Soldering - non-wetting, is it a defect?

Introduction

Nonwetting is an undesired phenomenon that can occur during the soldering process. Whether soldering copper wires, cables, or circuit boards, nonwetting is an issue that any engineer involved in these processes should understand and be able to distinguish from other phenomena, such as dewetting or cracking of solder joints.

This article explains nonwetting in soldering, which can occur during the soldering of cables, wires, and electronics soldering. This phenomenon also appears in other types of soldering, so every enthusiast of this technology will find something of interest here.

Wetting

Wetting in soldering is the effect where molten solder evenly (without gaps) covers the soldered surface, forming an intermetallic phase. The wetting angle between the solder and the soldered surface is less than 90 degrees. The molten solder "spreads" across the surface, creating a uniform layer.

Wetting depends on several key factors. The main ones include:

• Solderability of the surface. This is the property of soldered surfaces that allows them to be properly covered by the solder and form an intermetallic phase. Two standards for testing solderability have been developed in electronics: IPC-J-STD-002 (solderability of component leads) and IPC-J-STD-003 (solderability of PCB pads).
• Flux. Soldering flux is a chemical agent whose primary function is to remove metal oxides so that the solder can contact clean metal.
• Solder alloy. Currently, lead-free alloys are mainly used, though in some cases, lead-based alloys are still in use. The solder should have the appropriate composition with a minimal amount of impurities. Sometimes, the copper content in the alloy may increase due to a phenomenon known as copper dissolution during soldering.
• Temperature and soldering time. Proper wetting requires adequate heat, so parameters like solder temperature and soldering time are crucial. Different soldering methods have specific requirements, which are discussed further in articles such as Wave Soldering Profile and SMT Soldering Profile.

Wetting angle

The wetting angle is the angle formed between the flat surface of the material being soldered and the plane tangent to the surface of the solder at the point where they meet. In simpler terms, it's the angle where the edge of the solder alloy touches the soldered surface. A smaller wetting angle indicates better solderability, while a larger angle suggests worse solderability. If the wetting angle exceeds 90 degrees, proper wetting has not occurred. Note: there are exceptions to the 90-degree rule, but that's a subject for another discussion.

The illustration below demonstrates the concept of the wetting angle in soldering, with three tin (Sn) droplets on a copper (Cu) surface showing different wetting angles. The nonwetting effect occurs when the contact angle exceeds 90°

Nonwetting definition

Nonwetting is the opposite of wetting (I know... sounds very groundbreaking...).

The organization IPC defines nonwetting as "The inability of molten solder to form a metallic bond with the basis metal"[1][2]

Examples of Wetting and Nonwetting

Below is an image of proper wetting and two examples of complete nonwetting across the entire surface or around the lead.

Proper 360° wetting of the lead and PCB pad - acceptable class 1, 2, 3 according to IPC-A-610:

Nonwetting to connector leads - defect class 1, 2, 3 according to IPC-A-610:

Nonwetting to SMD resistor lead - defect class 1, 2, 3 according to IPC-A-610:

The IPC organization has defined criteria for evaluating solder joints, and minor instances of nonwetting are often acceptable. The permissible level of nonwetting depends on the product class and the type of solder connection. The detailed requirements are, of course, outlined in the IPC standards[2].

Summary

Nonwetting is an unwanted phenomenon that may occur during soldering. It should be minimized by using components with proper solderability. It is also essential to ensure an adequate amount of flux, the correct composition of the solder alloy, and appropriate thermal parameters (soldering temperature and time).

It is also important to make sure that the flux is not too aggressive, as this can lead to corrosion of electronics, particularly electrochemical migration. In addition, avoid overheating components, which can be another limitation in the soldering process.

In summary, achieving a proper solder joint requires considering many factors and maintaining the right balance throughout the soldering process.