding parameters of the hot air soldering station are actually the best combination of the welding surface temperature, welding time and the hot air volume of the hot air soldering station. When setting these three parameters, the number of layers (thickness), area, internal wire material, BGA device material (PBGA or CBGA) and size, solder paste composition and solder melting point should be considered mainly when setting these three parameters. The number of components on the printed board (these components need to absorb heat), the optimal temperature for soldering BGA devices and the temperature they can withstand, the longest soldering time, etc. In general, the larger the BGA device area (more than 350 solder balls), the more difficult it is to set the soldering parameters.
(2) Pay attention to the following four temperature zones during welding.
① Preheat zone (preheat zone). The purpose of preheating is twofold: one is to prevent one side of the printed board from being deformed by heat, and the other is to accelerate the melting of solder. For printed boards with larger areas, preheating is more important. Due to the limited heat resistance of the printed board itself, the higher the temperature, the shorter the heating time should be. Ordinary printed boards are safe below 150°C (not too long). Commonly used 1.5mm thick small-size printed boards can set the temperature at 150-160°C and the time is within 90 seconds. After the BGA device is unpacked, it should generally be used within 24 hours. If the package is opened too early, in order to prevent the device from being damaged during rework (produce "popcorn" effect), it should be dried before loading. The drying preheating temperature should be 100-110°C, and the preheating time should be longer.
②Medium temperature zone (soak zone). The preheating temperature at the bottom of the printed board can be the same as or slightly higher than the preheating temperature in the preheating zone. The temperature of the nozzle is higher than the temperature in the preheating zone and lower than that in the high temperature zone. The time is generally about 60 seconds.
③High temperature zone (peak zone). The temperature of the nozzle reaches its peak in this area. The temperature should be higher than the melting point of solder, but preferably not more than 200°C.
In addition to correctly selecting the heating temperature and time of each zone, attention should also be paid to the heating rate. Generally, when the temperature is below 100°C, the maximum heating rate does not exceed 6°C/s, and the maximum heating rate above 100°C does not exceed 3°C/s; in the cooling zone, the maximum cooling rate does not exceed 6°C/s.
There is a certain difference in the above parameters when CBGA (ceramic package BGA device) and PBGA chip (plastic package BGA device) are soldered: the diameter of the solder ball of the CBGA device should be about 15% larger than that of the PBGA device, and the composition of the solder is 90Sn/10Pb, higher melting point. In this way, after the CBGA device is desoldered, the solder balls will not stick to the printed board.
The solder paste connecting the solder ball of the CBGA device to the printed board can use the same solder as the PBGA device (the composition is 63Sn/37Pb), so that after the BGA device is pulled out, the solder ball is still attached to the device pin and will not adhere to the printed board. board
