How Factories Deal with the Tombstone Phenomenon?

In the PCBA processing industry in Shenzhen, most PCBA processing manufacturers will encounter an undesirable phenomenon. One end of the chip component is lifted during the SMT chip processing. This situation mostly occurs in small-size chip resistor-capacitor components, especially 0402 chip capacitors and chip resistors. This phenomenon is often referred to as the “tombstone phenomenon”.

Causes of the Tombstone Phenomenon:

(1) The melting time of the solder paste on both ends of the component is not synchronized or the surface tension is different, such as poor solder paste printing (one end is incomplete), offset, and the size of the solder end of the component is different. Generally, the end melts after the solder paste is pulled up.

(2) Pad design: The elongation of the pad has an appropriate range, too short or too long will easily cause tombstone phenomenon.

(3) The solder paste brush is too thick, and the components will float after the solder paste melts. In this case, the components are prone to tombstone formation due to the hot wind blowing.

(4) Temperature curve setting: The tombstone generally occurs at the moment when the solder joint starts to melt. The heating rate near the melting point is very important, and the slower it is, the better it is to eliminate the tombstone phenomenon.

(5) One solder end of the component is oxidized or contaminated and cannot be wetted. Pay special attention to components whose solder ends are single-layer silver.

(6) The pad is contaminated (with silkscreen, solder mask, foreign matter adhered, and oxidized).

How Does the Tombstone Phenomenon Happen:

During reflow soldering, the upper and lower surfaces of the chip components are heated at the same time. Generally speaking, the pad with the largest exposed area is first heated to a temperature above the melting point of the solder paste. In this way, one end of the component wetted by the solder is often pulled up by the surface tension of the solder at the other end.

Solution:

(1) Design: Properly design the pad-the extension size must be reasonable, and try to avoid the situation where the outer edge (straight line) of the pad formed by the extension length is greater than 45°.

(2) Production site:

•  Wipe the screen frequently to ensure that the solder paste performance graphics are complete.
• The placement of the patch is accurate.
• Use non-eutectic solder paste and reduce the heating rate during reflow soldering (control at 2.2°C/s).
• Reduce the thickness of solder paste.

3) Incoming materials: Strictly control the quality of incoming materials to ensure that the effective area of ​​the two ends of the components used is the same (the basis for surface tension).