Tombstoning is just one of the numerous issues Rush PCB and other PCB assembly workers face in the soldering process for PCBs. It is a problem that the majority of PCB manufacturers have to deal with. As contract makers, we generally inform our customers about tombstoning. However, it’s essential to understand the causes and ways to avoid it.
What is Tombstoning?
Tombstoning can be found in PCBs with passive components that are mounted on the surface, such as capacitors, resistors or even resistors. The typical component is soldered on one end onto the pad and the other one is lifted in an angle. Sometimes, the part is placed on its one end and appears to be the tombstone of a graveyard, hence the name. This is a significant problem, since the PCB assembly is not functional due to an open circuit.
The angle and the position of the component that is lifted could vary, but it doesn’t matter. The PCB is not functional until the operator has repaired the component affected. Tombstoning could be a quality issue as the PCB requires rework after post-soldering, and the purchaser is responsible for the costs.
What Causes Tombstoning?
Rush PCB has identified two major factors that have led to an increase in the frequency of Tombstoning. The two factors are:
- Lead-free solders are solders that have greater melting points and a lower capacity for wetting and
- Significantly less weight and size of the SMT or surface mount technology components.
A passive SMT device has two end caps, which sit on copper plates that are on the PCB. To conduct soldering the component, the manufacturer puts an adhesive paste onto the pad prior to installing the device. If the assembly is ready to reflow, the it is heated to melt the solder paste. the liquid solder has to be soaked into the copper pads and cap ends of the component completely. During the cooling process the molten solder hardens and secures the component onto the pad. Ideally, the component should sit close to the PCB’s surface after the soldering.
The wetting process could be different for the two pads. One pad may have solder paste that is heating and cooling at a different speed than one. On one pad the solder paste might melt prior to that on the other pad has the chance to. The melting solder creates the force of a surface tension on the part, whereas the other side doesn’t have pulling force.
The one-sided pull of the component is a tilt that lifts the other side of the board. This effect is stronger for smaller and lighter components due to the weight and size of a heavy component will effectively withstand the pull of one side which can prevent the lifting.
So, the most frequent causes of tombstoning are:
- A variation in temperature across the board causes solder to start and finish its melting and wetting processes at various time. A big component located close to could cast its shadow over an area of a smaller component, creating an uneven temperature on the smaller one.
- The non-uniform distribution of solder paste across each pad on a component could result in different times for the paste to melt and then wet the pads as well as the ends caps of the part.
- The designer might have not considered the tolerance requirements on the pads in the component, which resulted in a variance in the pads’ size. This can also result in different apertures on the stencil, which could result in the same amount of solder paste on the part.
How to Prevent Tombstoning?
Although a large part of the issue of tombstoning is due to the uneven temperature of an oven for reflow, the structure of the PCB is a major factor. To minimize the chance of tombstoning, there are few points that the PCB designer should be aware of.
Component Pad Dimensions
It’s not difficult to make mistakes with the dimensions of pads when creating footprints. One of the most frequent mistakes could be to create one pad bigger or smaller than the other pads. The extra copper functions as an energy sink in the process of reflow, taking in heat as the solder melts which slows down its melting. In the cooling process the extra copper releases the heat away, which slows down the process of solidification. This results in uneven wetting of the part, which increases the chance of tombstoning.
The designer should adhere to the specifications of manufacturers of components provide in their data sheets. Designers can also utilize the IPC-7351B footprint calculator to assist in making precise calculations of land patterns according to the actual dimension of the part.
PCB Surface Finish
The problem of tombstoning is most evident on smaller components, such as 0302 or 0201. In lieu of the conventional HOT Air Solder Leveling, or HASL for boards with these tiny components, a new surface finish could assist.
Even though HASL is the industrial standard for finishing the surface due to its low cost and broad access, HASL also causes uneven surface finish, which can affect the process of solder wetting. The better alternatives are OSP, tin or immersion or gold for immersion, based on the design limitations.
Routing and Placement
When heavy and bulky components are that are not susceptible to tombstoning various trace widths on the two pads might not be a problem. But for smaller lighter components the traces connecting their pads should be the same width. Because their pads aren’t huge, having different trace widths on each side can result in an unbalanced thermal effect.
For smaller pads, the tracks could have a dramatic impact, even if they have a different orientation. It is best to ensure that the tracks join the pads in a similar direction and either Xor direction of the Y. This will prevent the dissipation of heat by using tracks that are angled differently.
Placement is a major factor in determining the distribution heat in a reflow oven. Small components placed close to the larger one can result in a different spread of heat. The larger part can absorb heat from the soldering process and deprive the smaller components of heat which means they need longer to be soldered. Also, when it cools the larger part, it releases enough heat to keep the components in close proximity from cooling fast.
Copper Coverage
There is a huge copper land close to an SMT pad on the SMT device could increase the likelihood of tombstoning that component. The huge copper land located in the same plane or an inner plane, also functions as a heat sink delay the process of soldering one of the pads nearby.
Designers should ensure that copper land is covered on each side of the component equally, even if copper land is located on an entirely different layer.
Thin Solder Mask
Solder mask thickness that is less than the copper’s thickness is recommended and prevents tombstoning. Two ways that designers are able to open the soldering mask on a pad – solder mask specified or SMD and non-solder mask, or NSMD.
In the case of an SMD pattern the actual dimension of the pad could be bigger than what is needed. Solder masks opening determines the deposition of solder paste, because the solder paste’s opening will be smaller than pad’s size. This also means that the pad sizes are more likely to be different, increasing the chance of tombstoning.
When designing an NSMD pattern The solder mask opening is slightly bigger than what the pad actually is. This allows the designer to make two pads of the exact size, which reduces the possibility of tombstoning.
Conclusion
RUSHPCB.COM has vast experience in the design of circuit boards and PCBs for different industries. We’ve tried to offer an outline of how to prevent tombstoning to aid our customers to avoid this problem.