Is The Single-chip Welding Process Important

Many pcb engineers have paid little attention to the welding process in the early stage, and even rarely go back to check the solder joints when making samples. We usually think that the solder joints with unsightly appearance are poor soldering. For example, disturbed solder joints, cold solder joints, blow holes, etc. can lead to a decrease in mechanical strength. The sharpening of the solder joints indicates poor wetting. Too much solder can lead to the inability to judge the wettability of the pads, and so on.

If we are soldering, there is no problem with other processes on the solder joint, but there is an extra tip, will it affect the signal?

The answer is yes, if the frequency of the signal is very high it will have an effect. In a high-level digital circuit, whether it is a solder joint tip or a via stub will have a very large impact on the signal, and even cause the signal to be unusable.

Suppose our PCB board is eight layers, and the signal is connected from the first layer to the third layer. In the common plate making process, the vias are generally through the PCB, although we only need the first layer to the third layer to transmit signals. This part of the hole is enough, but in fact, the via hole still extends from the third layer to the other side of the PCB, and this extra via hole is the via hole stub.

In high-speed digital circuits, via stubs, like our previous TV antennas, will release electromagnetic radiation and receive interference, which will seriously affect the quality of high-speed digital signals. When the transmission rate exceeds 10Gbps, the via stub will affect the propagation integrity of the high-speed signal. Usually, when we design the circuit board, we will also provide a protection against this problem, use the backdrill process to remove the unused via metal, and minimize the via stub to less than 10mil.

In the above picture, the eye diagram on the left is the effect after the backdrill process is used to remove the via stub, and the right picture is the effect with the via stub. It can be seen that the data eye in the right picture is almost closed. The signal quality is relatively poor.