Die Leiterplattenbestückung ist ein äußerst komplizierter Prozess, Dabei kommt es immer auf Genauigkeit an. Selbst die kleinste Unvollkommenheit kann zu zahlreichen Problemen in der Elektronik führen. There are still a number of various defects that can negatively affect the functioning of PCBs and their application in electronics; one of the most widespread of them is the so-called “PCB tombstone” or “tombstoning. But do you know what is it? What should you do to avoid this issue? Let’s dive in to find the answer.
PCB tombstone is also commonly known as the Manhattan effect, Stonehenge effect, or drawbridge effect, which is a soldering defect that is likely to happen during the surface mount components assembly process. It’s termed this because an affected part looks like a tombstone in a graveyard in appearance. When tombstoning happens, one end of a surface-mount component (often a passive part such as a resistor, Kondensator, usw.) is kept lifted off the PCB-Pad, while the other end remains in the soldered position. This problem usually occurs on some smaller SMT components. The smaller the size, the greater the risk.
At its core, tombstoning itself is caused by an imbalance in the forces acting on the component during the soldering process. To understand this phenomenon, we need to explore the idea of “wetting” in the soldering process. Wetting refers to the process where molten solder forms a bond with the metal surfaces of both the component lead and the PCB pad. In an ideal scenario, wetting occurs simultaneously and uniformly on both ends of a component. jedoch, when wetting is uneven, it can lead to tombstoning. Several factors can contribute to the imbalance that causes tombstoning:
Temperature gradients within the PCB can make one end of a component to solder before the other end. This is caused by the inconsistent heat distribution in the reflow oven or the changes in the thermal characteristics of the PCB board materials.
When the amount of solder paste applied to the pad is inconsistent, it will cause uneven wetting. The reason for inconsistent solder paste amount is usually due to improper stencil design or improper solder paste printing process.
If pad sizes are mismatched or the layouts of the pads are not right, then it is probable to develop different surface tension forces. Even with slight differences in pad dimensions, tombstoning is likely to occur especially when it is a case of small parts.
Some of the shapes and dimensions of components can be more inclined to tombstoning due to their physical characteristics. Zum Beispiel, components with asymmetrical designs or larger aspect ratios are usually more sensitive.
Fluctuations in Wärmeleitfähigkeit across the board are one of the main causes of irregular heating. This is especially true in the case of multi-layer boards which have irregular copper density in inner layers.
Some of the PCB finishes influence solder wetting. Conventional Hot Air Solder Leveling (HASL) finishes are cheaper; jedoch, they cause surface imperfections that promote tombstoning.
Misalignment of parts on the PCB would affect the soldering process, which will lead to high risks of PCB tombstone.
While a tombstoned component might seem like a minor issue, its consequences can be severe and far-reaching. Zuerst, it would affect the function of the whole electronics. When tombstoning occurs, it leads to the formation of an open circuit which in turn leads to partial or in the worst of cases complete failure of the device. Zweite, even if a tombstoned component maintains some contact, it can lead to intermittent connections, resulting in unreliable performance over time. Zuletzt, detecting and reworking PCB tombstone is not an easy job that will increase manufacturing costs and time.
Glücklicherweise, there are several strategies that PCB designers and manufacturers can employ to minimize the risk of tombstoning:
Weiterführende Literatur: 8 Gängige Oberflächenbeschaffenheiten von Leiterplatten: So wählen Sie das Richtige aus?
PCB tombstone is still a problem that plenty of electronics manufacturing companies encounter. With device miniaturization and high numbers of circuits, solutions to this problem are important. By understanding the causes of PCB tombstone and the corresponding measures, tombstoning incidences can be prevented or at least reduced greatly by the manufacturers.
MOKO Technology is one of China’s leading PCB companies, with almost two decades of experience in creating PCB assembly. Our expertise guarantees the highest quality and reliability in PCB assembly services. Do not let tombstone defects get in the way of your creativity and innovation. GNehmen Sie Kontakt mit uns auf to find out how we can assist in your project!
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