Solder Joint: The Common Defects That Exist

About Solder Joint, Soldering is an essential process in electronics manufacturing.

And most soldiers available today are the lead-free variant (due to many recognizing lead as toxic).

Although lead-free solder melts at higher temperatures, it has various challenges—one being solder joint defects.

This article will explore the different solder joint types and the possible defects during electronics construction.

Ready? Let’s discover!

Table of Contents

Are All Solder Joints Bad?

A person soldering

A person soldering

First off, not all solder joints are bad.

Although incorrect solder joints present various severe effects, most are harmless and won’t cause any issues. In truth, good solder joints have shiny finishes without too much solder.

So, how do you detect a good solder joint?

Usually, they have a slightly concave contour around the solder. However, remember that applying excess solder on a joint will make it worse, as it can spill into another track.

As a result, you may have short circuits on the Printed Circuit Boards.

Common Solder Joint Defects to Watch Out For

Solder and soldering iron

Solder and soldering iron

Let’s examine some of the most widespread solder joint problems plaguing the PCB soldering process.

Open Joints

There’s a high chance of encountering open joints, especially when working with POP packages that combine and reflow solder Ball Grid Array (BGA) & Chip Scale Packages (CSP).

Further, package warping is one of the primary culprits of open solder connections, often leading to one or more open connections on a circuit.

Another culprit for open joint formation is poor solderability. Also, paste deposits reaching too far under the component’s body is another cause. 

Sadly, overflowing solder lifts the component’s lead above the solder paste’s surface when reflowing, making it wet and leaving the joint open.

Solderability Failure

This defect is common in Quad Flat No-Lead (QFN) and Land Grid Array (LGA) packages. Why? Because they usually come without solderable side terminations or solderable qualities from the manufacturing process.

Also, the edge termination’s lack of protective tin coating on its copper lead frame surface leads to poor solderability. 

Component Damage

Solder reflow often becomes a complex process for Moisture Sensitive Devices (MSD) that may cause unwanted damage.

For example, exposing such components to high humidity can cause popcorning or body cracking.

However, they won’t suffer mechanical damage from cracking or handling if manufacturers package them properly with preventive measures.

Solder Balls

Solder balls result from using a paste to handle components like BGA. In truth, excessive paste use on such parts can turn them into a ball during reflow.

And solder balls can reduce a circuit’s insulation separation distance, sending it far below the minimum requirements.

Thankfully, you can remove solder balls with high-pressure airlines. However, you must perform a follow-up inspection with an x-ray or optical device.

Note: Please proceed to remove solder balls with high-pressure airlines carefully to avoid damaging your circuit.

Non-coalescence of Paste

Technology keeps shrinking standard component sizes, causing manufacturers to use small paste deposits (which do not reflow fully). 

It’s a common issue with lead-free solder pastes, primarily when operators utilize traditional soak profiles.

Usually, the small paste deposits turn into balls that don’t reflow fully into most of the solder joint.

You’ll likely find this defect in 01005 and 0201 chip components, particularly during convection reflow.

Surface Corrosion and Copper Dendrites

Flux is not the only thing that can cause surface corrosion. In truth, lack of surface cleanliness is another significant culprit.

As a result, contamination testing and surface insulation resistance assessment are two essential methods for controlling and monitoring dangerous contaminants, helping to avoid PCB surface corrosion.

Also, copper dendrites formation can result in intermittent PCB failures.

However, lead-free soldering is a more common problem (while using VOC-free fluxes).

The problem is the flux residue sticking to the board’s surface and forming a conductive path through the moisture layer.

And this happens when you subject the process to high humidity and temperature.

Tin Whiskers

PTH PCBs feature tin coatings that sometimes result in whisker growth.

Tin whiskers are bad for circuit boards because they create unwanted connections between neighboring pads and tracks.

Although tin is the go-to finish option in the electronic industry (using lead-free soldering), whiskers remain a severe issue.

Usually, it happens when stress forms during the PCB plating process.

Insulation Contamination

This fault is more common in the wave soldering process.

So, insulation contamination occurs when soldering insulated leads to a circuit board directly, especially variants made from low-temperature material.

Sadly, when insulation pieces fall into a solder joint during the soldering process, it will result in contamination and a defect.

PCB Delamination

Delamination forms when water vapor from circuit board material escapes and expands while heating. Further, the higher temperatures required to melt lead-free solder push more energy on the components.

As a result, delamination or air gaps will occur when any moisture present in the board expands during the reflow process.

How do You Create Good Solder Joints?

Although it can be tricky, creating good solder joints is easier with practice. However, the process always needs careful follow-up inspection to ensure every solder remains problem-free.

Also, you’ll need the right equipment to make good solder joints. Here are a few tips to help with the process:

  • Make sure all solderable surfaces are clean and free from grease residues.
  • Always secure the items you want to solder, preventing them from shifting during the soldering process. Also, it’ll help avoid dry joints.
  • Apply some tin to the tip of your soldering iron. Then, use a damp sponge to clean it, and add a tiny amount of solder. Consequently, it will help to enhance the joint’s heat flow.
  • Place the soldering iron on the joint and add some solder quickly.
  • Then, allow sufficient solder to flow onto the joint to form a concave meniscus.
  • Now, remove the soldering iron as fast as you can. Leaving it on the joint for too long will exhaust the flux, oxidize the solder, and create a dry joint.
  • Finally, wait for the joint’s solder to cool and solidify before moving anything.

Rounding Up

Solder joints are like double-edged swords. They have their purposes on circuit boards but can quickly become the reason for failure.

While the problems listed above are not exhaustive, they are the most common issues in the industry. 

But that shouldn’t deter you. You can follow our tips above to learn how to create good solder joints.

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Emma Lu
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