FAQs

What is the difference between “soft soldering” and “silver soldering”?

Although both expressions use the word “soldering,” the terms “soft soldering” and “silver soldering” mean two different types of metal joining. “Soft soldering” refers to a process of metal joining that uses Tin-based alloys as the metal being used to join the two pieces of metal. “Silver soldering,” on the other hand, refers to a process whereby two pieces of metal are joined using a Silver-based alloy. Tin-based alloys melt at a lower temperature than Silver-based alloys; therefore, soft soldering requires lower heating temperatures than does silver soldering. These days, many people simply refer to soft soldering as “soldering” and silver soldering as “brazing,” which reduces the confusion of using the word “soldering” in both terms. A wide range of examples of soft soldering (or soldering) fluxes can be found at Electronics Soldering Flux and Industrial Soldering Flux. In addition, a wide variety of silver soldering (or brazing) fluxes can be found on our Brazing Flux pages. It should be noted that it is not uncommon for Tin-based solders to contain Silver. However, the amount of silver added is relatively small compared to the amount of Tin present in such solders. One thing that both soft soldering (or soldering) and silver soldering (or brazing) have in common is they both need flux! However, since the alloys and temperatures are substantially different in each process, it stands to reason that you need to choose the right flux for each process. A soldering flux won’t do you any good for a brazing application, and a brazing flux is not going to help if you’re soldering a Tin-based alloy.

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What’s the difference between a No-Clean Flux and a Water-Soluble Flux?

A No-Clean flux is typically a soldering flux used in electronics applications whose residues do not need to be cleaned off after soldering. This means that the flux is strong enough to facilitate the soldering at hand, but mild enough that its residues won’t cause damage to the components or printed circuit board (PCB) if left on after soldering and not cleaned off. For examples of No-Clean fluxes, please see the following pages on our website: Wave Soldering Flux – No-Clean, Alcohol-Based, Wave Soldering Flux – VOC-Free, and Selective Soldering Flux (though not all selective soldering fluxes featured are No-Cleans). In order to be accurately labeled as a No-Clean, a flux needs to undergo industry-standard testing to prove that its residues will not cause damage to the component or PCB if left on, uncleaned, after soldering. (It should be noted that although the term No-Clean is typically used for fluxes used in electronics applications, there are fluxes used in non-electronics applications that are designated as No-Clean, meaning their residues can be left on the part after soldering or brazing, with little or no concern that the residues will lead to problems stemming from corrosion. These so-called No-Cleans are subject to different standards than No-Clean fluxes formulated for electronics applications.). A water-soluble flux is a flux whose residues can be washed off with water after soldering. The water is sometimes agitated or heated (or both) to facilitate this washing, but the hallmark of a water-soluble flux is that its residues can be dissolved and cleaned off with water. Unlike a No-Clean flux, there is less concern about the residues harming the PCB or component, since they are going to be washed off after soldering anyways. This means that water-soluble fluxes tend to be more active than No-Clean fluxes, which makes them advantageous for applications that require a stronger flux. Of course, when using a more active, water-soluble flux, special attention must be paid to assuring that post-solder cleaning is done effectively, since uncleaned flux residues are likely to cause problems down the road. While the residues of a No-Clean flux are left on the board, there are No-Clean fluxes whose residues can be cleaned off with water after soldering. The residues of these water-soluble No-Clean fluxes are mild enough to be left on the board after soldering but water-soluble in case an application requires post-solder cleaning with water. As such, a water-soluble No-Clean flux will never be as active as a true water-soluble fluxes whose residues must be cleaned off after soldering.

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What’s the best flux for soldering electronics, such as components on a printed circuit board (PCB), or asked another way, are there soldering fluxes that you shouldn’t use when soldering something onto a PCB?

