Every electrical device has a fundamental component called a printed circuit board, or PCB. Every modern electrical device, from your TV to your dishwasher to your phone to your Alexa, is made up of this fundamental component.
A circuit board includes tiny components that must be assembled on the board and logically linked for the electronic device to work. How are these little parts connected to the board? PCB assembly engineers frequently employ a particular kind of paste to keep the board’s components together. Solder paste is the term for this substance.
If you are eager to find out what is solder paste and solder paste flux, what are the types of these pastes and which one to choose, you’ve come to the right place.
Contents
What is solder paste?
A gelatinous chemical substance called solder paste flux is often combined in a 50/50 ratio with solder powder. The paste is a greyish, putty-like substance once it has been blended.
This substance is used to join metal pieces. From metallurgy to the construction of electrical circuit boards to attaching your license plate to your automobile, solder may be utilized in a variety of undertakings.
Solder functions as a form of adhesive to attach metal surfaces once it has been heated and melted.
The term “solder joint” is widely used to describe the intersection where two surfaces are held together by solder paste.
Solder pastes are far more efficient for joining mechanical and electrical components than conventional glue. This is so that cables and other things may transport electricity unhindered because the molten solder enables the passage of electric impulses.
Types of solder paste
- Depending on the level of environmental protection
Lead-free and leaded
Flux paste and solder powder are combined to create leaded solder paste. Pb and Sn are the two primary ingredients. Sn63/Pb37 is the most widely used leaded solder paste.
Tin, silver, and copper make up the majority of lead-free solder paste, which has a lead concentration that meets the EU criteria of less than 1000 ppm.
- Depending on the temperatures needed for the application
high temperature, standard temperature, and low temperature
Low-temperature solder paste is often referred to as solder paste having a melting point of less than 138°C. It is mostly a tin-bismuth alloy. High-temperature solder paste melts at temperatures over 250°C, whereas standard-temperature solder paste melts at temperatures between 150 and 250°C.
- Depending on how often they need to be cleaned
environmentally friendly cleaner and disposable
The consistent blend of solder particles, no-clean flux, and additives in disposable solder paste mixture makes it the perfect material for the manufacture of SMT.
Water-soluble flux is used in solder pastes for environmental cleaning. The residue left behind after soldering interacts with water and may result in a short circuit if it becomes moist, thus it has to be cleaned.
- Depending on the activity
High RA, medium RMA, and low R
The low R type contains natural rosin (no additional catalysts have been introduced). Medium RMA type is a halogen-based catalyst used in addition to natural rosin. High RA is the catalyst addition that is quite powerful. Although there is good welding, rust is a major problem in the high RA type.
What is solder paste flux?
When soldering electronics, a chemical called solder flux is utilized. More precisely, flux shuts out the air to prevent further oxidation and removes oxidized metal from the surfaces. Additionally, flux enhances the wetting properties of the liquid solder.
Solder paste flux differs from a liquid flux in terms of both physical shape and intensity of activity. In comparison to liquid flux, which only comprises 2-25% active chemicals by weight, solder paste flux often includes 60-80%.
Flux enables the dissolution of oxide coatings on metal surfaces, preparing the surface for solder adherence by “wetting” it. Molten solder can become more sticky when it is wet, or in a condition of liquidus.
Then you may create a bond or solder junction by employing flux to change the surface tension of the metal’s molten solder. As a result, flux “cleans” the metal surfaces, enabling the formation of solid solder joints.
Flux is also essential for preventing the reoxidation of your metals after soldering. Metal objects develop oxides over time and after being exposed to oxygen, which prevents the development of stable solder joints.
Types of solder flux
Flux can be added to solder paste in a variety of forms, each with a specific function. It might have a solid, paste-like, or liquid flux. Before beginning the soldering process, it is vital to understand the variations in the bases of each type of flux.
Rosin based Flux
Rosin flux, which is made of a natural pine tree extract, may be removed off the board using solvents when the soldering is complete.
