The Tombstone Effect in Reflow Soldering
Tombstoning is one of the many issues that Rush PCB and other PCB assemblers face during the soldering process of PCBs. This is a common issue most PCB manufacturers face. As contract manufacturers, we usually inform the customer about tombstoning, but it is important to know what it is and how to prevent it.
What is Tombstoning?
Tombstoning occurs in PCBs using surface mount passive components like capacitors and resistors. Typically, the component has one end soldered to the pad, while the other end lifts at an angle. Sometimes, the component stands on one of its ends and looks like a tombstone in a graveyard—hence the name. This is a serious issue, as the PCB assembly is non-functional because of the open circuit.
The angle of the lifted component may vary, but that does not matter. The PCB remains non-functional until an operator has reworked the affected component. Tombstoning is a potential quality issue, as the PCB requires post-soldering rework and the customer must bear the expenses.
What Causes Tombstoning?
Rush PCB has identified two primary factors that have increased the occurrences of Tombstoning. These are:
- Use of lead-free solders with a higher melting point and lower wetting capability, and
- A significant decrease in the mass and size of surface mount technology or SMT components.
A passive SMT component has two end caps that rest on copper pads on the PCB. For soldering, the manufacturer deposits solder paste on the pads before mounting the component. When the assembly goes for reflow, heat melts the solder paste and the liquid solder must wet the copper pads and the end caps of the component thoroughly. During the cooling phase, the molten solder solidifies and anchors the component to the pads. Ideally, the component will lie parallel to the PCB surface after soldering.
However, the wetting process may be unequal for the two pads. One of the pads may have solder paste heating and cooling at a different rate than that on the other. On one pad, the solder paste may melt before the paste on the other pad can. The molten solder exerts a surface tension pull on the component, while the other side has no such pull.
The one-sided pull on the component tilts and lifts the other end from the board. The effect is more pronounced on lighter and smaller components, as the size and weight of a large component can effectively counter the one-sided pull, preventing the lift.
Therefore, the most common causes of tombstoning can include:
- An uneven temperature across the board, causing solder to begin and complete its melting and wetting process at different times. A large component close by can throw its shadow across a part of a smaller component, causing uneven temperatures on the smaller component.
- Non-uniform solder paste deposit across the pads of a component can lead to different times for the solder paste to melt and wet the pads and the end caps of the component.
- The designer may have ignored the tolerance specification for the pads of the component, resulting in a variation in the size of the pads. This would also reflect in varying apertures in the stencil, leading to an unequal solder paste deposit for the component.
How to Prevent Tombstoning?
Although a part of the problem of tombstoning depends on uneven temperatures in a reflow oven, the design of the PCB plays a greater part. To reduce the chances of tombstoning, there are a few things a PCB designer must take care of:
Component Pad Dimensions
It is easy to mess up pad dimensions during the process of creating footprints. The most likely mistake could be to make one of the pads larger or smaller than the others. The extra copper acts as a heat sink during the reflow process, soaking up heat while the solder is melting, thereby slowing down the melting process. Again, during the cooling process, the extra copper gives up its heat, slowing down the solidification process. This causes unequal wetting on the component, increasing the chances of tombstoning.
The designer must follow the specifications that the component manufacturers offers in their data-sheets. The designer may also use the IPC-7351B footprint calculator for calculating the land patterns precisely based on the actual dimensions of the component.
PCB Surface Finish
The tombstoning problem is more pronounced on smaller components such as of size 0402 and 0201. Rather than the traditional Hot Air Solder Leveling or HASL on boards with such small components, a different surface finish may help.
Although HASL is the usual industry norm for surface finish because of its low cost and wide availability, HASL also causes uneven surface finish impacting the solder wetting process. Better alternatives would be immersion tin, OSP, or immersion gold, depending on design constraints.
Routing and Placement
With large and heavy components not prone to tombstoning, different trace widths on the two pads may not be an issue. However, for small and light components, traces connecting their pads must be of the same width. As the pads of these components are not very big, different trace widths on either side can cause an unbalancing thermal effect.
For small pads, the tracks can have a profound effect even with their different orientation. It is preferable to ensure that the tracks connect to the pads in similar orientation, either in the X- or in the Y-direction. This prevents variable heat dissipation with differently angled tracks.
Placement plays a large part for determining the distribution of heat within a reflow oven. Placing small components very close to a large one may cause an unequal distribution of heat. The larger component can soak up heat during the soldering process, starving the smaller components of heat, so they take longer to solder. Likewise, when cooling, the larger component gives off enough heat to prevent the smaller components nearby from cooling quickly.
The presence of a large copper land near one of the pads of an SMT component may also increase the chances of tombstoning on that component. The large copper land, either on the same plane or an inner plane also acts as a heat sink, delaying the soldering process on one of the nearby pads.
Designers must ensure that copper land covers both sides of a component equally, even when the copper land is on a different layer.
Thin Solder Mask
Solder mask thickness lower than the thickness of copper is preferable and helps prevent tombstoning. There are two ways designers can open the solder mask on a pad—solder mask defined or SMD, and non-solder mask defined or NSMD.
For an SMD pattern, the actual size of the pad may be larger than necessary. The solder mask opening defines the solder paste deposition, as the solder paste opening is smaller than the pad size. However, this also means the pad sizes have more chances of being unequal, increasing the chances of tombstoning.
For an NSMD pattern, the solder mask opening is marginally larger than the actual pad size. This ensures the designer makes both pads of the same size, thereby reducing the chances of tombstoning.
RUSHPCB.COM has vast experience in designing circuit boards and fabricating PCBs for various industries. We have tried to provide a guideline on preventing tombstoning that will help our customers avoid this issue.