The presence of oxides and other unwanted materials on a PCB’s surface potential impedes the performance. This is because the soldering process wouldn’t be as smooth as it ought to be. A better way to tackle that is to use a cleaning agent. The choice is always between no-clean flux vs. water soluble flux.
The two have distinct features and their potential downsides. By comparing what they have to offer, you will arrive at a choice of the one that works best for you.
What is the Purpose of Flux for PCBs?
To better understand what the flux does for the circuit board, you first need to understand the reason for using it. The reason is that there are usually oxides or metal oxides on the PCB’s surface. Among many other challenges, these oxides tend to make the bonding of copper difficult, as well as littering the surface.
It is therefore imperative to use a cleaning agent to get ride of those and prepare the PCB’s surface for soldering.
By using a flux, you would be able to clean the PCB’s metal surface, remove the oxides and further enhance the bonding or creating of a stronger solder joint.
Types of Fluxes for PCBs
There are two major types of fluxes or cleaning agents you can use. They are the no clean and water soluble flux.
They help you with cleaning the metal surfaces and to remove any of the oxides likely to interfere with the soldering process.
What is Water Soluble Flux Used for?
Also called the organic acid flux, it is a type of flux used for cleaning the oxides off the PCB’s surface. Unlike the rosin flux, it is made from organic materials other than rosin.
Why Use Water Soluble Flux?
There are a couple of arguments on the efficacy of water soluble fluxes, considering that they can leave behind some residues. Nevertheless, they have a good number of features that make them worth using.
These are some of the benefits or advantages of using a water soluble flux for electronics:
Highly Active
The highly active feature is the reason why this flux has a long life, when used in the process. It also aids the excellent soldering performance.
Excellent Cleaning
Due to the aggressive nature, the water soluble flux makes a good cleaning agent for wiping off metal oxides. This way, it prepares the circuit board’s surface for soldering.
Zero Burn-Off
Water soluble fluxes rarely burn off during the soldering process.
These are some of the additional reasons why you want to use the flux for your PCB:
- It is made of high solid content, usually between 11% and 35%.
- Halide free
- Halide activated (pH neutral)
- Halide activated (low pH)
- It is occasionally solvent or water-based.
Downsides/Disadvantages of Water Soluble Fluxes
Here are some of the reasons why using this type of flux for getting rid of metal oxides is not recommended:
General Chemistry Aggression
This type of flux is generally aggressive and this often translates to corrosion. The fact that it remains even after soldering is a thing of concern because it can continue to react on the surface until it is fully wiped off.
Residue Deposition
Water soluble flux continues to react after soldering and this is primarily due to the residue it leaves behind. To this end, you must endeavor to wipe it all off.
Strenuous Removal Process
The process of removing the flux is not easy either. You must follow these processes to be sure the PCB’s surface is freed of the flux:
- Occasionally monitor the surface to find out if there are signs of ionic contamination. Sometimes, it is best to use either an Omegameter or Ionograph to test for this contamination.
- Sometimes, the PCB’s surface needs to be subjected to a machine-washing process to get rid of the lurking residue, faster. This often extends the lead time and causes additional expenses.
The Susceptibility to Dendritic Growth
The use of water soluble flux also exposes the board to dendritic growth. It infers to the metallic and hair-like growth that sprouts out between the short, adjacent conductor paths.
How to Use Water Soluble Flux
Despite the downsides, you can still maximize this PCB flux by washing it off as soon as the soldering process is over. If it still sticks, consider using the machine-washing process.
No-Clean Flux: What Is It?
It is the opposite of the water soluble flux, in the sense that it doesn’t need to be washed off. No-clean flux are meant to be on the circuit board’s surface after the soldering process is over.
Also called the low-solids, it is the type of cleaning agent to use when you don’t want to wipe off the PCB’s surface after soldering.
The features of no-clean include:
1. Zero Cleaning
When we say that the no-clean flux doesn’t need to be wiped off; it is because it leaves behind negligible residue. This means that the residue left behind is inconsequential and wouldn’t harm the surface.
Besides, the composition of less chemistry and activity means that you might not need to clean the PCB after soldering.
2. Low-Solid Composition
The composition of low-solids means that the no-clean flux can be available in different types of liquids. For example, you could variants based on alcohol and water.
The choice of the one to use depends on the performance.
Downsides of No-Clean Flux
In comparing the pros and cons of no-clean flux, always check the amount of residue left behind, as compared to the possible infringement on the board’s performance.
With that being said, these are some of the potential issues with using a no-clean flux for your PCB:
Less Chemistry and Activity
While the presence of more chemistry and activity potentially increases the residue deposition; it could also impart the board’s performance.
In this instance, the reduction of the activity and chemistry of the no-clean flux makes it more difficult for the board to be soldered. The higher the activity and chemistry of the flux, the higher the soldering capabilities.
Residue-Related Failure
Although this type of flux boasts of leaving some unharmful, negligible residue behind, it is not entirely so. With the advent of new ways to manufacture modern electronics, and considering the closing of the gaps/spaces between these components; it has become expedient to get rid of residues.