An important rule when choosing a flux for any soldering application is to always choose a flux that is sufficiently strong for your application. The word, sufficiently, is emphasized because just as you want to avoid a flux that is not active enough for your application, you also want to avoid a flux that is too active for your application. The reason for avoiding a flux that is more active than your application requires is that stronger fluxes tend to leave stronger residues, and you have to be mindful of flux residues, as they can cause problems after soldering. This rule is particularly important when it comes to soldering electronics, since electronic components and PCBs are sensitive and thus susceptible to being damaged by aggressive flux residues. Fortunately, there are numerous fluxes that are specifically suitable for electronics, including: rosin fluxes, No-Clean fluxes, organic acid fluxes, and water soluble fluxes that are designated for electronics. Superior Flux offers a full range of Electronics Soldering Flux. If you choose a No-Clean flux, the idea of avoiding an overly aggressive flux could be thought of as less of an issue, since such fluxes are formulated so that their residues can be left on after soldering. However, even when choosing a No-Clean flux you should always opt for the flux that is strong enough and not too strong. For example, choosing a 4% solids-content No-Clean flux when a 2% solids-content No-Clean flux would do the job could carry risks; even though both fluxes are technically No-Cleans, the higher solids-content flux is more aggressive and, however unlikely, more likely to cause problems if something goes wrong. If you’re using a water-soluble flux or another type of flux whose residues need to be cleared off after soldering (such as many rosin fluxes), then the principle of using a flux that is strong enough rather than too strong is particularly important. It is important to note that when using a flux that is designated as “water-soluble” make sure that it is suitable for soldering electronics. You’ll know this if the label specifically says that it’s suitable for electronics or if the label indicates that the flux contains ingredients that are not suitable for electronics, most notably Zinc Chloride, as will be elucidated below. You might ask why the relative strength of the flux matters if you’re using a flux whose residues are going to be washed off after soldering (with water or some other cleaner or solvent). We must keep in mind that post-solder cleaning, like most operations, is not without error. Even effective post-solder cleaning operations can sometimes leave flux residues on the PCB; this is particularly the case when there are nooks and crannies in the board where stubborn residues can take up residence and resist cleaning efforts. When such anomalies occur, it’s better for the life span of the PCB if such flux residues are from a less rather than more active flux. Back to the question as to whether there are fluxes that should not be used when soldering electronics, the answer is absolutely. As a corollary to the axiom that we should avoid fluxes that are too active for an application, we should categorically avoid fluxes that contain certain ingredients that are harmful to PCBs and thus should not be introduced to them. Chief among these ingredients is Zinc Chloride, a common ingredient in fluxes used for soldering copper pipes, roofing joints, stained glass, stainless steel, and other more industrial applications. If you’ve got a flux on your shelf that contains Zinc Chloride, don’t use it to solder electronics or PCBs. Zinc Chloride fluxes will do a bang-up job when it comes to soldering your electronics, but its residues can damage the integrity of PCBs or electronic components, and cleaning off the flux residues afterward is usually not enough to ward off this damage. Fortunately, due to safety regulations, product labels of fluxes that do contain Zinc Chloride should clearly announce the presence of this chemical in the flux. In general, we should carefully read labels for our own safety, but in this case, also for the safety of the things we are soldering.

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Can you solder Aluminum?

Yes, you can solder Aluminum, but it depends on the type of Aluminum you’re working with and you need to understand in advance that it’s not as easy as soldering Copper, Brass, steel or even stainless steel. The reason it’s hard to solder is because of the tough Aluminum Oxide layer that forms over the surface of the Aluminum and that prevents solder from bonding to the surface.That an oxide layer would impede soldering is hardly unique. The reason we use fluxes in the first place during soldering is to deal with the oxide layer that forms on every metal surface we want to solder (at least when soldering in an environment where Oxygen is present, which is most everywhere). It’s just that the oxide layer that forms on Aluminum is particularly tenacious and particularly resistant to soldering. However, as we led off, you can definitely solder Aluminum. You just have to keep the following facts in mind.

First of all, it depends on the type of Aluminum you’re using. There are eight different types of Aluminum or “series” as they are known. These Aluminum series are designated by a four-digit numbering system that begins in 1000 and ends in 8000, such that there are 1000-series, 2000-series, 3000-series, etc. ending in 8000-series. (The series may also be designated as 1xxx, 2xxx, etc.). Not all Aluminum series can be soldered, and even those that can be soldered, can be done so with various degrees of ease. For example, the 1000-series and 3000-series of Aluminum can be soldered with relative ease (with an emphasis on the word, “relative”). The popular 6000-series can also be soldered, but not as easily as the 1000-series and 3000-series. The rest of the Aluminum series, such as 2000-series, 4000-series, 5000-series, 7000-series, and 8000-series cannot typically be soldered. Fortunately, the 1000-series, 3000-series, and 6000-series Aluminum are popularly used types of Aluminum.

Once you’ve picked an Aluminum series that can be soldered, the next step is to find a soldering flux that is specifically formulated for Aluminum. You might think that an aggressive flux that is formulated to solder stainless steel should be strong enough to solder Aluminum, but it doesn’t work that way. You need a flux that is specifically formulated to deal with Aluminum Oxides. If the flux label does not specify that the flux is formulated for soldering Aluminum, then it probably is not. Superior Flux offers a full range of Aluminum Soldering Flux. Now that you have the right flux, you need to choose the right solder. Tin-Zinc solders are often thought of as good choices for soldering Aluminum; a common Tin-Zinc solder chosen for soldering Aluminum is the Sn91/Zn9 (91% Tin & 9% Zinc). Another good choice would be a Tin-Silver solder such as Sn96.5/Ag3.5 (96.5% Tin & 3.5% Silver). The presence of Silver helps in Aluminum soldering.

