Understanding Solder Joint Failure in PCB Manufacturing

Introduction

Solder joint failure is a critical issue that impacts the durability and performance of printed circuit boards (PCBs). In this article, we will explore what causes these failures, how they manifest, and why early detection is essential for ensuring PCB reliability and product longevity.

What is Solder Joint Failure?

At present, in the field of electronic materials, solder serves as a critical connection medium. It provides electrical, thermal, and mechanical continuity between silicon chips and their corresponding pads. The integrity of these solder joints is essential to the performance and reliability of electronic assemblies. As electronic components become smaller and more complex, solder joints are increasingly recognized as the most vulnerable points within the assembly structure.

Solder joint failure refers to the loss of electrical or mechanical connection at these joints. This failure can arise from various factors, including mechanical stress, thermal cycling, electrical overloads, radiation effects, and chemical degradation. Among these, thermal fatigue remains a leading cause of failure, driven by the constant expansion and contraction of materials with differing coefficients of thermal expansion (CTE).

Modern packaging technologies like Surface Mount Technology (SMT) Assemblies, Chip Scale Packaging (CSP), Ball Grid Array (BGA), and Multi-Chip Modules (MCM) have really ramped up assembly density while shrinking joint sizes. This change can lead to increased stress at the solder interfaces, which raises the chances of fatigue cracks and eventual joint failure. On top of that, the move to lead-free solders brings its own set of challenges, such as more intermetallic compound formation and faster copper dissolution, both of which can weaken joint durability. So, solder joint failure continues to be a major concern when it comes to ensuring long-term reliability in electronics.

Types of Common Solder Joint Failure

When you solder, you want a strong, clean connection. But sometimes, things go wrong. There are common types of solder joint failures you should know:

Resin Joint

A resin joint happens when a thin film of flux stays between the metals. The wire and pad don’t fully connect. Even if they seem fine at first, they’ll come loose later. This usually happens when heat doesn’t spread evenly during soldering. You can fix this by placing your soldering iron in the right spot. Make sure the heat reaches both metal parts evenly. If you don’t, the joint will fail when used.

Cold Joint

Cold joints are very common. You’ll notice them by their dull, rough surface. They don’t shine like a good solder joint. This weakens the connection. The joint can break with vibration or even a small impact. Cold solder joints often happen when you don’t heat the solder long enough. Or if you move the part before the solder cools. You should always let the solder melt fully and stay still until it solidifies.

Insufficient Solder

This happens when you don’t use enough solder. The wire or lead will look half-covered. Parts of it might still be exposed. This weak spot can break under stress. Dirty or oxidized surfaces also cause this problem. Uneven heating makes it worse. To prevent this, clean your parts before soldering. Also, place the iron tip where it heats the joint evenly.

Excess Solder

Excess solder means you used too much. The solder spreads out and covers the terminal completely. It might even drip off. On its own, it’s not a big issue. But if it bridges two terminals, it causes a short circuit. That can be a serious failure. This usually happens when you heat too long or feed in too much solder. Be careful to control how much you apply. A neat, small fillet is what you’re aiming for.

Common Reasons for Solder Joint Failure

Solder joint failure can happen for many reasons.  Here are some few causes so you know what to watch for during your PCB production:

Poor Solder Joint Design

If the solder joint design is wrong, it will fail. You need to pick the right joint type for the job. Think about where the joint is and what it does. Poor design can cause stress points. Things like bad component placement or board mounting can strain the joint. Always plan the design carefully to avoid this problem.

Poor-Quality Soldering

Good soldering needs skill and practice. If you use the wrong tools or bad techniques, the joint will be weak. Even good materials can fail if handled wrong. Make sure you heat the joint properly and let the solder flow. Make sure your manufacturer has skilled workers.

Extreme Temperature Cycling

Electronics heat up and cool down constantly. This is called temperature cycling. When materials expand and shrink at different rates, it stresses the joint. Extreme cycles make this worse. Over time, small cracks will form and grow.

Low-Quality Solder Materials

Cheap solder materials don’t last. They crack, fatigue, or corrode faster. Using low-quality solder might save money at first. But it leads to failures later. Make sure your manufacturer pick solder with good composition and proven reliability.