The inability to do so often exposes the PCB to residue-related failures. Besides, the deposition of little residues varies by the assembly process. An example is the Through Hole Technology (THT) soldering process that requires the removal of all residues from the board.
Types of No-Clean Fluxes
Different types of no-clean fluxes exist. The following is an explanation of the different options available in the market, as low-solids:
1. Alcohol-Based and Rosin, Low-Solids/No-Clean Fluxes
This toes the line of traditional rosin fluxes, but with a bit of sophistication. This type of no-clean flux works well in the following aspects:
- Low-heat applications
- Preheat and long heat exposures and;
- Higher heating processes
2. Alcohol-Based and Non-Resin, Low-Solids/No-Clean Fluxes
The removal of the rosin improves the volatilization capabilities of the active flux. They are best used for the applications that require minimal residues and less heat.
On the flipside, the non-resin, no-clean fluxes are susceptible to depletion, especially when subjected to the selective soldering process. This is due to the exposure to higher heat.
The best way to combat this is to either use a synthetic equivalent of rosin or leave behind a little flux of rosin to aid the encapsulation of the remaining ionic residues.
3. VOC-Free and Low-Solids/No-Clean Fluxes
These are water-based fluxes capable of handling more exposure to heat. The other features include being ideal for high-heat programs (due to the longer water evaporation time)): and the tendency to be a little more active.
On the flipside, using the VOC-free, no-clean flux is not advised due to the longer preheat cycle.
Why Should You Wipe-off the No-Clean Flux’s Residue?
Besides the fact that the little residue left behind could trigger residue-related failures; it is also likely to make the residues conductive.
Wiping the remaining residues on the PCB’s surface after soldering also helps in the following ways:
- It prevents the trapping of solder balls in the left-over no-clean flux, which is situated between the components and conductor paths.
- Wiping-off the remaining residues helps to prevent leakage of currents between the circuits.
- It aids the adhesion of the circuits to conformal coating.
- Contaminations triggered by residue deposition are reduced.
- It reduces the chances of moisture absorbing into the flux’s residue.
Key Considerations for Leaving Behind No-Clean Flux Residues
If you are still intent on not wiping off the remaining no-clean flux residues; here are some of the best practices to follow:
Low-Density Board Considerations
You may not encounter residue-related failures if the residues are left behind in the low-density or low-voltage PCBs.
No-Clean Flux Activation
The activation of the no-clean flux refers to the full temperature-cycling of the flux. This is often based on the solder profile from the solder paste’s manufacturer.
Hygroscopic Properties
Despite the activation of the flux’s residue, it is still likely going to be hygroscopic. The term refers to the flux’s residue’s ability to absorb moisture, while prevent conformal coating adherence to the PCB.
No-Clean Flux vs. Clean Flux
While there are differences between the no-clean and water soluble fluxes, you may also want to compare it to another – clean flux.
A clean flux refers to the type of flux or cleaning agent that combines water, organic solvents and wetting agents.
The features include but are not limited to:
- Consistency
- The ability to remove solder paste residues, easily.
- The clean flux is also notable for the improved wetting properties.
- It also uses safer organic solvents, which have the EPA’s approval.
On the flipsides, the clean flux might not be a good investment for getting rid of metal oxides on a PCB. It is so for the following reasons:
- It requires extra cleaning, especially after the reflow process.
- It also doesn’t last if no-clean flux is used after it.
No-Clean Flux vs. Water Soluble Flux for Electronics
In choosing the right cleaning agent or flux for your circuit boards, a couple of factors are worth considering. Some of these are:
Consider the Service Environment
What type of environment or condition are you exposing the flux to? Ideally, the environment needs to be open or secluded, depending on what you are working on.
But, the point is that the type of service environment could be a “breeding ground” for moisture. Besides, the following are also worth considering alongside the environment:
- The extent of the residue left behind
- The components’ density. Ideally, a circuit board with higher residue and more component densities is likely to be exposed to moisture than the one with little residue and a relatively low component density.
- For the best results, use the no-clean flux for the standard service environments and the water soluble flux for the harsh environment.
The Assembly Process
For PCBs assembled via the Surface Mount Technology (SMT); use the no-clean flux. If the board uses the Through Hole Technology (THT); use the water soluble flux.
Consider the Applications
Where the circuit boards are designed for use plays an important role in choosing between water soluble flux vs. no-clean flux.
Using the water soluble flux for the high-risk applications is better, and this is due to the affordable re-optimization and less time spent.
Component Standoff Height
The Component Standoff Height (CSH) refers to the height of the components. The rule of thumb is to use the water soluble flux for the higher CSH.
The following are some of the best practices for the Component Standoff Height:
- Avoid the use of water soluble flux with shorter components. This is because of the parts’ susceptibility to trigger solder voids, trapping of the flux activators and the potential for causing short-circuiting.
- The best way to determine the Component Standoff Height (CSH) is to compare both the soldering reflow profile and the bond pad.
Conclusion
The choice between the no-clean and water-soluble fluxes depends on the types of applications, the cleaning process and the circuit board components’ densities. You should also reach out to a professional PCB assembling company to help you fine-tune and smoothen-out the grey areas so you can make a better decision.
