Consider the Design -- The assembly process begins with a good design, but requires reliable solderable materials and diligent equipment maintenance.

What causes tombstoning and how can it be prevented?

A: When one end of a chip component lifts from the corresponding pad during reflow, generating an open, the defect created is referred to as tombstoning or drawbridging. The major cause of this defect is the surface tension and unequal wetting forces in action during the reflow process. Many factors can contribute to unequal wetting forces-at each end of the chip component-during the soldering process. Two major factors promoting tombstoning are: 1) differential wetting forces at the pads and 2) improper design of component pads.

The Usual Suspects

In some instances, unequal wetting forces can be a direct result of inadequate component or circuit board termination solderability characteristics. Solder paste deposition volume variations, or oxidized or dried out solder paste, can also contribute to inadequate soldering conditions. The solder paste printing process and equipment can be the cause of many cases of tombstoning. Adequate stencil preventative maintenance, assuring reproducible and desired solder paste volume, is important in all cases-particularly when small, discrete components are in use.

Design is the first step in the manufacturing process, and pad design may be the major cause of tombstoning. Shorter, wider pads seem to be more forgiving than long, narrow pads. Referring to IPC 782 Surface Mount Design and Land Pattern Standard gives a more detailed explanation. In essence, pads that extend too far beyond the component can allow the component to slide during the solder wetting process-pulling the component off the pad at one end.

With small, discrete chip components, the perceived need to design in a different pad size for one end of the component, or the connection of one of the pads to a ground plane, can also lead to tombstoning. The use of different pad sizes can result in unequal pad heating and solder paste flow times. During reflow, the component literally floats on the liquid solder and reaches its final position when the solder solidifies. Differential wetting forces at the pads can result in lack of adhesion and rotation of the component. In some cases, lengthening the time above liquidus can result in reduced tombstoning. Extending the time above liquidus can result in a more uniform temperature across the component lands.

Other Culprits

Other cited causes of tombstoning include fabricated board conditions. Uneven solder mask thickness, under the component body, may lift one component termination off the pad. Solder mask on the pad may also reduce the wetting on the pad at one end.

The placement process can also be the source of tombstoning defects. Rotational errors and misplacement of the component on the pads can result in component movement during solidification. Ballistic, forceful placement of the component can extrude solder paste unevenly, from under the pads, and produce uneven wetting during reflow. In addition, fast shuttle acceleration and deceleration of the assembly, during the placement process, can displace the component and cause insufficient contact of the termination to the fabricated board pad.

Conclusion

As with many characteristics of the electronics soldering process requiring continuously evolving materials, components and equipment, no one solution exists to solve or eliminate a defect. Eternal vigilance, attention to detail and continuous education are the keys to success.

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Reference

1. IPC-S-816, SMT Process Guideline and Checklist, July 1993, p. 22, paragraph 8.2.5.

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Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; [phone]; e-mail: [e-mail].

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