Rosin flux is the most dependable flux.
However, activated and weakly active flux can sometimes leave behind traces of chloride ions or other corrosive leftovers.
To avoid any long-term failures—you don’t want the corrosion on your boards to be caused by the flux you used—these must be taken off of every circuit board after soldering.
Additionally, sticky flux residues can attract dust, which can lead to electrical shorts or failures. Increased use of lead-free solder alloys and highly active fluxes assist prevent oxidation at higher soldering temperatures.
Inorganic acid Flux
Stronger metals like copper, brass, and stainless steel operate better with inorganic flux than other metals. It consists of an amalgam of more potent acids, including hydrochloric acid, zinc chloride, and ammonium chloride.
After using inorganic flux, thoroughly clean the surfaces to eliminate the corrosive residues which, if left on the surfaces, would weaken or ruin the joint.
Note: for electrical or electronic assembly operations, this type of flux shouldn’t be utilized.
Plumbing flux
It is frequently used to connect two solder joints that have been soldered together. The pipes are being “sweated” in this situation. Flux plays a significant role in the sweating (or soldering) process when paired with solder.
It is produced from organic or inorganic components and is available in liquid and paste form. Which you choose relies on the characteristics your project requires.
Water-Soluble flux
This type of flux is simple to clean and is often composed of glycol and other organic materials. There is no need to utilize additional alcohol-based fluxes because many water-soluble fluxes are created with water-based solutions.
Water-soluble organic fluxes can minimize VOC emissions, notably for PCB makers that must adhere to the strictest restrictions. Aside from organic acids, monobasic and dibasic organic salts, amides, and halogenated compounds of chlorine and bromine are other common acid activators utilized in water-soluble fluxes.
To guarantee lifespan and dependability and to reduce the likelihood of corrosion, these activators should be taken out of any circuit board. Some of the best soldering conditions are provided by water-soluble fluxes, which also practically never burn off during the soldering procedure. In comparison to other fluxes, they are also significantly safer on metals.
No-clean flux
No-clean flux, which is made of rosin and other solid wastes, is favored for its affordability.
Genuinely manufactured no-clean fluxes might have additional activators in addition to the natural flux rosin’s beneficial characteristics. Any soldering rosin with extra activators might produce corrosion-causing residues, but not a no-clean flux.
No-clean fluxes were created to improve the quality of life for PCB assemblers by lowering the time and cost associated with cleaning a board after soldering. A no-clean flux does not interfere with board function or result in long-term corrosion, leaves fewer traces than a residual flux, and does not produce stains.
Residues left by residual fluxes may feel tacky or attract dust, no-clean fluxes, on the other hand, may hinder conformal coating from adhering well.
How to apply flux solder?
To apply solder pastes, you must first thoroughly clean the surfaces before adding any paste to successfully glue your preferred surfaces together.
In most cases, the flux application procedure starts with preparing or purchasing a pre-mixed bottle of flux-containing paste. Either a machine or a person can apply the flux. You can use a pen that technicians use to apply solder paste flux if you’re doing it by hand. You might also use your fingers or a little paintbrush.
You can use the following techniques to apply the mixture when applying flux to electrical surfaces:
- A procedure called solder reflow is applied to surfaces or circuit boards.
Solder reflow is the process of melting solder paste and flux together by utilizing a reflow oven, infrared heat, or even a unique pencil-shaped desoldering hot air device. Controlling the preheating, soaking, and reflow heating steps of such a process necessitates close attention.
- The ideal technique for soldering lead objects into SMT components that are attached to circuit boards is wave soldering.
On average, the procedure uses more solvents than other application techniques. Before the solder wave, the flux is sprayed over the surface.
The flux cleans the soldered pieces by eliminating any undesired oxidized layers once it has been applied to the surface.
You must thoroughly prepare the surface if you decide to utilize a less aggressive flux combination. Before you start, think about cleaning the surface more than once.
