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How to prevent open defects and tombstones during PCB assembly

PCB Tombstoneing is an art that requires precise knowledge of the components’ placement. This process is most effective if we place the components evenly and with similar orientation and width. If the components are not in the correct position, the copper coverage will be uneven, leading to uneven heating. These mistakes can be avoided during the tombstoning process by following a few essential steps.

Stencil design

PCB Tombstone is a problem caused by the non-uniform thermal conductivity of a PCB. Another cause is poor weldability or oxidation of components. The stencil’s opening or thickness also influences the tombstone. Other factors affecting the quality of the solder paste printing are the printing machine parameters and component placement.

One way to improve the stencil printing process is by making it thinner or thicker. POR stencils have a smaller aperture size than BAE stencils. Both stencils have similar volumes, but BAE stencils are considered better at limiting tombstone defects. Generally speaking, the BAE aperture has better results with 0.4mm prints than POR stencils. However, the BAE aperture is significantly better at limiting tombstone defects with new solder paste.

To avoid tombstoning, you must ensure proper component placement. A good stencil design will prevent this problem by ensuring that the copper pads are evenly covered. Also, the solder mask thickness must be appropriate to avoid oxidation and tombstoning. If it’s too thick, it could cause solder beads.

The stencil design for PCBs is an essential part of the assembly process. Any mistakes here can lead to failure. Therefore, a good stencil design is essential to get consistent printing results. While we can print most PCBs without a complex stencil, certain situations call for a more complex stencil design.

The stencil design also affects solder paste release. The stencil material should be four to eight thou thick to hold solder paste. In addition, the thickness of the stencil should be large enough to allow five solder particles to span the smallest aperture.

Offset printing

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PCB Tombstone occurs because of large differences in the wetting force at the solder paste ends of a device. One end of the device may be well soldered while the other is not. The solder paste on the poor end has a lower wetting force than the melted solder paste on the other end. As a result, the solder paste on the poor end has an uneven wetting force. This force imbalance is what causes the tombstone.

Offset printing is another problem, resulting in ineffective solder paste contact. Moreover, the printed-out board will not be complete. This will result in a tombstone that is not fully functional. Fortunately, there are some simple solutions to avoid this issue. The key is to control the paste application process and maintain a proper thermal balance.

There are two main causes of PCB tombstones. First, the pads do not have the proper design. As a result, they often do not cover more than 50% of the component’s terminals. In addition, the spaces between the pads are too wide. When these problems occur, reflow soldering can create a tombstone.

Second, before the soldering process begins, you must preheat the board’s surface. A significant temperature difference at the end of the board could lead to the formation of tin beads, which could make the PCB look like a tombstone. A reflow soldering process with a controlled temperature profile will reduce tombstones.

Another problem associated with tombstoning is the wetting force. The wetting force at one end is much greater than the combined force of the two ends. The result is that one end of the component will rise, and the other will fall. To avoid this problem, choose smaller and lighter-weight components.

Uneven wetting force

PCB Tombstone is a type of PCB defect resulting from uneven solder wetting force. The uneven wetting force is because of significant differences in solder paste temperature between two ends of the device. This makes solder paste on one end melt while it does not melt on the other. As a result, the tombstone’s wetting force is lower than that of the other end.

The most common cause of tombstones is an imbalance in the wetting force of two terminals. The force generated at one end of the component is greater than the combined force from the two ends, causing the component to rise out of the solder. This is particularly problematic in smaller and lighter-weight components.

Another common cause of tombstone is the non-uniform thermal conductivity of the PCB. This condition may result from improper solder application or insufficient solder paste thickness. Therefore, solder paste printing machines should also calibrate to ensure uniform thickness.

Uneven wetting force of PCB Tombstone is a common problem when reflowing the solder on components. This uneven wetting force causes the component to stand up on one end and overpowers the other. It is often the case when the components are small and located in a small package. A proper balance of the wetting force between the two ends can prevent this problem.

Uneven wetting force of PCB Tombstone can reduce by adjusting the pad size. A larger pad size increases the surface area of the molten solder, which causes a stronger wetting force on the component’s end. This also reduces the likelihood of component separation. The IPC standard provides recommendations for pad sizes. However, pad sizes vary from manufacturer to manufacturer.

Manhattan effect

The PCB Tombstone defect, also known as the Manhattan effect or the Stonehenge effect, is serious in surface mount devices. It is due to an imbalanced wetting force and can cause an open circuit. This issue often occurs when the solder paste on one end of a device is much hotter than the other. This causes a significant force imbalance and results in a tombstone.

One important factor in preventing tombstoning is component placement. When the component is small, it is essential to place it evenly across the pads. In addition, the traces must be of the same width and orientation. If they are unequal, the solder will not reach the component. This problem is common in low-cost, low-margin products but can also occur on high-margin devices.

Another factor affecting PCB Tombstone is uneven wetting force. During PCB reflow soldering, uneven wetting forces can cause the chip component to stand up. This condition is known as the Manhattan effect. Improving process design, quality control, process modulation, and straight-through assembly rate can reduce it. Good solderability of the solder paste is also essential. It is possible to control solder end size error, but this is more sensitive to the size of the PCB pad.

An imbalance causes the Manhattan effect in solder paste applied to the two sides of a PCB. When this happens, the solder paste on one end of a device may melt before the other. As a result, it pulls the part to the melted side. The Manhattan effect, also known as the drawbridge effect, can cause the tombstone effect.

Soldering rework

Tombstone is a soldering defect resulting in partially or fully lifted passive SMT components from the PCB pad. It typically occurs when one end of a chip is soldered to a PCB pad while the other end is not. As a result, the component’s terminals are not fully covered by the solder paste, resulting in uneven heating.

This defect can result in intermittent electrical issues. Fortunately, you can repair it by turning it into a pillow defect. First, remove the component from its attached pad. Then, place it on the board’s other side and apply flux. After applying flux to the component, heat the first side of the board.

A hot air rework station includes a wand holder and a reflow station. A polarized component is placed backward, shifted x degrees to one side, or has an unintended solder jumper. This component is easier to replace when placed before the reflow process. A second common issue is a component that is missing. This can happen during the reflow process or before it occurs. In the case of a missing component, hot air can help replace it.