This article introduces the PCBA double-sided back welding process (SMT) and parts placement considerations

Posted on 2022-04-25

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。


(顺便说一句,如果没有空间限制,其实单板制程可以节省一个SMT 制程。如果将材料成本与SMT工时成本进行比较,也许单板制程会节省更多成本.)
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:
(再题外话,为什么第一面已经镀锡的小零件大部分在第二面通过回流炉时没有重新熔化和脱落?为什么只有较重的零件脱落?)
哪些SMD零件应该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,所以当板子在高温下进入回流区时,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 SMD 零件应该通过第二面的回流炉?这应该是重点。
较大或较重的元件应放置在第二面,以避免在回流过程中零件掉入回流炉的风险。
LGA和BGA零件尽量放在第二面,避免第二遍时出现不必要的锡重熔风险,从而减少空焊/假焊的机会。如果有细脚且不排除较小的BGA零件,建议将其放置在第一面的回流炉中。
BGA应该通过回流炉放置在第一面还是第二面,在电子制造业中一直存在争议。虽然放置第二面可以避免二次回流后重新熔锡的风险,但通常第二面通过回流炉时,PCB会变形比较严重,影响吃锡质量,所以工作熊会说不排除脚细的BGA可以考虑放在第一面。但另一方面,如果PCB已经严重变形,只要是薄脚零件,放在第二面进行贴片肯定是个大问题,因为锡膏印刷位置和锡膏用量会变得极其不准确。所以重点应该是如何避免PCB变形,而不是因为变形就考虑第一面的BGA吧?
板子的变形不仅会影响零件吃锡的质量,还会使BGA在组装成整机时失效。因为板子组装时板子是竖立起来的,板子没有变形,所以会尽量把变形的板子恢复到原来的状态。变形状态。

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.
PIH/PIP parts 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, causing 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.
Card reader side A Card reader side B
参考文章前面SD卡板的正反两面图片,应该可以很清楚的判断并指出零件通过回流炉的第一面会布置在哪一边,而那面会放在第二面,面糊糊糊的!