PCBA Tech - SMT resistance, capacitance tombstone empty welding

Regarding the problem of free soldering of small SMD parts such as smt resistors and capacitors, the reason for its generation is the same as the cause of the tombstone. To put it simply, the melting time of the solder paste at both ends of the part is inconsistent,and eventually the force is uneven.The result of cocked.

Usually when the PCB enters the reflow oven and begins to heat up,the more the surface of the copper foil,the faster it will be heated,and it will reach the ambient temperature in the reflow oven faster,while the larger the inner layer of copper foil will be more heated.Slow,it will reach the ambient temperature in the reflow furnace more slowly.When the solder paste on one end of the part melts earlier than the other end,the part where the solder paste melts first will be used as the fulcrum to lift the part,causing the other end of the part to be empty.Soldering,as the time difference between the melting of the solder paste increases,the angle at which the part is lifted will increase,resulting in a complete tombstone result.

The method to solve the empty welding of the tombstone:

1.Design solution

Thermal resistance can be added to one end of the large copper foil to slow down the problem of excessive temperature loss. Reduce the size of the inner distance of the solder pad, and minimize the distance between the solder pads at both ends without causing a short circuit,so that the solder paste at the slower melting tin end has a larger view and can stick to the body and prevent it from standing up., Increase the difficulty of erecting the monument.

2.Process solution

The temperature of the wetting zone of the reflow furnace can be increased to make the temperature closer to the melting temperature.It can also slow down the heating rate of the reflow zone. The purpose is to make the temperature of the printed circuit boards reach the same level,and then melt the tin at the same time.

3.Deactivate nitrogen

If nitrogen is turned on in the reflow furnace, you can evaluate to turn off the nitrogen and try it. Although nitrogen can prevent oxidation and help solder, it will also worsen the difference in the original melting temperature and cause the problem of melting tin first in some solder joints.

The following are also possible reasons that may cause the tombstone to be erected:

Unilateral oxidation of parts or pads

Parts placement offset Feeder (Feeder) is unstable resulting in inaccurate suction

Solder paste printing misalignment (the solder paste printing misalignment also needs to consider the problem of puzzles, the more puzzles, the larger the probability of printing misalignment)

Poor accuracy of smt placement machine

In the same situation,the capacitor (C) is more prone to tombstone disconnection than the resistor (R). This is because only three sides of the resistor terminals are plated with solder, while the capacitors are plated with solder on five sides.On the left and right sides, the capacitor is generally thicker than the resistance, and the center of gravity is higher,so it is easier to be lifted under the same force.