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The introduced & precautions of smt PCB double - sided reflow process

At present, SMT industry mainstream circuit board assembly technology should be "the whole board reflow soldering (Reflow)" none other, of course, there are other circuit board welding methods, and this full-board reflow soldering can be divided into single-plate reflow and Double-sided reflow soldering, single-sided reflow of the board is now very few people use, because the double-sided reflow can save the board space, that is to say so that production can be smaller, so the market to see most of the board Belonging to double-sided reflow process.
(If the words, if there is no space constraints, in fact, single-panel process can save a SMT process, if the cost of materials and SMT working hours comparison, maybe single-panel but also more cost savings.

Because the "double-sided reflow process" need to do the relationship between the two reflow, so there will be some process restrictions, the most common problem is the board went to the second reflow furnace, the first side of the above parts will be Gravity relationship and fall, especially when the board flow to the furnace back to the high temperature, this paper will explain the double-sided reflow process parts placed Note:

(Again a question, why the second side of the back to the furnace, the original first side of the tin has been on most of the small parts will not re-melt and fall down why why only the heavier parts will fall?

Which SMD parts should be placed on the first side of the backwheel?

In general, relatively small parts of the proposed placement on the first side of the backwashing furnace, because the first side of the back welding furnace PCB deformation will be relatively small, solder paste printing accuracy will be higher, so it is Put smaller parts.

Second, the smaller parts will not fall in the second time when the risk of falling back furnace. Because the first side of the parts in the second side will be placed on the bottom of the circuit board directly down when the board into the backwash area when the high temperature will not be too heavy because of heavy fall from the board.

Third, the first panel of the parts must be twice back to the furnace, so the temperature must be able to withstand the temperature of two reflow, the general resistance and capacitance is usually required at least three times can be reflow high temperature, which is to In line with some of the board may be due to the maintenance of the relationship, need to re-take a reflow furnace and do the request.

Which SMD parts should be placed on the second side of the backwheel? This should be the focus.

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

LGA, BGA parts should be placed in the second side of the furnace, so as to avoid the second furnace when the unnecessary re-melting tin risk, to reduce the air / false welding opportunities. If there are fine feet and smaller BGA parts do not rule out the proposed placement on the first side of the backwashing furnace.

BGA placed in the first side or the second side of the furnace has been very controversial, placed on the second side although it can avoid the risk of re-melting tin, but usually the second side of the PCB after the back of the deformation will be more serious, But will affect the quality of eating tin, so the work will be said that the bear will not rule out the foot of the BGA can be considered on the first side. But in turn think, if the PCB deformation is serious, as long as the fine parts, placed on the second side of the patch must be a big problem, because the solder paste printing position and the amount of solder paste will become inaccurate, so the focus should be Think of how to avoid PCB deformation, rather than because of the deformation and consider the BGA on the first side, is not it?

Parts can not be resistant to too many high temperature parts should be placed on the second side of the back to the furnace. This is to avoid too many parts of the high temperature damage.

PIH / PIP parts should also be placed in the second side of the furnace, unless the length of its welding feet will not exceed the thickness of the board, or its protruding part of the PCB surface will be with the second side of the steel plate interference, Paste printing of the steel plate can not be affixed to the PCB caused by solder paste printing problems occur.

Some components will be used inside the case of soldering operations, such as LED lights cable connector, you must pay attention to the temperature of the parts can be twice back to the furnace, if not have to be placed on the second side of the fight.

But the parts placed on the second side of the pieces of the patch back to the furnace, it means that the circuit board has been a reflow furnace high temperature baptism, this time the number of circuit boards have been some warping and deformation occurred, that is tin The printing volume of the paste and the printing position will become more difficult to control, it is also likely to cause problems such as empty welding or short circuit, so on the second side of the furnace parts, it is recommended not to put 0201 and fine feet (fine pitch parts, BGA should also try to choose a larger diameter of the ball.

With reference to the front and back sides of the SD card in front of the picture, you should be able to clearly judge and point out that side will be arranged in the first side of the parts back to the furnace, and that side will be placed in the second Wiping pieces of patchwork!

In addition, in the mass production of electronic components to be assembled in the circuit board assembly, in fact, there are many kinds of process methods, but each process process is actually in the beginning of the circuit board design has been decided, because its circuit board The placement of the parts will directly affect the assembly of the welding sequence and quality, and wiring will indirectly affect.

At present, the welding process of the circuit board can be divided into the whole board welding and local welding. The whole board welding is divided into Reflow Soldering and Wave Soldering, while the local welding of the circuit board is carried by wave welding Carrier Wave Soldering, SelecTIve Soldering, and non-contact laser soldering.