Unintended Stresses from Polymeric Materials

Materials like coatings and potting compounds can cause hidden stresses. As they expand and contract, they pull on the solder joints. Under BGAs or QFNs, this stress can lift components off the board. This stretches the solder joints until they crack.

Stress from Potting, Underfills and Conformal Coating

When you apply potting or coatings unevenly, it can trap stress inside the assembly. These materials expand when heated. If they press against solder joints, they can cause fractures. Always apply coatings carefully to avoid these.

Mechanical Stress 

Dropping a device or pressing too hard on parts can rip solder joints. These sudden impacts create shear forces. The joint might break clean off the board. Your manufacturer should handle assemblies with care to prevent mechanical shocks.

PCBA Conditions

If you force a circuit board into a tight space, it can bend. This creates stress on the solder joints. Over-constraining a PCBA during mounting can lead to joint fractures. The boards are mounted without forced bending during the assembly.

Tensile Fracture

This happens when a joint is pulled too hard. It can result from mishandling or poor assembly processes. A sharp pull or drop can rip the joint apart.

Creep Failure

Creep is slow, permanent deformation under constant load. Over time, a solder joint under steady mechanical stress will sag and crack. High temperatures speed up this process. Analyze your PCB design so joints aren’t under continuous stress.

Fatigue Failure

When a joint faces repeated stress cycles, it weakens. Temperature swings or vibrations can cause fatigue cracks. Each cycle adds more damage until the joint finally fails.

Corrosion

Moisture and chemicals attack solder joints over time. Corrosion can come from the air, liquids, or even flux residues. Galvanic corrosion happens when different metals touch and get wet. Voltage-assisted corrosion can also create shorts. Clean assembly and good sealing can reduce this risk.

Solder Balls

Solder balls are tiny blobs of solder stuck where they shouldn’t be. They happen from messy solder paste printing or bad reflow settings. While small, these balls can cause shorts if they bridge connections.

Inadequate Wetting

Wetting means the solder flows smoothly over the pin and pad. If it doesn’t, the joint will be weak. Inadequate wetting happens when heat isn’t applied properly or the surface is dirty. Your manufacturer should clean the board and heat both surfaces evenly.

Solder-Starved Joint

A solder-starved joint simply lacks enough solder. The connection looks thin or incomplete. This weak joint might work at first but will fail over time. Heating the lead fully and applying enough solder can fix this problem.

Industry’s Solder Joint Failure Definition

Where do Cracks Appear if the PCB has Solder Joint Failure?

1. Cracks Start Under the Component – The first crack usually forms under the component. It starts near the edge where the metal layer, called metallisation, ends. This crack is very small and hard to spot during inspections.
2. Cracks Spread to the Solder Fillet Surface – Next, the crack moves outward to the solder fillet. This is the curved area of the solder joint. You’ll often see it first at the corners of the component, where stress builds up.
3. Cracks Grow from Corners to the Joint Center – As the crack gets worse, it spreads from the corners towards the middle of the joint. Over time, this weakens the entire solder connection.
4. Cracks Follow the Component-Solder Interface – In some cases, the crack runs along the line where the component and solder meet. This area is called the interface. Cracks here are dangerous because they can fully separate the connection.

For different types of components, cracks tend to appear in certain spots:

Leadless Surface-Mount Devices:

• Under big components, where the solder touches the metal layer.
• On the side where the solder fillet meets the metal end of the component.
• Under small chip components (like capacitors or resistors), where the solder touches the copper foil.
• Inside the bulk of the solder fillet itself.

Through-Hole Components:

• Around the wire, where the solder fillet meets the wire (called annular cracks).
• Inside the main body of the solder fillet.
• Between the solder joint and the PCB pad.

Solder Joint Reliability Simulation with Circuit Elements

Use the Junction Temperature

This method is straightforward. You take the component’s junction temperature as a stand-in for the solder joint. Heat from components like MOSFETs or processors flows into nearby joints. So, their temperatures get very close. If you use verified component models, you can estimate the solder joint temperature easily. It’s a quick way to see if the joint can handle the heat. This method works well for steady-state simulations. You won’t catch every small fluctuation, but you’ll get a reliable overall temperature reading.

