What is Solder Non-wetting?

Proper soldering techniques are essential to creating electronic devices that function properly. In the electronics industry when a PCB is manufactured, the immediate next step is to solder all the electronic components to the PCB traces. However, sometimes the process doesn’t go as planned and which eventually leads to soldering issues and poor electrical joints.

This is a comprehensive article that discusses the causes of Solder Non-wetting, and problems that arise due to this phenomenon. The article also contains information on how you can prevent a Solder Non-wetting situation by being cautious, and well-prepared.

What is Soldering Non-wetting? 

A non-wetting defect is a soldering defect where the molten solder fails to wet the base metal on the board. In this case, when the bond fails to materialize, the solder may not stick to the terminals of the components or the PCB pads to allow them to stick to each other properly. The surface material of the board will be exposed, and the solder itself may appear grainy or lusterless. This also leads directly to voiding — the creation of holes in a solder joint that have no solder material.

Non-wetting on a PCB creates unstable solder joints. When the joints on the circuit board are unstable, they may break or cause poor electrical conductivity. When solder does not adhere to the base metal, then components and PCB pads will not be firmly fixed to the board. They may come loose and form poor connections, or they may fall off, either immediately or when subjected to stress. Without its components firmly in place, the PCB may malfunction.

Causes of Solder Non-wetting in PCBs

Solder non-wetting defects have several causes that can affect the PCB. Below are many reasons non-wetting can occur on a PCB:

• Inadequate PCB Finish or Plating: Poor-quality finishes or insufficient plating thickness on the PCB, such as thin tin/lead coatings or improper copper plating, hinder solder flow and adhesion. Without a smooth and adequately coated surface, solder cannot bond to the base metal, leading to non-wetting.
• Oxidation: Oxide buildup on the PCB pads, component terminals, or soldering surface prevents the solder from bonding properly. This is the result of improper storage and exposure to air for some time, poor storage, or multiple thermal cycles; hence, it does not wet appropriately.
• Degraded or Wrong Flux: Flux is an important ingredient in the cleaning of oxides and contaminants in soldering. Flux, if old, degraded, or of low activity, fails to clean the oxides properly. The same happens when the wrong flux is used for a particular application.
• Improper Solder Paste: Poor wetting may be due to issues with solder paste, such as insufficient volume, expired paste, or using a type that is incompatible with the PCB's plating material. Generally, high-activity solder pastes are more effective in ensuring good adhesion and reducing the likelihood of defects.
• Storage and Handling Issues: Extended storage times can degrade the PCB finish, while mishandling, such as smearing of resin onto critical soldering areas, can obstruct solder flow. Both issues result in solder failing to bond correctly to the PCB surface or components.
• Temperature-Related Issues: The right soldering temperatures and controlled soak times are critical. Too little or erratic heat prevents the flux from activating fully, and excessive soak times exhaust the flux before soldering is completed. Both conditions compromise the ability of the solder to bond well. 
• Contaminated Materials or Equipment: The presence of contaminants within solder paste, flux, or even on the PCB surface will impede soldering by not allowing proper flow and adhesion. Similarly, contaminated materials will also have residues that create additional disconnections to solder bonding leading to increased non-wetting defects. 

Effects of Solder Non-wetting on PCBs 

Solder non-wetting can have severe implications for PCB performance and reliability. The effects extend to the physical stability of the components, the electrical functionality of the circuit, and the overall longevity of the device. Below are the primary impacts of solder non-wetting: 

• Unstable Solder Joints: Non-wetted solder joints do not possess structural integrity for stable connections. In such cases, mechanical stress, vibration, and thermal cycling tend to detach or create intermittent connections among the components. 
• Poor Electrical Conductivity: It is often associated with inadequate coverage of solder, thus providing exposed surfaces that interrupt electrical continuity. This increases the resistance within the joint, reducing the circuit performance and sometimes completely terminating the circuit. 
• Component Misalignment or Detachment: When solder fails to bond properly to PCB pads and component terminals, components may not stay securely in place. Over time or under stress, these components can shift, misalign, or completely detach from the board. 
• Voiding in Solder Joints: This might end in voids in soldered connections or what is called voids or cavities that do not include solder. Such voids weaken the connection, easily resulting in cracking or breakage if stressed mechanically or thermally. 
• Increased Repair and Rework Costs: Non-wetting solder joints result in additional repair or rework, which increases the cost of manufacturing. In some cases, extensive defects may make a PCB irreparable, thus resulting in wastage. 

How to Prevent Solder Non-wetting? 

We have seen the effects of Solder Non-wetting and how harmful it can be for electronic devices and electrical components connected to a PCB. Some of the ways we can prevent Solder Non-wetting: 

• Surface Preparation: Proper cleaning and removal of contaminants from the surface to be soldered with a variety of methods are highly important to avoid the contaminants hindering the solder wetting or solder wetting behavior. It can be carried out, for instance, by using cleaning agents or a mechanical or plasma-based approach. 
• Optimized Soldering Parameters: In order for the soldering process to come up with good-quality results, the right temperature, time, and pressure have to be applied to the soldering materials. 
• Flux Selection: The selection of the right flux is a critical step. The flux and the solder alloy should be compatible, and there should be the right amount of activity to remove the surface oxides and it help the wetting. 
• Surface Finish Selection: In selecting an appropriate surface finish that works well with the caving solver and yet provides excellent wetting properties it may help avoid solder non-wetting. 
• Solder Alloy Selection: Another method is by supplying the solder alloys that are recognized to possess excellent wetting properties as a means of how to prevent the problem of soldering non-wetting.

Click here to learn more about other solder defects like solder dewetting.