Common problems and solutions of reflow welding

1. Virtual welding
It is a common welding defect that some IC pins appear in virtual welding after welding. Reason: pin coplanarity is poor (especially QFP, due to improper storage, resulting in pin deformation); Poor solderability of pins and pads (long storage time, yellow pins); During welding, the preheating temperature is too high and the heating speed is too fast (easy to cause IC pin oxidation).

2, cold welding

It refers to the solder joint formed by incomplete reflux. Reason: insufficient heating during welding, insufficient temperature.
3. Bridge
One of the common defects in SMT, which causes a short circuit between components and must be repaired when the bridge is encountered. Reason: solder paste collapse; Too much solder paste; The pressure is too large during the patch; Reflux heating speed is too fast, the solvent in the solder paste too late to volatilize.

4. Erect a monument
One end of the chip component is lifted and stands on its other end pin, also known as the Manhattan phenomenon or suspension bridge. Reason: The fundamental is caused by the imbalance of the wetting force at both ends of the component. Specifically related to the following factors:
(1) The design and layout of the pad is unreasonable (if one of the two pads is too large, it will easily cause uneven heat capacity and uneven wetting force, resulting in unbalanced surface tension of the molten solder applied to the two ends, and one end of the chip element may be completely wet before the other end begins to wet).
(2) The printing amount of the solder paste in the two pads is not uniform, and the more end will increase the heat absorption of the solder paste and lag the melting time, which will also lead to the imbalance of the wetting force.
(3) When the patch is installed, the force is not uniform, which will cause the component to be immersed in the solder paste at different depths, and the melting time is different, resulting in uneven wetting force on both sides; Patch time shift.
(4) When welding, the heating speed is too fast and uneven, making the temperature difference everywhere on the PCB large.

5, wick suction (wick phenomenon)
Resulting in a virtual weld, or bridge if the pin spacing is fine, is when the molten solder wets the component pin, and the solder climbs up the pin from the solder spot position. It mostly occurs in PLCC,QFP,SOP. Reason: When welding, due to the small heat capacity of the pin, its temperature is often higher than the temperature of the solder pad on the PCB, so the first pin wetting; The solder pad is poor in weldability, and the solder will climb.
6. Popcorn phenomenon
Now most of the components are plastic sealed, resin encapsulated devices, they are particularly easy to absorb moisture, so their storage, storage is very strict. Once the moisture is absorbed, and it is not completely dried before use, at the time of reflux, the temperature rises sharply, and the internal water vapor expands to form the popcorn phenomenon.
7. Tin beads
It affects the appearance and also causes bridging. There are two types: one side of a chip element, usually a separate ball; Around the IC pin, there are scattered small balls. Reasons: the flux in the solder paste is too much, the solvent volatilization is not complete in the preheating stage, and the solvent volatilization in the welding stage causes splashing, resulting in the solder paste rushing out of the solder pad to form tin beads; The thickness of the template and the size of the opening are too large, resulting in too much solder paste, causing the solder paste to overflow to the outside of the solder plate; When printing, the template and the pad are offset, and the offset is too large, which will cause the solder paste to overflow to the pad. When mounting, the Z-axis pressure causes the component to be attached to the PCB, and the solder paste will be extruded to the outside of the pad. When reflux, the preheating time end and heating rate are fast.
8. Bubbles and pores
When the solder joint is cooled, the volatile matter of the solvent in the internal flux is not completely dispatched. It is related to temperature curve and flux content in solder paste.
9, solder joint tin shortage
Reason: printing template window is small; Low metal content of solder paste.
10, solder joints too much tin
Cause: The template window is large.
11, PCB distortion
Reason: PCB itself material selection is improper; PCB design is not reasonable, component distribution is not uniform, resulting in PCB thermal stress is too large; Double-sided PCB, if one side of the copper foil is large, and the other side is small, it will cause inconsistent shrinkage and deformation on both sides; The temperature in reflow welding is too high.
12. Cracking phenomenon
There is a crack in the solder joint. Reason: After the solder paste is taken out, it is not used up within the specified time, local oxidation, forming a granular block, which is difficult to melt during welding and cannot be fused with other solders into one piece, so there is a crack on the surface of the solder joint after welding.
13. Component offset
Cause: The surface tension of the molten solder at both ends of the chip element is unbalanced; The conveyor vibrates during transmission.
14, the solder joint dull luster
Cause: The welding temperature is too high, the welding time is too long, so that the IMC is transformed into.
15, PCB solder resistance film foaming
After welding, there are light green bubbles around the individual solder joints, and in serious cases, there will be thumbnail-sized bubbles, affecting the appearance and performance. Reason: There is gas/water vapor between the solder resistance film and the PCB substrate, which is not completely dried before use, and the gas expands when welding at high temperature.
16, PCB solder resistance film color changes
Solder resistance film from green to light yellow, cause: the temperature is too high.
17, PCB multi-layer board layering
Cause: Plate temperature is too high.