When soldering Aluminum, it ‘s not enough to have the right flux and right solder. Temperature plays a big role in soldering Aluminum. When you’re soldering an easy-to-solder alloy like Copper, you generally aim to heat the joint to the point that the solder turns molten. However, when Aluminum soldering it’s not enough to melt the solder; you actually need to heat up the surface of the Aluminum being soldering to an optimal soldering temperature, which depending on the flux, can range from 220 – 380°C (or 430 – 715°F). You not only need to achieve the recommended temperature, but you have to do without a certain period of time, otherwise, the flux will “burn out” and will no longer be effective. Typically, if the area on the Aluminum being soldered does not achieve optimal temperature within eight minutes, the flux will burn out and it’s game over.

Aluminum soldering fluxes are active; they have to enable soldering of Aluminum. This means a couple of things. For starters, while safety considerations are always a concern with even the mildest of fluxes, these considerations are even more serious when working with Aluminum soldering fluxes. Protect your skin, your eyes, and your lungs. In addition to wearing proper personal protective equipment (PPE), it is important to have good ventilation since Aluminum soldering fluxes give off strong fumes during soldering. Secondly, since Aluminum soldering fluxes are very active, you need to take special care in removing flux residues after soldering; otherwise, these flux residues can cause damage to whatever surfaces they are left on. Fortunately, most Aluminum solder flux residues are water-soluble, meaning they can be washed off after soldering with water.

Often people ask if you can solder Aluminum to other alloys, such as Copper or steel, and the answer to that one is yes but as with everything else with Aluminum, it’s conditional. On one hand it’s typically more difficult to solder Aluminum than whatever other alloy you’re soldering it to, so in that regard, once you’ve soldered to the Aluminum, soldering to the other alloy should be the easy part. That being said, when soldering two different alloys or metals together, you have to be mindful of the different ways that each metal responds to being soldered. For example, the amount of heat it takes to solder Aluminum is substantially higher than the amount of heat needed to solder Copper. As a result of this heat differential, you can end up providing more heat than is necessary for soldering the Copper, and this could lead to problems. This segues nicely into our final point pertaining to Aluminum soldering, and that is that you need to temper your expectations when soldering Aluminum, especially if you’re accustomed to soldering metals that are easier to solder, such as Copper. Soldering Aluminum requires greater attention to all the key inputs of soldering, including soldering alloy, flux, and temperature. However, if you’re working with the right series of Aluminum and carefully choose your inputs and manage your soldering parameters, you can achieve impressive Aluminum soldering results.

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What is the difference between white silver brazing flux and black silver brazing flux?

Let’s start with how these flux types are alike. Both the so-called white silver brazing flux and black silver brazing flux are formulated, as their names suggest, to braze silver-containing braze alloys. (The term “braze alloy” refers to the alloy you heat up to molten form in order to join two pieces of metal; it may also be called a filler metal or filler alloy.) The two fluxes are almost identical except the black brazing flux is fortified with a chemical called Elemental Boron, with the description “Boron-modified” sometimes used to describe the black brazing flux. Actually, if we’re being accurate, then the Boron-modified flux would really be called “brown silver brazing flux,” since the addition of elemental Boron causes the flux to turn brown and not black in color, but the name black brazing flux has stuck. The addition of Boron to brazing flux gives the black brazing flux several key advantages. For starters, it gives the flux the ability to withstand higher brazing temperatures as well as the ability to withstand prolonged heating cycles. This enhanced temperature tolerance is particularly important when brazing larger pieces or when brazing more difficult-to-braze alloys, both which require higher temperatures during brazing and heating that is sustained over a greater period of time. Boron also increases the oxide removing ability of the flux, thus promoting better wetting or flow during brazing. Boron-modified brazing fluxes are particularly suited for brazing certain specialized metals, such as stainless steel, carbide steel, and alloys containing Nickel. The white and black silver brazing fluxes are available in both paste and powder form, with the paste form being more common in many markets. Additionally, the white and black silver brazing fluxes are often differentiated through designations set by various industry standards. For example, the Aerospace Material Specification (AMS) designates white silver brazing flux as AMS 3410 and the black silver brazing flux as AMS 3411. The American Welding Society (AWS) designates white silver brazing flux as FB3-A and the black silver brazing flux as FB3-C. To learn more about fluxes and their various specifications, go to Find a Flux by Specification.

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