Topic

Tombstoning

This effect has a number of names, which include ‘Manhattan effect’, ‘drawbridging’, and ‘Stonehenge effect’, all of which describe chip resistors or capacitors that are soldered at one end only and lifted at an angle of up to 90° from the board by the surface tension of solder.

Figure 1: Component lifting: a ‘drawbridged’ capacitor

Figure 2: Component lifting: a ‘tombstoned’ resistor

As well as the traditional tombstoning shown in Figure 2, an alternative defect which is becoming increasingly common now that chip components are getting nearer square in cross-section is ‘bill-boarding’, where both ends of the chip component are connected, but the component is mounted side-on to the laminate, at 90° to its intended orientation.

Tombstoning and bill-boarding of chip components are caused by a lack of symmetry in the forces applied by the molten solder to the component (Figure 3). There may be several reasons for this:

Increasing with smaller components, where the pads are themselves small (to take advantage of the small chip size) and often little bigger than the component, the major cause of tombstoning is reported to be poor placement accuracy.

Figure 3: Mechanism of tombstoning and drawbridging

 

 

The effect is noticeably more frequent with short, wide chips, and with thin devices such as resistors. This is because it is the weight of the chip, acting through its centre of mass (usually the geometric centre of the device), which counteracts surface tension forces in the molten solder: these forces vary with chip dimensions; the downward forces vary with the square of the chip dimensions. The problem was sufficiently common in the 1980s to have promoted the 1206 chip component in preference to the 0805 size.

Because tombstoning is related to uneven wetting, it is affected by anything that changes the amount of solder available to make the joint. For example, if solder is drained away to a nearby via. Even when a via is filled, as with via-in-pad, differences in wetting can be sufficient to increase the percentage of tombstone defects.

Tombstoning is also related to speed of wetting, as reported by Benlih Huang and Ning-Cheng Lee in Effect of lead-free alloy composition on tombstoning in Global SMT & Packaging, October 2004. Their investigation into the effect of alloy composition using SAC solder indicated that:

They concluded that it was beneficial to use an SAC alloy with a relatively low silver content (say 2.5%).

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