In the  Multi-layer PCB SMT  processing industry, most  Multi-layer PCB SMT  processing manufacturers will encounter the undesirable phenomenon. During the SMT patch assembly process, one end of the chip component is lifted, causing the component to stand up. This situation often occurs in small-sized chip RC components, especially 0402 chip capacitors.

1. Formation Reasons:

1.1. The melting time of the solder paste at both ends of the component is not synchronized or the surface tension is different, such as poor solder paste printing (one end is incomplete), misalignment, and the size of the component soldering end is different. Usually the melted end of the solder paste is pulled up.

1.2. Pad design: There is a suitable range for the extension length of the pad. Too short or too long is prone to tombstone phenomenon.

1.3. The solder paste is too thick, and the components will float after the solder paste melts. In this case, the components are easily tombstone due to hot air blowing.

1.4. Temperature curve setting: Tombstone generally occurs when the solder joint begins to melt. The heating rate near the melting point is very important. The slower it is, the better it is to eliminate the tombstone phenomenon.

1.5. One solder end of the component is oxidized or contaminated and cannot be wetted. Pay particular attention to components with single-layer silver terminations.

1.6. The pad is contaminated (with silk screen, solder mask ink, foreign matter adhered, oxidized).

2. Formation Mechanism:

During reflow soldering, the top and bottom of the chip components are heated at the same time. In general, the pad with the largest exposed area is always heated to a temperature above the melting point of the solder paste first. In this way, one end of the component that is later wetted by the solder tends to be pulled up by the surface tension of the solder at the other end.

Solution:

2.1. Design aspects

Reasonable design of the pad - the overhang size must be reasonable, and try to avoid the situation where the wetting angle of the outer edge (straight line) of the pad formed by the overhang length is greater than 45°.

2.2. Production site

2.2.1. Wipe the screen frequently to ensure that the solder paste results in complete graphics.

2.2.2. The placement of the patch is accurate.

2.2.3. Use non-eutectic solder paste and reduce the heating rate during reflow (control at 2.2°C/s).

2.2.4. Reduce the thickness of the solder paste.

2.3. Incoming materials

Strictly control the quality of incoming materials to ensure that the effective area at both ends of the components used is the same size (the basis for generating surface tension).