Common defects of SMT process and analysis of their causes

A. Bridge solder:

1. The solder paste is too thin, including low metal or solid content in the solder paste, low solubility, and the solder paste is easy to explode.

2. The solder paste particles are too large and the surface tension of the flux is too small.

3. Too much solder paste on the pad.

4. The reflow temperature peak is too high.

B. Open (Open):

1. The amount of solder paste is not enough.

2. The coplanarity of the component pins is not enough.

3. The tin is not wet enough (not melted enough, the fluidity is not good), and the solder paste is too thin to cause the loss of tin.

4. Pins sucking tin (like rushes) or there are connection holes nearby. Pin sucking can be prevented by slowing down the heating speed and heating more on the bottom and less on the top.

5. The solder does not wet the pins, and the drying time is too long to cause the flux to fail, the reflow temperature is too high, and the time is too long to cause oxidation.

6. The pad is oxidized, and the solder does not melt the pad.

C. Tombstoning/Part shift:

Tombstones are usually the result of unequal wetting forces that make components stand up on one end after reflow, generally the slower the heating, the smoother the board, and the less it happens. Reducing the rate of temperature rise of the assembly through 183°C will help correct this defect.

D. Tombstone effect:

The tombstone effect occurs with three forces: the gravity of the part pushes the part down; the molten tin under the part also pushes the part down; the molten tin on the outside of the part on the tin pad pushes the part up

1. Improper pad design - pad design optimization

2. The two ends of the part have different tin ingestion --- the better part has tin ingestion

3. Uneven heating at both ends of the part --- Slow down the heating rate of the temperature curve

4. The temperature curve heats up too fast ---preheat to 170℃ before Reflow

E. Holes:

A defect usually found by X-ray or cross-sectional inspection of a tin spot. Voids are tiny "bubbles" within a tin point, possibly trapped air or flux. Voids are generally caused by three curve errors: not enough peak temperature; not enough reflow time; too high temperature during the ramp-up phase. Causes the non-volatile flux to be trapped in the tin point.

In this case, in order to avoid the generation of voids, the temperature curve should be measured at the point where voids occur, and appropriate adjustments should be made until the problem is resolved.

F. Air welding of SMD parts with feet:

1. Part feet or solder balls are not flat --- Check the flatness of the part feet or solder balls

2. Too little solder paste --- increase the thickness of the steel plate and use a smaller opening

3. Lamp core effect ---PCB is first baked

4. Part feet do not eat tin --- parts must meet the needs of eating tin

G. Air soldering of SMD parts without feet:

1. Improper design of solder pads---separate solder pads with solder mask, appropriate size

2. Uneven heating at both ends --- the size of the tin pads of the same part should be the same

3. The amount of solder paste is too small --- increase the amount of solder paste

4. The parts have poor tin-eating properties --- parts must meet the needs of tin-eating

H. Cold solder joints :

Cold soldering means that the solder joints do not form an intermetallic layer or the resistance of the solder joints is high, and the peel strength (Peel Strength) between the solder objects is too low, so it is easy to pull up the part feet from the tin pads.

1. Reflow temperature is too low---minimum reflow temperature is 215℃

2. Reflow time is too short --- the solder paste should be above the melting temperature for at least 10 seconds

3. Pin's tin-eating property --- check Pin's tin-eating property

4. Pad's tin-eating property --- check Pad's tin-eating property

I. SMD part float (drift):

1. Uneven heating at both ends of the part ---Separation of tin pads

2. Poor tin eatability at one end of the part --- Use parts with better tin eatability

3. Reflow method --- preheat to 170℃ before reflow

J. Cracks in components (cracking):

1. Thermal Shock --- Natural cooling, smaller and thinner parts

2. Pressure generated by PCB board warping --- avoid PCB bending, directionality of sensitive parts, and reduce placement pressure

3. Improper design of PCB Lay-out --- individual pads, the long axis of the part is parallel to the direction of the folding board

4. Solder paste amount --- increase the amount of solder paste, proper solder pads.