How to Prevent Poor Solder Wetting

In many cases, inadequate solder wetting is responsible for subpar joints and unreliable soldering processes. Nonetheless, the prevalence of this issue doesn't imply inevitability. By employing effective strategies, you can proactively prevent inadequate solder wetting, ensuring durable and reliable joints.

 

 

What Is The Soldering Wetting Process?

 

Let's begin by precisely defining the wetting process.

 

During soldering, two metals are joined using a solder alloy, forming a permanent fusion between them. However, behind the scenes, a crucial wetting process must occur.

 

Wetting involves the solder metal bonding with the metal surfaces of your PCB or components. As the solder melts, it transforms into a fluid state and adheres to the component, establishing the essential solder joint for your process.

 

For the wetting process to occur successfully, specific environmental conditions must be met, including:

 

A clean copper surface devoid of any contaminants.

Attainment of the optimal temperature.

 

It's worth noting that some metals exhibit greater receptivity to the wetting process, while others pose more of a challenge.

 

 

What Is Solder Wetting?

 

Solder wetting refers to the ability of molten solder to spread and adhere to the surfaces of the materials being joined during the soldering process. It is essential for creating strong, reliable solder joints. Efficient wetting ensures that the solder forms a smooth, uniform bond with the substrate, promoting good electrical and thermal conductivity. Inadequate wetting results in solder that does not spread evenly or adhere properly, leading to weak joints, poor electrical connections, and increased risk of failure.

 

Effective solder wetting, and consequently, metal bonding, necessitates a specific environment for proper execution. This process relies on a clean copper surface devoid of contaminants and heated to the appropriate temperature. Below, we will delve into the significance of achieving optimal solder wetting and outline strategies to prepare your operation for successful outcomes.

 

 

Why Is Good Wetting Soldering Important?

 

Effective solder wetting is essential for achieving proper metal joining. Without it, metals may fail to adhere adequately and are unlikely to meet industry standards for acceptable use, rendering them essentially defective. Optimal solder wetting results in well-made solder joints capable of withstanding the test of time.

 

Distinguishing good solder wetting is often possible by its appearance. It produces a shiny, smooth-looking solder that has reached maximum flow. Conversely, poor solder wetting is frequently identifiable. The solder may appear grainy, dull, and porous, indicating inadequate adhesion to components. Such solder is often unusable in commercial settings, leading to wasted time, capital, and productivity.

 

 

What Causes Poor Wetting?

 

Several factors can contribute to poor solder wetting, including:

 

Oxidation on solder surfaces: Leaving a solder tip exposed to air can cause oxidation, leading to inadequate wetting.

Incorrect soldering temperatures: If the temperature is too low, the solder may not achieve proper fluidity and fail to make sufficient contact with components. Conversely, excessively high temperatures can cause rapid vaporization of solder, hindering proper wetting.

Prolonged solder tip contact: Holding the solder tip against components for too long can result in burned flux and damaged components.

Insufficient wetting: Dirty circuit boards or failure to apply heat evenly to both the pad and pin can result in insufficient wetting and poor bonding.

 

 

 

Wetting Soldering Vs. Dewetting

 

The defect of non-wetting is distinct from that of dewetting. Dewetting occurs when solder pastes initially cover the terminals of components and pads on the PCB but subsequently retreat from certain areas, leaving behind thin solder surfaces on metal alloys and thick, irregular clumps in others. While the PCB surface material is generally not consistently exposed, dewetting primarily impacts solder joints and fillet quality when it does occur.

 

On the other hand, non-wetting predominantly arises with solders lacking lead, such as copper-silver-tin solder. Some metals readily exhibit non-wetting behavior, while others do not due to variations in material properties, including spread and wicking characteristics. Bare copper circuits with organic solderability preservatives are susceptible to non-wetting on PCB pads, especially after undergoing multiple thermal cycles. Pure tin promotes good spread and minimizes non-wetting, as does silver immersion surface finishes. Similarly, gold-and-nickel alloys facilitate proper soldering and reduce non-wetting provided they are free of impurities.

 

 

How to Prevent Poor Solder Wetting During Reflow？

 

Improving wetting in soldering involves specific steps, although not all may be feasible for every scenario. Therefore, it's crucial to select the most appropriate strategy for your particular design.

 

1.Opt for a solder paste with higher activity

 

Such pastes typically offer superior wetting, especially beneficial when dealing with challenging surface finishes or oxidized surfaces like immersion silver, tin, or OSP. These surfaces become more problematic to wet after initial reflow. Choosing a different solder paste can significantly impact wetting performance, as explained in a paper addressing surface finish effects on solder paste performance. We provide a range of solder pastes tailored to diverse manufacturing needs, all designed to promote maximum wetting with good activity.

 

2.Reduce oxides on soldered surfaces

 

Whenever possible, work with less-oxidized surface materials. While this isn't always feasible, alongside using a higher-activity paste, employing nitrogen during reflow can help minimize oxide formation during the reflow cycle.

 

3.Increase solder paste volume or lead size

 

To address poor wetting, consider adjusting the stencil to apply a greater volume of solder paste. If this isn't an option, selecting components with larger leads can compensate for the low paste volume and aid in completing the solder joint.

 

4.Fine-tune the reflow profile

 

Extending soak time can help equalize soldering temperatures, resolving trouble spots and facilitating good wetting of both pads and component leads. Additionally, increasing the time spent above the liquidus stage ensures complete solder flow before solidification begins.

 

5.Clean and Well-Maintained Environment

 

The presence of contaminants, dirt, or moisture on your PCB can hinder the wetting process. It is advisable to clean the PCB before soldering to remove any grease or dirt that could affect solder wetting. Solvent wipes or PCB cleaning products are suitable for this purpose.

 

6.Flux Choice

 

The flux used in your soldering application plays a crucial role in improving wetting. It has a cleaning effect that helps to clear surface oxides, especially when a stronger flux is utilized. Flux can also enhance heat transfer, particularly in rework applications.

 

7.Preheated Soldering Iron/PCB

 

Preheating your PCB offers several benefits for solder wetting. It helps to activate the flux more effectively and prevents oxidation, thus enhancing the wetting process. Additionally, preheating is an essential aspect of PCB assembly and offers various other advantages.

 

 

To Conclude

 

We trust that this post has clarified the significance of the solder wetting process, a crucial element in every soldering task.

 

Understanding the factors contributing to poor wetting, along with practical tips to enhance it, will undoubtedly elevate your soldering proficiency.

 

Keep in mind, encountering poor wetting in a soldering joint isn't a catastrophe. You can rectify it by allowing the joint to cool, eliminating the flux, and reheating the joint to initiate anew.