Dewetting in soldering: What it looks like and why it happens

Introduction

Dewetting is a physical phenomenon that occurs in various technological processes, and it is described within the framework of fluid mechanics. In this article, we discuss dewetting in the context of soft soldering, particularly in processes such as electronics soldering and cable and wire soldering.

A professional soldering engineer should be able to visually differentiate dewetting from other phenomena, such as nonwetting, solder joint cracks, or other soldering anomalies.

What is Dewetting?

Dewetting in the soldering process refers to a situation where the molten solder initially wets the surface but then recedes, leaving behind irregular areas resembling "solder islands" separated by flat surfaces.

Dewetting indicates that the wettability of the surface has significantly deteriorated, which can negatively impact the reliability of the solder joint.

The IPC-T-50 standard defines[1] dewetting as:

"A condition that results when molten solder coats a surface and then recedes to leave irregularly shaped mounds of solder that are separated by areas that are covered with a thin film of solder and with the base metal not exposed."

The following image shows a PCB pad surface where dewetting has occurred:

Causes of Dewetting

There are many possible causes of dewetting in soldering processes. Below are some examples:

• Base material diffusion to the surface. Component leads or PCB pads are coated with a solderable material (gold, tin, etc.). Under this layer is the base material, which often contains copper (Cu), sometimes zinc (Zn), or other elements. If these substances migrate to the surface and oxidize, then the wettability of that surface can degrade significantly. The solution is to maintain a sufficiently thick finish layer, use intermediate layers (usually a nickel barrier), and control the storage conditions of components and PCBs (i.e., time, temperature, humidity).
• Surface oxidation. Over time, a layer of oxides can form on the surface intended for soldering, which may be difficult for the flux to clean. Using a more active flux can help in such cases, but it may lead to corrosion problems or a phenomenon known as electrochemical migration.
• Incorrect soldering profile. Too high a soldering temperature can cause the flux to degrade and the soldered surfaces to reoxidize. A similar effect can occur with a soldering time that is too long. Therefore, ensuring the correct SMT reflow profile and wave soldering profile is essential.
• Incompatibility of solder alloy. Using a solder alloy with properties not suitable for the substrate or process parameters can lead to dewetting. Some lead-based alloys and lead-free alloys can contribute to this phenomenon.
• Contamination of solder alloy. Contamination of the solder alloy by foreign elements, such as arsenic, zinc, copper, etc., can cause dewetting. These contaminants can come from the solder pot or from contact with component leads and PCBs. A typical phenomenon that increases the concentration of copper in the solder pot is known as copper dissolution.
• Large pad areas. Very large areas covered with solder (e.g., grounding pads or heatsink mounting areas) may exhibit symptoms of dewetting. This phenomenon is typically acceptable if the area is not intended for a soldered connection.

Standards

Dewetting is discussed in the following standards:

• IPC-T-50 Terms and Definitions for Interconnecting and Packaging Electronic Circuits.
• IPC-A-610 Acceptability of Electronic Assemblies.
• J-STD-001 Requirements for Soldered Electrical and Electronic Assemblies.
• IEC 61191-1 Printed Board Assemblies - Part 1: Generic Specification - Requirements for Soldered Electrical and Electronic Assemblies Using Surface Mount and Related Assembly Technologies.

Summary

Dewetting is an unwanted phenomenon. If it occurs on component leads or PCB pads where a solder joint is supposed to form, then it is classified as a defect condition according to the requirements of IPC standards, regardless of the product class[2][3]. However, if dewetting occurs on surfaces not intended for soldering (e.g., heatsink mounting areas, screw attachment, markings), it is generally not a problem.[3]

Elimination of dewetting effect from the soldering process may be challenging, so understanding the causes of this phenomenon is very important for developing effective corrective actions.