All About Tombstoning
Tombstoning is an issue occurring during the soldering process when all components on a board are anchored to its bare surface. While there are many that will point fingers of blame on fabricators and assemblers for the tombstoning issue, the truth is designers also have a role to play. In this article, Rush PCB Inc discusses what tombstoning is, and how designers can prevent it when designing the PCB.
What is Tombstoning?
As the name suggests, tombstoning resembles slanted slabs of granite sometimes found at cemeteries. For a PCB assembly, tombstoning is related mostly to two-pin surface mount components, typically passive, like capacitors and resistors. The component rests partially on the PCB with one end soldered to its pad, while the other end is lifted up from its pad as an open circuit.
While the angle of the lifted tombstone component can vary, the net result is the same—an open circuit, leading to an unusable PCB. Most manufacturers will correct the issue with post-soldering rework but charge extra for it. Tombstoning was a frequent issue with vapor phase reflow techniques. Although this problem has reduced with the decline in use of vapor phase reflow, it has not entirely ceased to exist.
While manufacturers around the world are struggling with tombstoning, there are two primary reasons for an increase in its occurrences:
- Component Size — PCB assemblies now use components with significantly reduced size and mass. Lighter components are more prone to shift and lift during soldering.
- Newer Regulations — PCB assemblies must meet RoHS regulations, for which the assemblers use lead-free solders that melt at higher temperatures.
The above two variables put together, are a certain recipe for tombstoning to occur.
Why does Tombstoning Occur?
Before coming to why tombstoning occurs, it is necessary to understand the process of anchoring components to a bare board with solder, and the concept of wetting. Wetting is the process of molten solder spreading over and covering the entire pad, and in the process, attaches to the pad and the component lead. Once the solder cools, the component is firmly attached to its pads. A proper solder joint has a smooth concave meniscus, with a wetting angle of about 40-70 degrees.
Typically, the molten solder wets both pads of the component, and completes its wetting process simultaneously on both ends—without resulting in tombstoning issues. The problem starts when the wetting process is unbalanced. If the wetting process on one of the pads completes before that on the other one does, it can result in solder on one side of the component solidifying while the other is still in the process, and the component starts facing a twisting action due to the meniscus pull.
For instance, a simple SMD resistor has two pads. In an ideal situation, solder will wet both pads simultaneously, thereby attaching both ends to the board at the same time. However, if the wetting process is unbalanced, the pad on which the wetting process has completed first starts pulling up the other end. As this end is still in the process of wetting, the entire component lifts on this end, and looks like a tombstone. While there are many variables leading to tombstoning during a typical wave reflow process, here are the most common causes:
- Uneven Temperatures — If the temperature of the reflow oven is uneven, it can cause the solder to begin and complete its wetting process on the PCB at different times.
- Non-Uniform Solder Paste — If the amount of solder paste deposited on the pads of the board are not uniform, it can lead to different wetting times.
- Non-Uniform Pads — If the pads of a component are of different sizes, they can affect the accuracy of the solder paste deposition, leading to different wetting times.
Sequence of Tombstoning
One of the main causes of tombstoning in a practical design is when an SMT components with identical pads connects to two different trace widths. For instance, while the right pad connects via a wide trace to a ground plane, the left pad attaches only to a thin trace. In the process of heating up inside a reflow oven, the left pad is going to heat up way more than the right pad.
Therefore, the left pad will complete its wetting process first, as it has heated up more. During the solidification process, solder on the left pad will likely assert a greater meniscus pull on the component, thereby lifting the right end of the component from the board, creating the tombstoning effect.
Causes of Tombstoning
Although manufacturing process imperfections can and do cause tombstoning, designers can also influence the occurrence with some factors. Mostly, these include non-uniform pads, or a PCB with uneven copper mass on the pads. But, there can be other causes also that lead to uneven heating, and thereby to tombstoning:
- Multiple tracks connect to a pad
- Large copper area connects to a pad
- PTH close to a pad
- Via in a pad
- Uneven pad sizes
- Legend under component
- PCB finish
Track to Pad Issues
A pad that has multiple tracks connecting to it will remain cooler than the pad with a single track connected to it. This is because the multiple tracks will conduct away most of the heat. This will lead to the wetting process being completed at different times, increasing the probability of tombstoning.
It is common for designers to use a thicker track when connecting a pad to a large ground or power area. However, this will result in the wetting process for the pad with the thinner track being completed before the pad with the thicker track can, resulting in tombstoning.
