August, 2019
www.us-tech.com Overcoming the Tombstone Effect By Marc van Stralen, IMDES Creative Solutions T
ombstoning is a defect that occurs when an SMT component is partially or com- pletely lifted from the PCB. Generally, one end of the component is soldered to the PCB, while the other lifts off, resembling a cemetery tombstone.
This effect is mainly observed in small and light two-pole SMD components, such as 008004, 01005, 0201, and 0805 (0201, 0402, 0603, and 2012 metric) resistors and capacitors. Possible cause of tombstoning include inac- curate layout, incorrect stencil geometry, incor- rect placement, and poor surface quality. In gen- eral, the better (the more oxygen-depleted) the soldering process, the lower the interface ten- sion. High surface tensions lead to uneven mass- es, lifting two-pole SMD components.
Tombstones in Vapor Phase Soldering
The oxygen-free vapor blank in vapor phase soldering prevents the re-oxidation during soldering. This implies a significantly lower oxida- tion of the metallic surfaces during transit into reflow, compared with reflow processes under normal process environment conditions. Vapor phase soldering ensures a clean surface for optimal wetting. The fast, initial wetting does not leave time to reduce ∆Ts. However, an additional delay in initial wetting to reduce ∆Ts is exactly what is need- ed to minimize tombstones. There - fore, in addition to the obvious bene- fits of vapor phase soldering, other precautions must be taken to mini- mize tombstones.
There are basically two causes of tombstoning. First, there are defects related to the surface of the PCB and components, such as solder- ability, oxidation and physical dam- age to the plating. Secondly are defects related to the temperature (∆T) and heat dissipation.
Effect of Solder Paste Alloy
The greater the pad geometry and the area of the molten solder, the stronger the effect of surface tension, which is why attention must be paid to the thermal mass of the component connection surfaces.
The elimination of tombstones, or at least a significant minimization, can be achieved in part by using the correct SnAgCu lead-free solder paste. This can include: the use of sol- der paste with a thermally stable flux system that provides and maintains stickiness; and a paste with metal particles with two different eutectic points — 50 percent melts at 422.6°F (217°C) for Sn95, 5Ag3, 8Cu0, 7, and the remainder at 429.8°F (221°C) for Sn96Ag4.
The tombstone effect, which is created by the force of a faster first wetting in a joint is mechanically impeded by the solid particles of the alloy, which melt at 429.8°F (221°C). This gives the alloy, which melts at 422.6°F (217°C) on the other pad, the fraction of a second longer to melt and to create balance in the wetting force. SnPb (tin-lead) solder alloys are another issue. This requires: the use of solder paste with a thermally stable flux system that provides and main- tains stickiness; a paste with metal particles with two different eutectic points — 50 percent melts at 354.2°F (179°C) (Sn62, 5Pb36, 5Ag1, 0) and the remainder at 361.4°F (183°C). Tombstoning is created here also by the force of a faster first wet-
ting in a joint, but is mechanically impeded by the solder particles of the alloy that melt at 361.4°F (183°C). This then gives the alloy, which melts at 354.2°F (179°C) on the other pad a fraction of a second more time to melt and create a balance in the wetting force.
Other Ways to Reduce Tombstoning
Since pads cannot always be changed, other ways to reduce tombstoning include:
l A reduction of the solder paste quantity by reducing the stencil thickness. l Modifications to the stencil openings to
wedge or strip openings. l The paste quantity can be reduced, but the
Diagram of the tombstone effect in SMD devices. Continued on page 65
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