What Is a Tombstone in Soldering?

Are you familiar with the crocodile or Manhattan effect? If not, then you're in luck. These terms describe the concept of PCB tombstone, one of the most common issues manufacturers and technicians face during soldering and assembly. It's up to you whether you see it as a crocodile with its head out of the water, an erect buiding perpendicular to the ground, or a grave's tombstone.

The idea is to describe the phenomenon in which a printed circuit board's end escapes the welding areas and stands upright during soldering due to bad design or poor melting technique. In this post, we'll expand tombstoning PCB assembly in detail to help you prevent this common soldering issue.

PCB Tombstone Explained

To understand why components sometimes stand up like tombstones, we must look at the soldering process, especially the wetting phase. Wetting happens when the soldering paste converts to liquid and sticks to the component and the PCB pad. However, to make a strong bond, you need to ensure all parts get wet simultaneously.

When wetting occurs, the liquid solder paste pulls on each electronic component, like a Tug-of-War. This pulling helps the component stay centred between its pads if wetting happens simultaneously. But, on some parts, if wetting isn't synchronized, it can lead to issues like tombstoning, where one part gets pulled up like a standing tombstone.

The key factor between tombstoning and not tombstoning is how fast the pads heat up and cool down. This affects how quickly the solder paste turns liquid and then solid again. While some assembly issues can cause tombstoning, we'll focus here on problems related to how the PCB is laid out and how to avoid them. It's worth noting that issues during assembly are usually in the control of the person doing the assembly.

The Most Common Causes of PCB Tombstoning PCB Assembly

To simplify, PCB tombstoning involves partially or completely lifting the circuit board's surface or PCB pad. Essentially, one end stands up while soldering due to many reasons. However, all the reasons result in imbalanced torque during the wetting phase. Here are the most common causes:

• Bad PCB pad designs - Poorly designed PCB pads contribute to tombstoning. Uneven pad sizes, inadequate spacing, or improper dimensions can result in uneven wetting during soldering, leading to components standing up.
• Nitrogen presence - The presence of nitrogen during the soldering process can impact the wetting process. Nitrogen affects the solder's flow and can lead to uneven wetting, causing tombstoning issues in surface mount components.
• Inconsistencies in pad sizes - Variations in pad sizes on the PCB can cause tombstoning. If the pads for a component are not uniform, the wetting process may not occur simultaneously, resulting in one end getting pulled up.
• Temperature fluctuations - Sudden changes in temperature during soldering can lead to tombstoning. Inconsistent heating and cooling affect the solder paste, causing imbalances in wetting and potentially resulting in components standing vertically.
• Poor chip placement – Another PCB tombstone cause is poor chip placement. If a component is not properly aligned or placed, the wetting process may not occur uniformly.
• Inconsistent thermal capacities of welded materials - Differences in the thermal capacities of soldered materials can cause tombstoning. If the materials heat up or cool down at different rates, the wetting process may not synchronize, resulting in components standing up unevenly.

How to Prevent PCB Tombstoning

Here's how you can prevent PCB tombstone during soldering and improve the efficiency of electronic component assembly:

1.      Welding at Both Ends at the Same Time

Make sure to weld component ends at the same time. This helps reduce differences in melting between the two ends. Steer clear of situations where one end gets fully welded while the other is not melted.

2.      Good Pad Design

When designing the PCB pads, keep them consistent in size. Different-sized or uneven pads can cause issues with solder paste quantity. Pay attention to design details, like having a wide welding length, a short pad space length, and an appropriate pad width.

3.      Keep Solder Paste Active

Take care of the solder paste, which is a sensitive material. It can be affected by contamination, oxidation, or moisture absorption. Store it in a cool place (5-10℃), let it warm to room temperature before use, stir it well, and cover it properly to avoid defects like tombstoning.

4.      Contamination Prevention

Be cautious about keeping the PCB surface clean. Contamination can lead to poor wetting and tombstoning. Both manufacturers and users should prevent contamination, ensuring cleanliness during the manufacturing and use of PCB components.

5.      Consider a Different PCB Finish

When dealing with small components prone to tombstoning, like 0402 or 0201, consider using something other than the common Hot Air Solder Leveling (HASL) finish. HASL is cheap, but it can make surfaces uneven, affecting how solder sticks. Check out finishes like immersion tin, immersion gold, or OSP based on your design needs.

6.      Watch Component Placement and Routing

For tiny parts, look closely at where you put them and how you connect them. Make sure all the paths connecting to the pads are the same width. Connect paths in the same direction to avoid different heat levels with differently angled paths.

7.      Check Inner Layers for Copper

Tombstoning issues might not only come from the top and bottom. Inner layers can hide problems, too. An inner layer with uneven copper on a pad can make heating uneven during soldering. Always ensure copper on the inner layers covers both sides of a pad.

8.      Double-Check Pad Dimensions

When making parts, be careful with pad sizes to avoid problems. Mistakes like making pads too small or big or having uneven sizes can act like a heat sponge during soldering. This makes one side finish sticking earlier than the other.

Conclusion

To summarise, a PCB tombstone is when components on a circuit board stand up like tombstones instead of lying flat. It can happen during assembly, affecting the connection and performance of electronic devices. Taking preventive measures in the design and manufacturing process is crucial to avoid this issue and ensure the printed circuit board's reliable and efficient operation.