Introduction and Cautions of PCB Double - side Resoldering Process ( SMT )

         Introduction and Cautions of PCB Double - side Resoldering Process ( SMT )


       The current mainstream SMT circuit board assembly technology should be "full plate reflow soldering (Reflow)", as well as other other circuit board welding method, and the whole plate reflow soldering can be divided into single and double plate panel reflow reflow reflow, single board now rarely used, because double reflow can save board space, that is to say, let the production be smaller, so the market to see the board mostly belongs to the double reflow process.


      Because the "double reflow process" to do the relationship reflow two times, so there will be some restrictions on the process, the most common problem is that the board went second times reflow oven, the first above parts because of gravity and fall, especially the flow to the welding area of high temperature back board the furnace, this paper will illustrate the attention of double sided reflow soldering parts placed:


      What SMD components should be placed on the first side via the reflow furnace ?


      Generally speaking, the smaller parts are recommended to be placed on the first side over the reheating furnace, because the amount of deformation of PCB will be relatively small when the first side passes over the reheating furnace, and the accuracy of solder paste printing will be relatively high, so it is more suitable for placing smaller parts.


     Secondly, small parts are not in second times of reflow oven when falling risk. Because the first surface of the parts will be put to the bottom surface of the circuit board directly down on the second surface, when the board back into the welding zone of high temperature is not because the weight is overweight and falling down from the board.


     Which SMD parts should be placed in the second side back welding furnace? This should be the focus.


     Large or heavier components should be placed on the second side of the furnace to avoid the risk of parts falling back into the welding furnace.


    The LGA BGA parts should be placed on the second side of the furnace as far as possible so as to avoid the unnecessary risk of remelting tin during the second pass. In order to reduce the chance of empty / false welding. If there is a fine pin and smaller BGA parts are recommended to be placed in the first side of the return welding furnace.


     BGA placement on the first or second side of the furnace has been controversial, placing the second side, although it can avoid the risk of re-melting tin, but usually when the second side of the back furnace PCB deformation is more serious. Instead, it affects the quality of the tin eaten, so the working bear would say that not ruling out the finer foot of the BGA could consider putting it on the first side. Conversely, if the PCB is seriously deformed, as long as it is in fine parts. Placing the patch on the second side must be a big problem, because the printing position and amount of tin paste will become inaccurate, so the focus should be on how to avoid PCB distortion. Instead of thinking about putting BGA on the first side because of the deformation, right?


     Parts which can not withstand too many high temperatures should be placed in the second side back welding furnace. This is to avoid the damage of too many high temperatures.


    The parts of PIH/PIP must be placed on the second side through the furnace. Unless the length of the welding foot does not exceed the thickness of the plate, the foot that extends the PCB surface will interfere with the second surface steel plates. It will make the second solder paste printed steel plates unable to be pasted on PCB, resulting in the abnormal problem of solder paste printing.


    In some components where soldering is used internally, such as wire connectors with LED lights, it is important to note whether the parts can withstand the heat twice and, if not, to be placed on the second side.


   Just put on a second plane hit piece parts over patch reflow, said circuit board has been a reflow furnace baptism, circuit board at this time have some warpage and deformation, that is to say the amount of printing and printing paste position will become more difficult to control, so it is easy to by air welding or short circuit, so put a furnace in the second face parts, suggestions so as not to be placed 0201 and thin legs (fine pitch) parts, BGA should also try to choose the larger diameter of the solder ball.


    In addition , in mass production , the electronic parts are welded and assembled on the circuit board , in fact , there are a number of process methods , but each process is actually decided at the beginning of the circuit board design , because the placement position of parts on the circuit board can directly influence the welding sequence and quality of the assembly , and the wiring is affected indirectly .


    At present, the welding process of circuit board can be divided into full board welding and local welding. The whole plate welding is divided into reflux welding (Reflow soldering) and wave soldering (wave soldering). On the other hand, there are carrier Wave soldering and selective wave soldering. Laser soldering, etc.