Preface

 

 

 

Recently, a fan asked about the problem of solder empty on SMD small chips (smallchips) such as resistors and capacitors. Although I have repeatedly checked all the reflow profile settings and confirmed them to be correct, I still often find that these types of resistors and capacitors have empty soldering, and they also appear in fixed positions. Even if I observe the soldering conditions, I can't find it. The situation of non-wetting or de-wetting is puzzling?

 

 

 

 

 

 

In fact, the problem of air welding of this type of parts is not that difficult to understand! The reason for its formation is the same as the reason for erecting a tombstone. To put it bluntly,

 

The solder paste melting time at both ends of the part is inconsistent.

 

 

 

Eventually, the force is uneven and one end becomes warped.

 

 

 

If you carefully observe the copper foil wiring on the problematic PCB, you can usually find that there is a large piece of copper foil connected to one end of such parts, but not to the other end. Usually when the PCB enters the reflow furnace and starts to be heated, the copper foil on the surface will be heated faster, that is, it will reach the ambient temperature in the reflow furnace faster, while the large copper foil on the inner layer will be heated. It will be slower, that is, it will be slower to reach the ambient temperature in the reflow oven. When the solder paste on one end of the part melts earlier than the other end, the part will be lifted with the end where the solder paste melts first as the fulcrum, causing The other end of the part is open-soldered. As the time difference between solder paste melting increases, the angle at which the part is lifted will become larger, eventually forming a complete tombstone.

 

Friends who are interested in in-depth study of the real reasons for the formation of the monument can start from the perspective of mechanics. When the pulling force at one end is higher than the other end, the weight of the part should be added and the position of its center of gravity should be calculated. Then one end can be used as the fulcrum to move the part. Lift the other end.

 

The following two pictures are examples of air welding pictures for your reference.

 

 

 

 

 

 

The methods to solve this type of empty welding of tombstones include:

 

 

 

1. Solved through design. [Thermal relief] can be added to the end points of the large copper foil to slow down the problem of excessive temperature loss.

Reduce the inner spacing of the solder pads and minimize the distance between the solder pads at both ends without causing a short circuit. This will allow the solder paste at the slower melting end to have more space to stick to the body and prevent it from standing up. , that is, increasing the difficulty of erecting monuments.

 

 

 

2. Solved through manufacturing process. The temperature of the infiltration zone of the reflow furnace can be increased to bring the temperature closer to the molten tin temperature. It can also slow down the heating rate of the reflow area. The purpose is to make the temperature of all the lines on the PCB reach the same level, and then melt the tin at the same time.

 

 

 

3. Disable "Nitrogen (N2)". If nitrogen is turned on in the reflow oven, you can evaluate and try turning off the nitrogen. Although nitrogen can prevent oxidation and help soldering, it will also worsen the original melting tin temperature gap, causing the problem of some solder joints melting tin first.

 

 

 

Other notes:

 

The occurrence of the above-mentioned empty welding and monument erection is only one of many possible reasons. The following items are also possible reasons for the erection of monuments:

 

§ Unilateral oxidation of parts or pads

 

§ Part placement offset

 

§ Feeder (Feida) is unstable resulting in inaccurate material suction

 

§ Solder paste printing offset (Solder paste printing offset also needs to consider the issue of panelization. The more and larger the panels, the higher the chance of printing offset)

 

§ The accuracy of the placement machine is poor

 

Secondly, in the same situation, the capacitor (C) is more likely to have tombstone breakage than the resistor (R). This is because the terminals of the resistor are only plated with solder on three sides, while the capacitor is plated with solder on five sides. There are two more left and right sides. In addition, capacitors are generally thicker than resistors, so the center of gravity is relatively high, making it easier to lift under the same force distance.