During turnkey pcb manufacturing, double-sided reflow soldering process considerations

At present, the mainstream circuit board assembly technology of turnkey pcb manufacturing industry should be "full-flow reflow soldering". There are other board soldering methods, and this full-board reflow soldering can be divided into single and double sided reflow soldering, single-sided reflow soldering boards are rarely used now, because double-sided reflow can save board space, which means that can be made smaller, so most of the boards belong to the double-sided reflow process in the market.

Because the "double-sided reflow process" requires two reflow , there are some limitations on the turnkey pcb manufacturing process. The most common problem is when the board goes to the second reflow oven, the first side. Parts will fall due to gravity, especially when the board flows to the high temperature of the reflow zone of the furnace. This article will explain the consideration for the placement of parts in the double-sided reflow process:

1. Which SMD parts should be placed on the first side of the reflow oven?

In general, the relatively small parts are recommended to be placed on the first side of the reflow oven during turnkey pcb manufacturing, because the deformation of the PCB will be smaller when the first side passes through the reflow oven, and the precision of the solder paste printing will be higher. Therefore, it is better to place smaller parts.

Second, the smaller parts do not risk falling down during the second reflow oven. Because the first surface part will be placed directly on the bottom surface of the circuit board when facing the second side, when the board enters the high temperature of the reflow area, it will not fall off the board due to excessive weight.

Third, the parts on the first panel must pass through the reflow oven twice during turnkey pcb manufacturing, so the temperature resistance must be able to withstand the temperature of the two reflows. The general resistance and capacitance are usually required to be reflowed at least three times. This is in order to meet the requirements that some boards may need to be re-routed back to the furnace because of the maintenance .

2. Which SMD parts should be placed on the second side of the reflow oven?

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

LGA and BGA parts should be placed on the second side of the furnace as much as possible. This avoids the risk of unnecessary re-soldering during the second pass to reduce the chance of empty/false welding. During turnkey pcb manufacturing, if there are small and small BGA parts, it is recommended to place them on the first side of the reflow oven.

It is always controversial that the BGA is placed on the first side or the second side. Although the second side can avoid the risk of remelting the tin, the PCB will be deformed more severe when the second side passes through the reflow oven. On the contrary, it will affect the quality of tin, so that the BGA can be considered placing on the first side. However, conversely, if the PCB is severely deformed, as long as the fine parts are placed on the second side, the patch must be a big problem, because the position of the solder paste and the amount of solder paste will become inaccurate, so during turnkey pcb manufacturing so it’s important to find ways to avoid PCB distortion, rather than consider placing BGA on the first side because of deformation, isn't it? Parts that cannot withstand too much heat should be placed on the second side of 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 of the furnace, unless the length of the solder fillet does not exceed the thickness of the board, otherwise the foot protruding from the surface of the PCB will interfere with the steel plate on the second side, which will make the second side Solder paste printed steel plates can not be flat on the PCB, causing abnormal solder paste printing problems.

Some components inside may have soldering work. For example, a cable connector with an LED lamp must pay attention to whether the temperature resistance of such a component can pass through the reflowing furnace twice. If it is not workable, it must be placed on the second surface.

Only when the parts are placed on the second side of the patch to pass the reflow oven, it means that the circuit board has passed the high temperature of the reflow oven. At this time, the circuit board has some warpage and deformation, that is, tin. The amount of paste printed and the position of the printing will become more difficult to control, so it is easy to cause problems such as empty welding or short circuit. Therefore, during turnkey pcb manufacturing parts placed on the second side of the furnace, which is recommended not to place 0201 and fine. For fine pitch parts, the BGA should also try to choose a larger diameter solder ball.

In addition, in the mass production of electronic components to be soldered to the board, there are many technics ways, but each process is actually determined at the beginning of the board design, because placement position of the parts will directly affect the welding sequence and quality of the assembly, while the wiring will have an indirect effect.

At present, the welding process of the turnkey pcb manufacturing circuit board can be roughly divided into full board welding and partial welding. The full board welding is roughly divided into reflow soldering and wave soldering, while circuit board partially soldered includes Carrier Wave Soldering, Selective Soldering, Laser Soldering, etc.