PCB monument: Common causes and how to prevent this kind of assembly defect?

Some defects may occur when soldering components to the PCB, especially surface mount components, because they are small. If you do not handle the assembly process, tombstone PCB is very common.

Atomization may cause electronic components to fail, which is essentially the death of the components. Let's analyze this kind of welding defect to see its common causes and how to prevent it during the assembly process.

What is the tombstone in the PCB?

The tombstone effect, also known as the Manhattan effect, is an assembly defect that affects passive SMT equipment with two mounting connection points.

Unbalanced wetting can lead to this phenomenon, that is, the solder on one terminal is fully wetted before the solder on the other terminal. This will cause the molten solder to attach to the pads and components.

The joint forms an inter-metal bond, and the meniscus tension is applied to the terminal of the element.

If this tension occurs before the other side melts, and the same happens, the assembly will be pulled. This tension causes it to rise and separate from the side that has not yet been glued.

The unsoldered side can be raised a few degrees to 90 degrees from the circuit board. This makes the component look like a tombstone.

Common causes of PCB tombstones

Incorrect placement of PNP components

When placing SMT components on a circuit board, machine inaccuracy may cause one terminal to go deeper into the pad than the other.

This will cause uneven wetting force during reflow, causing the side farther from the pad to be pulled more and the other side to be lifted.

Improper pad design

Improper pad design involves several aspects. These problems include too far away, too many copper traces connected to a pad or through holes in the pad.

It may also include having different trace widths or electroplating through holes/large copper areas too close to a pad.

Screen printing between legend/pads

Although the screen printing layer is very thin, keep in mind that tiny SMT components are placed almost flat on the PCB.

Therefore, the tiny legend layer between the pads may slightly raise the SMT device, causing it to bend to one side when placed. This bending may cause the side with little contact to rise during reflux.

Unbalanced reflow soldering heat curve

This imbalance usually occurs when the assembler fails to fully preheat the circuit board during the reflow soldering process. It will cause the joint to be heated unevenly, resulting in wear-resistant phenomenon.

Poor-quality reflow soldering machines or large reflow soldering machines that process multiple circuit boards at the same time may cause uneven internal heat distribution, resulting in uneven wetting on the pad.

Incorrect solder paste deposition

When an inferior template is printed on one side or more solder is applied, inconsistent solder paste deposition will occur.

This defect may cause logical deletion in two ways. First, the side with more solder may take more time to fully melt, resulting in a difference in melting time, causing the other side to lift it up.

In the second case, if the wetting speed time difference is minimal, the side with more solder will exert greater tension, which may be enough to lift the other side.

Different solder paste ingredients

Solder is an alloy containing different metals. If different components of solder are used on each pad, the alloy will melt at different temperatures, which means that one side will be wetted in a shorter period of time. This problem is not common, but it may occur if you use both leaded and lead-free solder pastes.

Component design

Thicker components are more prone to monument phenomenon, because if the solder on one side melts faster, they have a larger terminal surface area for pulling. Those with uneven weight distribution or different thermal characteristics of each terminal are also more likely to become tombstones.

Oxidation of circuit boards or components

Due to the oxide layer above the metal, oxidation of the pad or component terminal will hinder wetting. This will increase the wetting time on one side, thereby increasing the possibility of erecting a monument.

Warping of circuit boards or components

Warped components or PCBs may cause poor or minimal contact between a terminal and the pad. This allows another solder to apply greater tension to lift the device.

Presence of nitrogen

Nitrogen has some benefits in the welding process, such as reducing welding beads. However, this is a secret of the tombstone effect, because the gas enhances wettability.

If it is more available on a cushion or terminal, it will moisten that side faster and accumulate enough force to lift the other side.

Solder resist layer or PCB quality

Improperly stored PCBs absorb moisture, and moisture may escape from the pad area in the form of vapor during reflow. The poor-quality solder resist layer may also contain solvents, which will escape through the pad area when it is hot, resulting in a tombstone effect.

Narrow pad spacing

If the pads are too close, the solder paste can easily slide between the pads during the printing process. This will cause the component to float above the solder paste and cause the tension on the terminal to be unbalanced during reflow.

How to prevent tombstone phenomenon during PCB assembly?

Optimize the reflow heat curve

It is essential to optimize and maintain a balanced reflow heating curve to ensure uniform heating of solder joints. Remember that when the circuit board enters the reflow soldering oven, it must first be preheated and ensure that the oven heats all PCB parts evenly.

Optimize solder paste deposition

The main equipment used in solder paste deposition is the template, which plays an important role.

It must match the pad pattern design of the component, be placed flat on the board, and not be too thick. Therefore, the design of steel mesh is essential, and engineers should check its aperture size, spacing and shape.

Check component design

Component selection is also essential to minimize tombstone. These SMT devices should have the same terminal size and balanced weight at both ends.

Check component placement

Automated assembly lines must be equipped with precise pick-and-place machines with high-quality vision systems to prevent inaccurate positioning of components on pads and solder.

Consider changing the PCB surface treatment

The standard surface finish of most PCBs is hot air soldering leveling. However, it may cause uneven surface finish of the pad, which affects wetting and causes tombstone.

Therefore, you may need to consider other surface treatments, such as immersion gold or tin.

Ensure that the pad and inner copper layer have the correct design

The size and design of the pad should be the same to prevent uneven heating. This design optimization should go further into the traces, where each pad should have approximately the same number of connections to have the same “cooling water tank”. In addition, avoid placing some pads near large copper areas.

Make sure the solder resist layer is thin

If the thickness of the solder resist layer is higher than the pad pattern, it will hinder the flow of the solder when it melts.

When covering the terminals of the components or weak connectors, the results will be inconsistent. It is best to keep this layer thin and lower than the pad.

Conduct regular process audits and implement quality control

You need to monitor and calibrate design, manufacturing, and assembly steps that may cause logical deletion over time to eliminate this problem.

At the same time, you should implement the required quality control measures to prevent or detect this situation as soon as possible. These measures include visual and automatic optical inspections.