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. 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.
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. 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.
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. 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.
