Introduction of SMT Manufacturing of Circuit Board Reflow Soldering Process
Introduction to PCB double-sided reflow process (SMT) circuit board assembly technology. Now the full-board reflow is quite popular in the industry. This technology can be divided into single-sided reflow and double-sided reflow. Single-sided reflow is less used because double-sided reflow can save space on the circuit board. Double sided reflow requires two times of reflow. Due to process limitations, some problems may arise. For example, when the board goes to the second reflow furnace, the parts on the first side may fall due to gravity, especially when the board flows into the reflow zone of the furnace at high temperatures.
Which kind of SMD parts should be placed on the first side of the reflow furnace?
First of all, it is recommended to place the smaller parts on the first side to pass through the reflow oven. When the first side passes through the reflow furnace, the deformation of PCB board will be relatively small and the accuracy of solder paste printing will be relatively high so it is more suitable to place small parts. Secondly, the smaller parts will not be at risk of falling during the second pass through the reflow oven. Because the parts on the first side will be placed on the bottom side of the circuit board directly when hitting the second side. It is less likely to fall off the board due to the heavy weight when the board enters the reflow area and the high temperature. (For more information about PCB design, welcome to visit Jiepei PCB.) Thirdly, the parts on the first panel must pass through the reflow furnace twice, so its temperature resistance must be able to withstand the temperature of the reflow twice. Generally, the resistance capacitance is usually required to pass through the reflow high temperature at least three times, which is to meet the requirements that some boards may need to go through the reflow furnace again due to maintenance.
Which SMD parts should be placed on the second side to pass through the reflow furnace?
1. Large or heavy components shall be placed on the second side of the furnace to avoid parts falling back into the welding furnace when passing the furnace. LGA and BGA parts shall be placed on the second side of the furnace as far as possible. This can avoid the unnecessary risk of re melting tin during the second pass to reduce the chance of empty / false welding. If there are small BGA parts with fine pitch, they can also be placed on the first side through the reflow furnace as long as they can effectively avoid PCB board deformation.
2. Parts that cannot withstand repeatedly high temperature should be placed on the second side of the reflow furnace to avoid damage caused by too many times of high temperature. PIH / PIP parts shall also be placed on the second side of the furnace if the weld leg length does not exceed the plate thickness. Otherwise, the foot protruding from the PCB board surface will interfere with the steel plate on the second side which will make the steel plate printed by the second side of solder paste unable to be pasted on the PCB board and thus result in abnormal solder paste printing.
3. Soldering can be used inside some components, such as the network cable connector with LED lights. It must be noted whether the temperature resistance of this kind of component can pass the welding furnace twice. If not, it must be placed on the second side. When the parts are placed on the second side and pasted through the reflow furnace it means that the circuit board has been baptized by the high temperature of the reflow furnace and the circuit board has some warpage and deformation. That is to say, the printing volume and printing position of the solder paste will become more difficult to control which may easily cause problems such as empty welding or short circuit. Therefore, It is recommended not to place 0201 part and fine pitch part as far as possible on the second side of the furnace and part BGA should also try to select tin balls with larger diameter.
At present, the welding process of circuit board can be roughly divided into full board welding and local welding. The full board welding is generally divided into reflow soldering and wave soldering. While the local welding of circuit board includes carrier wave soldering, selective wave soldering, non-contact laser soldering, etc.