The aim is to match the track widths connecting to both pads of a component. This unified track width must run at least 0.5 mm from the pad edge before any changes to the widths. It is also a good practice to run the traces in a similar orientation. This helps to synchronize the wetting process.
Mismatched track width leading to tombstoning is not restricted to the outer layers alone. The designer must inspect the inner layers of the board also. If an inner layer has a large copper area covering only one of the pads of the component, but not the other, then the probability for tombstoning to occur increases, as there will be uneven heating during the wave soldering process. A safe way would be for the designer to allow the copper area to cover both pads identically.
PTH Close to Pad
A PTH or plated through hole or via placed too close to a pad will have two repercussions. One, it will draw heat away from the pad, keeping it cooler than the pad without a PTH or via nearby. This will unbalance the wetting time for the pads, increasing the chances of tombstoning.
The second effect of an unplugged via or PTH close to a pad is solder wicking. Capillary action and meniscus effect will cause molten solder from the pad to wick down into the hole, leaving the pad without solder to wet it. The other pad that has no via or PTH nearby, will complete the wetting process faster, and its pull on the component may cause it to rise up and form a tombstone.
There can be three solutions to the above problem:
The first is to position the via or PTH a safe distance away, about 0.5 mm, from the pad edge. This will prevent the via or PTH from drawing away heat from the pad, thereby synching the wetting process.
In addition to the above, filling the via or PTH with epoxy will prevent it from wicking the solder away from the pad. This will allow the wetting process to complete at the same time as for the other pad, reducing the chances of tombstoning.
It is also possible to obtain the same effect as that of filling the via or PTH by using a soldermask dam between the pad and the via or PTH. The dam will prevent the solder from flowing towards the via or PTH. However, none of the above solutions are possible if the designer has placed the via or PTH within the pad itself.
Via in Pad
Miniaturization in electronics is compelling designers to increase the component density on a board and to use close-pitch components like BGAs. As a result, there is a significant reduction in the number of channels available for routing. One of the solutions to this problem is placing vias within the component pads. However, this increases the chances of solder wicking down the hole, resulting in an uneven wetting process.
Even if the PTH has a component lead in the hole, there is still a greater chance of one pad completing the wetting process before the other pad can, leading to tombstoning. Even if there is no tombstoning, there is still insufficient solder to properly anchor the component.
For via in pad, the only solution is via filling, as this not only reduces the chances of dry soldering, but of tombstoning also.
Uneven Pad Size
Ideally, designers use the IPC-7351B standard for assigning shapes and sizes of component pads to avoid problems like tombstoning. However, there can be inadvertent mistakes and one of the pair of pads may be different in size and shape from the other. This will inevitably lead to different wetting times, and possibly to tombstoning. Even when the pads are of the same size and shape, but larger than those necessary for the component, the result may be a shifted or rotated component.
Legend Under Component
Placing legends under a component is clearly inappropriate, as it defeats the purpose of having a legend, since the component will hide it anyway. Moreover, placing the legend below a component also increases the chances of tombstoning. This is because the total thickness of a copper track, soldermask, and the legend layer could be more than the height of copper pads, thereby preventing the component from sitting flat on the board. The effect becomes more critical as the component size decreases.
The only solution to this problem is to not place legend under components.
The same situation can arise if the soldermask under a component is thick or uneven. Although soldermask is a great protective layer, if its thickness on the board is more than that of the pads, the chances of tombstoning will increase. Designers should confirm from their manufacturers that the soldermask thickness they plan to use will be below the level of the land patterns on the board.
Because of the trend towards miniaturization, designers prefer using tiny component sizes, like 0402 or even 0201. However, smaller components have an increased tendency toward tombstoning. Moreover, traditional surface finishes like HASL or hot air surface levelling on a board does not offer an even finish. This affects smaller components more than it does the larger ones, and leads to an increase in tombstoning by impacting the wetting process.
To get rid of tombstoning due to uneven surface finish, designers can consider using other board finishes, such as immersion tin, OSP, or immersion gold, as they offer a more even surface finish.
Following the above guidelines from Rush PCB Inc will go a long way towards avoiding the tombstoning issue on a design. However, this is not to say that the above guideline alone is adequate for preventing the occurrence of tombstoning. A careful analysis is necessary to decide whether the issue is one of design or a process problem at the manufacturer’s end.