Use it as a Parasitic Element

Here’s another approach. You model the solder joint as a tiny resistor in your circuit. This simulates the joint’s parasitic resistance. No metal is a perfect conductor. So, solder joints naturally resist a bit, which generates heat. By adding this element into your simulation, you can track how much heat builds up directly in the joint. You can run DC sweeps or use signal sources to see how the temperature changes. This method gives you a more detailed look at the joint’s own heating behavior, not just the component’s.

Tools for Solder Joint Failure Detection

SPICE Simulation

SPICE simulation is a handy way to study how temperature affects solder joints over time. You can simulate thermal shock and stress as they happen. It’s great for catching sudden temperature spikes or stresses that hit the joint during use. You’ll need to program some parameters from your component models. But once it’s set, you can see how heat builds up and impacts the joint. This is perfect for simulating what happens during startup, shutdown, or rapid power changes.

Smoke Analysis

Smoke analysis is about finding how much heat a solder joint can handle before it degrades. You compare the steady-state temperature of the joint with your target derating value. This helps you balance tradeoffs between component temperature, board heat, and how much the board expands. Smoke analysis gives you a clear idea if the joint can survive long-term use or if it’s at risk of failure.

3D Multiphysics Solver

If you want a deeper look, you use a 3D multiphysics solver. This tool models heat flow and stress right inside the solder joint. It’s a more powerful method because it shows exactly where cracks or strains will form. But it takes more time and computing power. You don’t need it for every project, but for complex designs, it can be a lifesaver. It helps you catch failures that simpler models might miss.

Acoustic Micro Imaging (AMI)

AMI uses sound waves to see inside solder joints. It’s like an ultrasound for your PCB. You can spot internal cracks, voids, or delamination without cutting the board open. This is useful for through-life monitoring. You can scan the joints after vibration tests or thermal cycling. AMI shows you how the joints degrade over time. The images are processed to track damage progression, making it easier to decide when a joint is near failure.

Conclusion

This PCBMay blog explained solder joint failure and its effects on PCB reliability. It discussed common failure types, causes, and how cracks form. It also covered detection tools like SPICE simulation and AMI. You learned how temperature, stress, and poor materials led to joint failure, and how to detect problems early.

If you need help with PCB or PCBA manufacturing, PCBMay is here for you. Our production line meets strict quality standards to prevent solder joint failures. We manufacture PCBs under IPC Class 1, 2, and 3 to fit your project’s needs. You can request a free quote and a free DFM (Design for Manufacturability) analysis to ensure your design is ready for smooth fabrication. Just send us an email at sales@pcbmay.com. We make sure your PCB gets the durability and performance it needs.

Frequently Asked Questions

• How to Tell If a Solder Joint Is Bad?

First, look at it. A good joint looks shiny and smooth. If it’s dull, rough, or has cracks, that’s bad. Also, check the shape. It should be a small, neat cone. Too much solder or weird shapes mean trouble. Look for gaps, bubbles, or burn marks. Cracks at the edges are also a sign the joint is weak. If it looks messy, it’s probably bad.

• Can You Repair a Solder Joint?

Yes, you can. If the joint isn’t too bad, you can just reheat it with a soldering iron. This melts the solder and makes it flow again. But if the solder is dirty or cracked, you’ll need to remove it first. Use a solder sucker or wick to clean it off. After that, clean the area with isopropyl alcohol. Don’t forget to apply flux. Then, add fresh solder while heating the pad and lead together. Let it cool without moving it.

• How to Protect a Solder Joint?

You can protect a solder joint by covering it. Heat shrink tubing works great for insulation and strain relief. If you need waterproofing, a dab of hot glue helps. For special jobs like RF cables, use strain relief boots or ferrules.

• How Do You Clean a Solder Joint?

First, heat the joint with your soldering iron. Then, use solder wick or a solder sucker to remove old solder. After that, clean off any leftover flux or dirt with isopropyl alcohol and a small brush.