Analysis of welding defects of wave soldering in smt patch processing A-957

Common wave soldering process problems during smt patch processing are: non-wetting/semi-wetting, solder balls, disturbed solder joints, cold soldering, tipping, bridging, and virtual soldering. The main reasons are as follows.
(1) Non-wetting/semi-wetting
Non-wetting refers to the formation of discontinuous solder on the surface of the pad after wave soldering, and sometimes the bare base metal can be clearly seen. Semi-wetting means that in wave soldering, the solder begins to wet the pad and then shrinks due to poor wetting. The reasons for non-wetting and semi-wetting are: ① poor solderability of pads and pins; ② insufficient flux activity; ③ grease pollutants on the soldering surface; ④ oxidation of pads and pins.
(2) Tin ball
Due to the cohesive force of the solder itself, the solder forms a discontinuous ball around the pad. The reasons for this defect are: ①The solder paste moisture is too heavy; ②The surrounding environment is too large; ③The outgassing during the soldering process causes the molten solder to splash.
(3) Disturbed solder joints
Disturbed solder joints usually have rough appearance and are powdery. The reasons for disturbing the solder joints are: conveyor belt vibration or shaking. Before the molten solder is solidified, the components are displaced, resulting in uneven solder joints; the wind speed is too large.
(4) Cold welding
During wave soldering, there is no or insufficient wetting between the base metal and the solder, and the appearance of the solder joints is not smooth and wrinkled. The reasons include: ①The transmission speed is too fast; ②There are components with large heat capacity on the PCB, and some parts have not reached the melting point of the solder paste, and the solder paste has not completely melted.
SMT chip processing
(5) Tip
The tip of the tip is icicle-shaped at the top of the solder joint. The reasons for this phenomenon are: ①The PCB transmission angle is too small. When the angle is 6°~7°, it can effectively reduce the sharpness; ②The solderability of the pad or component is poor, and the activity of the flux is not enough; ③The temperature curve is not Reasonable; ④The solder in the solder is oxidized or contaminated.
(6) Bridge
Bridging refers to the connection of two or more adjacent solder joints together. The reasons are: ①The circuit design is too close, and the pads are too close; ②The surface of the PCB is not clean; ③The solder paste is contaminated; ④The flux activity is not enough; ⑤The PCB eats too much tin; ⑥The component layout is unreasonable; ⑦The component leads The foot protrudes too much through the hole; ⑧The transmission angle of the PCB is too small; ⑨The shape of the wave crest is unreasonable. After welding, an “air knife” device is generally installed to reduce the occurrence of bridging.
(7) Welding
Virtual solder generally refers to solder joints with reduced mechanical strength and poor conductivity, and ideal intermetallic compounds are not formed between the solder and the pad or pin. The reasons for false soldering are: ①The peak temperature is too low or the time above the liquidus line is too short; ②The solderability of the component leads or pads is poor; ③The solder temperature in the solder tank is too low.