What Could Cause a Solder Joint to Crack?

Every part of a PCB can have reliability issues, including solder joints on SMD and through-hole parts. Components in a PCB are placed with a soldering process, and placements must have the ability to withstand vibration, mechanical shock, thermal cycling, and even the assembly process.

Over time, when a device fails, an inspection might reveal a worn or cracked solder joint. When a solder joint cracks, a deeper investigation could reveal the root cause, which might require a simple design change to prevent in future builds. Here are some of the main causes of solder joint cracking as they relate to a device’s operating environment and assembly processes.

What Could Crack a Solder Joint?

Solder joints are eutectic alloys that could crack for several possible reasons. The root causes of a cracked solder joint could be mechanical, thermal, or chemical, and the failure can take a significant amount of time to manifest itself. Cracked solder joints could even present intermittent failure, such as fractured solder joints below a BGA. When cracked solder joints do occur, they require an investigation to determine the root cause of failure. Typically the assembler will perform this investigation, although simulations could be helpful for identifying contributing factors (see below).

Before investigating solder joint failure, it’s important to ask a few important questions that aid investigation. These include:

• Is the failure intermittent or persistent?

• Does the failure occur repeatedly in the same assembly?

• Does the failure occur repeatedly across multiple assemblies?

• What steps would be needed to reproduce the failure?

The last question in the list is usually answered only after some investigation identifies the root cause. Below, we outline instances of SMD and through-hole solder joint failure and some potential causes.

SMD Solder Joint Failure

An SMD joint failure, most commonly caused by a cold joint, is the most common form of failure. The presence of a weak joint can be verified visually (particularly cold joint), and these joints will have lower mechanical strength. The result is that the cold joint is more susceptible to failure when the PCB is exposed to mechanical impact or vibration. A related is a disturbed joint, which is sometimes confused with a cold joint due their similar appearance.

These problems arise due to variances during the soldering process. For example, excessive vibration of the board during reflow can cause a disturbed joint, while temperature excursions or insufficient temperature could cause a cold joint. Both are addressed by examining the solder process and correcting inaccuracies.

Solder Joint Failure Under a BGA

Insufficient solder, poor planarity, and insufficient wetting could all lead to a failed solder joint on a BGA package. These problems would often present as intermittent failures as other solder balls tend to hold the package into electrical contact. In some HDI boards, an intermittent failure is not due to solder at all, and is instead due to a failed microvia in the stackup.

If a BGA appears to be failing, it should be inspected in an X-ray inspection machine. These failures are often intermittent and appear at a higher temperature (after the package expands), so X-ray may need to be performed after heating to spot the break. If no solder defect is found, then the failure may be internal to the board, such as in a buried via.

Through-Hole Solder Joint Fracture

Through-hole vias are generally much stronger than SMD components, largely due to the fact of how a solder joint forms. The bond forms on both sides of the board and inside a plated hole, so there is much more solder to hold the part in place compared to an SMD part.

Through-hole solder joint failures could result from the root causes listed earlier, but there is an additional issue that can occur during assembly which contributes to failure. During soldering, the soldering process may not provide enough solder into the through-hole, which is especially problematic with hand soldering. Selective soldering may also produce this problem. Over time, vibration or temperature cycling can pull on the partially attached lead on the through-hole and eventually cause the via plating to crack or delaminate. This could present as an intermittent or persistent failure.

Check your soldering process for sufficient solder to eliminate through-hole solder defects.

Mechanical Simulations For Solder Joints

Solder joint reliability requires mechanical simulations and measurements to estimate the lifetime of solder joints under various thermal and mechanical loads. On the mechanical side of reliability testing and simulation, solder joints are qualified under the following conditions:

• Thermal cycling

• Mechanical shock

• Vibration

Thermal cycling requires a multiphysics approach that can illustrate the thermomechanical behavior of solder joints. The other two types of simulations are purely mechanical and are performed at fixed temperature.

These simulations give information about the mechanical reliability only of the solder joints, they tell us nothing about the weld to the copper and its surface coating. However, more complex simulations could be performed to determine if delamination of copper from the PCB plays a role in solder joint failure under certain conditions.

Whenever you need to simulate the mechanical behavior of your PCB, use the complete set of the system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. Cadence PCB design products also integrate with a multiphysics field solver for thermal analysis, including verification of thermally sensitive chip and package designs.

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