![]() ![]() The following is an explanation of the kind of faults that can occur when yields are low despite everything appearing to be set correctly (ie, perfect screen print, no board damage, and accurate placement).Ī major cause of defects is when either the PCB or the component warp during reflow. The reason behind the yield problems was due to the plastic BGA warping during reflow, caused by excessive source temperature on the topside of the PCB. A better optimised profile than that in Figure 2. The requirements for a solder profile are generally well understood they are normally dependent on the materials being used (solder paste or flux) and would typically be as shown in Table 1.įigure 3. In some cases, particularly on larger PCB assemblies, failure may still occur, giving low yields when all of the conditions for rework are apparently correct. This article discusses the influential factors and the potential pitfalls to be avoided. Selective heating of an area of board can inadvertently cause process failure even when all the parameters appear to be correct. The nature of the operation dictates that we only wish to reflow the component being reworked and therefore we have to heat a specific area of the circuit board. ![]() The normal way to approach the rework is to try and emulate the production reflow profile for the individual component being removed or replaced. When we consider reworking an array package such as a BGA (ball grid array) or CSP (chip-scale package), it is recognised that process control is of paramount importance to achieve a successful result. The capacity of the oven would be appropriate to the size or thermal mass of the PCB and the production throughput requirements. This is accomplished by evenly heating both the top and bottom sides of the whole board, generally through a multi-zoned convection reflow oven. ![]() If we consider profiling a PCB (printed circuit board) for reflow in production, we are generally concerned with achieving an even delta-T (ΔT) across all of the joints being soldered. All credit goes to their rights holder.Board profiling for production versus rework If you have components that require low temp those should be all on same side and that side should be last to reflow to ensure that you don’t damage them by overheating.Ĭredit: images lifted off google search results. Of course this also requires attention to components maximum temperature. The process will still remain the same though - first reflow one side then reflow the other side. This type adhesive is cured by heat and allows for double sided assembly where you have heavy components on either side. #Smd reflow oven manual#For manual process it is sold in syringes same as solder paste and can be (very carefully) applied to the board. Good SMA house will have machine that dispenses adhesive. Adhesive can be placed either under the component if there is no bottom pad or can be placed at the corners (for BGA components). For heavy parts there is special heat curable adhesive that will ensure that those stay in place. Now the question in my mind is, is it true that if we have to do 2-sided soldering of such a PCB it would be that once the one side has been soldered in the oven and we place components on the opposite side and put into oven again, the components would fall off from the first side due to heat in the oven? If this is true, how is this mitigated?Īs other answers mentioned the light parts are usually not a problem. For manufacturing at industrial scale the component placement would be done by a machine. For prototype board the components would be placed by hand and then the board would be put into the oven. If we are to do a complex board then we would be using a reflow oven. The soldering could be done by hand if the component side allows or we could use solder paste with hot air gun. It could be done the other way around but the thing is that, it will be done on one side at a time. If we have a 2-sided PCB with SMD components on the top and also bottom, first we shall put components on the top side and solder them and then on the bottom side and solder them. As far as I am aware, and please correct me. ![]()
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