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

目前SMT行业主流的电路板组装技术应该是“全板回流焊（Reflow）”。当然，还有其他电路板焊接方法。回流焊”和“双面回流焊”，现在单面回流焊板用的比较多，因为双面回流焊可以节省电路板的空间，也就意味着可以把产品做得更小，所以市场看到大部分到货的板子都属于双面回流工艺。

因为“双面回流工艺”需要两次回流，所以会有一些工艺限制。最常见的问题是当板子进入第二个回流炉时，板子的第一面已经被标记了。由于重熔锡和重力的关系，零件可能会掉落，特别是当板子流到回流炉的回流高温区时。本文将解释双面回流工艺中放置零件的注意事项：

哪些SMT零件应通过第一面的回流炉？

一般而言，建议将较小的零件放置在通过回流炉的第一面，因为第一面通过回流炉时PCB的变形会比较小，并且锡膏印刷的精度会更高，所以更适合摆。放置较小的部分。

其次，较小的零件在第二次通过回流焊炉时不会有掉落的风险。因为第一面的零件会在第二面被击打时将电路板底部朝下放置，所以当板子进入高温回流区时，由于其本身的特性，它不太可能从板上脱落。沉重的重量。

第三，第一个面板上的零件必须经过两次回流焊，因此它们的耐温性必须能够承受两次回流焊的温度。一般电阻和电容通常要求在高温下至少回流3次。这是为了满足部分板子可能因维修需要再次经过回流焊的要求。

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.
另外，在大批量生产中，在电路板上焊接和组装电子零件的工艺方法有很多种，但每个工艺实际上都是在电路板设计之初就决定的，因为它的电路板零件的放置会直接影响组装的焊接顺序和质量，而接线会间接影响它。

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。