- A solder that is dripped or sprayed onto a surface is referred to as selective soldering.
Solder is applied automatically using this technique to specific board holes and pads. The device either uses a nozzle to deposit molten solder or a laser to melt solid solder that is supplied into the junction. The surfaces that need to be soldered should be sprayed or applied in single drops.
- A soldering iron is a tool used to solder and apply solder paste to metal surfaces.
The tip of the soldering iron rod is composed of metal. Solder paste is applied with the help of the tip. This iron may be connected to a power outlet or a soldering machine. The iron is heated by electricity being drawn to it.
The iron’s handle is insulated. The paste is melted through the hot iron’s tip. The solder paste can also be placed at the desired position using the tip.
What are the differences between solder flux and solder paste?
Between soldering flux and solder paste flux, there is a narrow line, yet each has distinct qualities. The physical characteristics and chemical makeup of the paste and liquid solder variants are the main distinctions for the majority of uses.
You may mix a material called solder paste flux with solder powder to get a gray paste. Solder paste flux, as you might expect, typically necessitates more cleanup.
The way that they function is the other distinction between solder paste flux and liquid flux. While the jelly-like paste has between 60% and 80% active chemicals, liquid solder has up to 25% active ingredients.
One of the most remarkable characteristics of solder paste flux is still its chemical makeup.
A paste can contain up to 90% metal after being combined, which seems like a lot to handle.
However, the non-metallic components make up around half of the product’s volume and have a lower density.
Frequently asked questions
Can you use soldering paste instead of flux?
Flux is a collective term for materials that aid in soldering. Therefore, whatever you utilize turns into a flux! Some are good for particular metals and some are superior to others. The list of compounds that are utilized as flux is extensive, and it includes solder paste flux.
Solder paste flux is a mixture of flux and solvent in liquid form. Small solder balls are present in the flux and solvent solution. A circuit board’s pads may be printed using solder paste flux thanks to the consistency’s pasty nature.
What is solder paste used for?
The paste is utilized in the production of printed circuit boards to join surface mount technology components to board pads. By printing the paste in and over the holes, it is also feasible to solder through-hole pins in paste components.
When the board is heated, the solder melts, creating a mechanical bond as well as an electrical connection, holding the components in place momentarily. The paste is applied to the board using a jet printer, stencil printer, or syringe, and the components are then assembled either manually or with a pick-and-place machine.
Is soldering paste necessary?
Yes, it is indeed necessary.
Before use, paste or flux should always be mixed, especially if it has been sitting in storage for a while. When heated with a torch, the flux or paste causes the solder wire to flow through the fitting and tubing junction.
To remove the tarnish, all solder joint components need to be cleaned with sandpaper or a sanding cloth. To facilitate better water flow, all tubing inner ends should be reamed free of the inside tubing ridge.
Without reaming, turbulence will develop, leading to the tubing wearing out a few inches from the joint, creating pinholes, and eventually causing a leak in the water supply.
Is soldering fluid the same as flux?
Soldering flux and solder paste are two distinct things.
Metal bodies are joined together using solder paste or soldering flux. The various metal alloys and their component pieces may be glued and joined together using a solder paste, which is a mixture of products and substances.
Typically, the flux is applied to a metal surface before the soldering process begins. Flux might, however, also be included in the paste. Flux is used throughout the soldering process to clean the metal components that are being connected.
Conclusion
As technology advanced, the method for soldering components onto printed circuit boards changed. Things have advanced significantly from hand soldering to the usage of solder pastes. In most large-scale PCB assemblies, solder paste flux — a mixture of tiny metallic balls and flux — is now chosen.
There is also such an option as soldering flux which is needed to clean metal surfaces before soldering them together. The main purpose of flux is to deoxidize metal surfaces without dissolving since any oxides that are left on a metal surface might lead to poor solder connections.
We hope that this article helped you distinguish solder paste from soldering flux, and find out about their components and types. Make sure to use that knowledge!