Component "tombstone" phenomenon in the reflow soldering often occurs, refers to the chip components in the soldering process stand up the situation, also known as "drawbridge" or "Manhattan" phenomenon. This phenomenon is a common defect, the root cause of the component ends of the wetting force is not balanced, resulting in an imbalance of torque, which triggered the "monument" phenomenon. As shown in Figure 2. If M1>M2, the element will stand up to the left; if M1<M2, the element will stand up to the right.

Figure 1. "Tombstone" phenomenon.

Figure 2. Unbalanced moments at the two ends of the element lead to the "monument" phenomenon.

Any of the following situations can cause an imbalance in the wetting force between the two sides of the component during reflow soldering.

Pad design and layout is not reasonable

1. components on both sides of one of the pads connected to the ground or one side of the pad area is too large, uneven heat capacity at both ends of the pad;

2. PCB surface of the temperature difference is too large so that the component pads on both sides of the heat absorption is not uniform;

3. large devices QFP, BGA, heat sinks around the small piece of component pads at both ends will appear uneven temperature.

Solution: improve the pad design and layout.

Solder paste and solder paste printing

Solder paste activity is not high and poor solderability of components, solder paste melting, the surface tension is not the same, the same will cause unbalanced pad wetting force. Two pads of solder paste printing is not uniform, more than one side will be due to increased heat absorption of solder paste, melting time lag, so that the wetting force is not balanced.

Solution: Select the higher activity of the solder paste, improve the solder paste printing parameters, especially the window size of the template.

Patch

Uneven force in the Z-axis direction will lead to uneven depth of the component immersed in the solder paste, melting will be due to the time difference between the two sides of the unbalanced wetting force. Component patch displacement will lead to standing monument.

Solution: Adjust the process parameters of the mounter.

Furnace temperature curve

PCB heating work curve is not correct, so that the temperature difference on the board is too large, usually reflow oven furnace body is too short and the temperature zone is too small will appear these defects, defective operating temperature curve as shown in the figure.

Solution: According to each different product to adjust the appropriate temperature profile.

Figure 3. Defective furnace temperature operating curve.

Oxygen concentration in N2 reflow soldering

The use of N2 protection reflow soldering will increase the wetting power of the solder, but more and more reports illustrate that the occurrence of standing monument phenomenon in the case of oxygen content is too low rather than increased. It is usually considered appropriate to control the oxygen content at about 100~500ppm.

Through the above solutions, can effectively reduce or avoid the chip components in the reflow soldering process, "monument" phenomenon, improve soldering quality and reliability.