Introduce PCBA double-sided reflow process (SMT) and precautions for component placement

Posted on 2022-04-27







哪些 SMT 零件应该通过第二面的回流炉?这应该是重点。

Large components or heavier components should be placed on the second side to avoid the risk of parts falling into the reflow oven during reflow.
LGA and BGA parts should be placed on the second side as far as possible to avoid the risk of unnecessary re-melting of tin during the second pass, so as to reduce the chance of empty/false soldering. If there are thin feet and smaller BGA parts are not excluded, it is recommended to place them in the reflow oven on the first side.

Whether the BGA should be placed on the first side or the second side through the reflow oven has been controversial in the electronics manufacturing industry. Although placing the second side can avoid the risk of re-melting tin after the second reflow, but usually when the second side passes through the reflow oven, the PCB will be deformed more seriously, which will affect the quality of tin eating, so the work bear will say It is not ruled out that the BGA with thin feet can be considered on the first side. But on the other hand, if the PCB has been seriously deformed, as long as it is a thin-footed part, placing it on the second side for patching must be a big problem, because the solder paste printing position and amount of solder paste will become extremely inaccurate. So the focus should be on how to avoid PCB deformation, instead of considering placing BGA on the first side because of deformation, right?
The deformation of the board will not only affect the quality of the parts eating tin, but also make the BGA fail when it is assembled into the whole machine. Because the board is erected when the board is assembled, the board is not deformed, so it will try to restore the deformed board to its original state. deformed condition.
Parts that cannot withstand too many high temperatures should be placed on the second side to pass through the reflow oven. This is to prevent the parts from being damaged by too many high temperatures.
The parts of PIH/PIP should also be placed on the second side to pass the furnace. Unless the length of the solder feet does not exceed the thickness of the board, the feet protruding from the surface of the PCB will interfere with the steel plate on the second side, which will cause the second side The solder paste printed steel plate cannot be flatly attached to the PCB, resulting in abnormal solder paste printing.
Some components may use soldering inside, such as a network cable connector with LED lights. It is necessary to pay attention to whether the temperature resistance of this part can pass through the reflow oven twice. If not, it must be placed on the second side.
It is only that the parts are placed on the second side and the patch has been passed through the reflow oven, which means that the circuit board has passed the baptism of the high temperature of the reflow oven. At this time, the circuit board has been somewhat warped and deformed, that is to say, the tin The printing amount and printing position of the paste will become more difficult to control, so it is easy to cause problems such as empty soldering or short circuit. Therefore, it is recommended not to place 0201 and fine spacing for the parts that are placed on the second side to pass the oven. pitch) parts, BGA should also try to choose solder balls with larger diameters.

目前,电路板的焊接工艺大致可分为全板焊接和局部焊接。全板焊接大致分为回流焊和波峰焊,而电路板的部分焊接包括载波波峰焊(Carrier Wave Soldering)、选择性焊接、非接触激光焊接等。