The tombstone is a universal indicator marking the end of a person’s life, keeping their memory permanently on display. As long as their name lives, so, in essence, does their memory. But the idea of tombstones transcends human and pet graveyards, as the terminology has spanned the spectrum of indication of standing entities to commemorate those laid to rest.
One essence in which the idea of a raised “tombstone” is applicable is in the assembling of printed circus boards (PCB). This term is applied to a common SMT component defect in which it lifts off from the board on one end while staying down on the other. This can be the result of a number of causes, which we will discuss here. Additionally, we will explore methods by which the tombstoning problem can be maximally mitigated or avoided entirely.
What Exactly Is “Tombstoning”?
“Tombstoning” is a standard industry term for what is generally referred to as “Chip Lifting,” but goes by multiple other names including a Manhattan Effect, Stonehenge Effect, or Drawbridge Effect. All these terms refer to the notable attribute of this common defect in operating with SMTs (surface mount technologies) on PCBs. Normally, a chip component is soldered onto the copper pad on the PCB on both ends.
However, due to several potential issues, one end does not remain adhered to the board, while the other does. The appended end remains attached, but the other lifts from the board, causing the component to erect on one side, resembling a tombstone rising out of the ground.
The lift of the component could be complete or it could be partial and once it occurs on a surface mount device, much like an actual tombstone, it can mark its function’s fatal ending.
Causes Of Tombstoning
Generally, solder paste is used when it has a toothpaste like consistency, then applied to a stencil which will allow contact with the board and the ends of a component. The idea is to apply these uniformly, but too often the soldering is unbalanced, resulting in the wetting process holding one side of the component more readily to the surface of the PCB while the weaker side lets go, resulting in the component fully or partially standing. The molten solder pad lets go on one side, resulting in an elevated component that resembles a rising tombstone.
Because solder paste formations vary, their wetting speed varies as well. When solder paste wets quickly, one side outraces the other to completion, creating an uneven torque between the two sides of the component. When alloys are mixed to generate solder joints, the wetting process is poorer, resulting in solder balling and a duller appearance.
Certain components, such as inductors and capacitors, which are both thicker in size, are more prone to tombstoning than thinner ones because the components have more leverage to stand upright when one side wets faster than the other. The surface can also be responsible as the copper pad layout on the circuit that contains elements that are proximate to holes or the two ends attached to separate materials does not promote an even wetting process due to how quickly or slowly it allows the solder to finalize. Similarly, when pads are too far from each other, the components may get misaligned resulting in tombstoning. If the pad exceeds the size of the component, the pulling force increases the likelihood of the component erecting on one end.
Solder quality can also be affected by oxidation on either side of the board. If one side of the board experiences higher oxidation than the other, it will throw the solder wetting into an imbalance state, resulting in the component detaching from one end. The same idea applies to the oxidation of either side of the component being soldered. Therefore, chip placement is extremely important to assure that both sides are thoroughly soldered. The components themselves present a problem when they are too small in size, as it is very challenging to solder both sides of something extremely small in an efficient manner.
Solvent vapors from the PCB or flux during the reflow process can also push a component up on one side. Certain solvents start to evaporate more quickly resulting in the production of vapors. These vapors push from underneath the component causing a vertical rise or a horizontal push to occur. The nature of the tombstoning effect relies heavily on the paste’s chemical formulation. In that sense, tombstoning can manifest in several different ways.
Manufacturers have taken steps to mitigate the “tombstoning” effect with several strategies by focusing on the specific root causes. Several primary areas encompass the bulk of these issues. The first is the negative effect on the thermal mass of the solder joints by using pads that are not sizably suited for the small passive components on a PCB. When pads have less mass, they will drive faster reflow more quickly than larger ones. The solution for this is that the manufacturers must recommend industry standard sizes in their supplied datasheets.
The passive parts must also have the correct footprint in their build. When the components are not positioned centered on the pads. The pads must also be appropriately spaced to assure that both ends or pins of the components are situated exactly on the same portion of the pads on the PCB. In this case, the solution is once again to build the PCB footprints during the SMT according to manufacturer recommendations.
Routing must also be balanced, as not doing so could result in “tombstoning” problems even if the CAD footprint is perfectly balanced and correctly sized. This can happen when one pad is connected with a thick trace while the other is connected with a thin one. The reason for this is that a thicker trace slows down the soldering process, causing an essential heat sink. When soldering on one end takes longer than the other, tombstoning commonly occurs as a result.
A similar issue occurs when the pad is embedded with a power plane. This causes a larger metal area to pull in more heat, disproportionately affecting the wetting speed of the solder. Great care should be taken when routing between two pads, especially when connecting the pad to the metal plane.
The other aspect is the solder itself. Needless to say that the solder should be applied both evenly to each component side. However, its consistency must also be identified as is the mix from which it is formulated.
Finally, it is important to ensure that the stencil is well-designed and that the nozzle has been adjusted to dispense the solder at the correct and consistent pressure, with no adjustments happening in between the soldering application sessions, especially to the same component. When the design and settings of the stencil are optimal, the application of the solder is also the most effective, reducing the chance of the dreaded tombstoning effect.
In summation, the effect of “tombstoning” is when a component on the PCB rises on one side due to uneven soldering. The effects can be due to an imbalance in the solder applied, the heat conductivity of the board, the placement of the component pads, the chemical inconsistency of the solder applied, pad size variance, uneven temperature flows, and a variety of other issues common during the SMT.
Component and pad placement is essential to a balanced PCB, as is the assurance that the solder is formulated to an optimal chemical state, distributed evenly on components’ sides, and applied to a surface with a uniform heat absorption propensity. The thickness of the solder mask will not only prevent oxidation but will also ensure that the components seal to the PCB evenly, without the drawback of potential tombstoning. The stencils should be optimally tuned and adjusted to apply the solder evenly as well.
“Tombstoning” is one of the most common and fatal flaws of products that utilize PCBs. Any steps that are taken to prevent it result in a more optimally designed product and better functioning electronics that the PCBs assist in controlling.