Basic Understanding of Solder and Soldering Applications

Flashlight Modding and DIY Parts
1

Excellent review on the very basics and terminology…

Basic Understanding of Solder and Soldering Applications

Description

Soldering is a well known and widely used process where two or more metal items are joined together using a fusible alloy with a melting temperature that is lower than their own. The most commonly used solder is a fusible alloy consisting essentially of a tin and lead mixture. It is the solvent action (the solder actually dissolves a small amount of the metals surface, at a temperature that is well below its melting point and joins with it) of the solder alloy that causes it to fuse with and attach to the surface of the metal items being joined. The solvent action that takes place, between the solder and the metal, makes the joint chemical (not just physical) in nature and causes the properties of the joint to differ from the original solders properties and from those of the surface of the metal items being joined. When metal parts are joined by solder, a metallic continuity is established as a result of the interfaces where the solder is bonded to the metallic surfaces.

Basic Differences

The metal joining process that is generally referred to as soldering (or soft soldering) requires temperatures between 361°F to 842°F. The joining of metals at temperatures above 842°F (and below the melting point of the metals being joined) is more commonly referred to as brazing (or hard soldering). The actual melting and fusing of the metal items that are being joined together is considered welding. There are, of coarse overlapping situations that may occur when classifying a process. The actual joining characteristics that take place are physically different in each of these processes. Soft solders attach to metals by what is referred to as a solvent action that takes place at relatively low temperatures. Hard solders, or brazing alloys contain metals that require higher temperatures to cause the solvent action to take place and fuse the alloy with the metal being joined. Because welding involves actually melting and fusing the surface of the metals that are being joined together, a filler, or fusible material is not always used.

Uses

Soldering is used primarily when the expected operating temperature of a joint will not exceed around 300oF and thermal or electrical continuity cannot be adequately achieved, or maintained, by the use of a mechanical joint. It is one of the most ideal methods available for the creation of a physical, electrical, or hermetically sealed bond between various metal items that are being joined together. Soldering is quite often used, in addition to other mechanical methods (twisting, crimping, etc.) to improve electrical continuity, to help protect the joint from the effects of vibration, or to encapsulate the joined metals preventing oxidation. Although soldering may be used to provide some minor support to an assembly, the solder should not (excluding sheet metal applications) be used as the primary mechanical support of a finished joint.

Ingredients and Methods

The soldering process may be accomplished in a wide variety of ways, but the four primary ingredients required will remain the same:

a base metal (or metal items being joined)

a type of flux (or a method of cleaning and maintaining the surface to be soldered)

solder

a source of heat

It is important to match the soldering method and the equipment that will be used, to the soldering application that is being considered.

The base metal is the metal that is in contact with the solder and forms an intermediate alloy. There are many metals that will react willingly with solders to form a strong chemical and physical bond, while others can be very difficult, or even impossible to solder.

Flux is used to eliminate minor surface oxidation and to prevent further oxidation of the base metals surface during the heating process. Although there are many types of flux, each will include two basic parts, chemicals and solvents. The chemical includes the active portion, while the solvent is actually the carrying agent. It is the solvent that determines the cleaning method required to remove the remaining residue after soldering.

Solder is the alloy used to create the solvent action, which generates the bond between the base metals. The type and form of the solder is very important and must be determined by the individual application being performed, as well as the base metals and soldering method being employed.

There are several methods, as well as a wide variety of tools available to perform the task of soldering. Some of the current methods that are available include induction, conduction, ultrasonic, flame, dipping, resistance, oven and wave soldering. Some of these methods involve the use of small inexpensive hand tools, while others may require large and expensive machinery, equipment and tools. It is a good idea to become educated on the various methods and tools that are available, in order to insure that you are utilizing the best, safest, most efficient and economical means available for your specific soldering application.

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Creating a Quality Solder Joint: A Simple How-to-Guide
Soldering is the process that uses solder (a metal alloy usually consisting of tin mixed with other metals) for the metallurgical joining of metal components to form an electrical, mechanical or hermetically sealed bond at temperatures (less than 840oF) that are well below the melting temperature of the individual components that are being joined. The soldering equipment (used to create the required heat) and other materials (solder, fluxes, heat sinks, fixturing, etc.) should always be properly matched to the intended soldering application. The equipment and materials used may vary, but the basic soldering techniques that are required will usually remain the same.

One of the most important rules to remember about soldering is “keep it clean”. This includes, not only the items being soldered, but also the materials used. Choose quality solders and fluxes without unnecessary impurities. Surface oxidation, contaminants and other impurities are some of the most common reasons for poor quality solder joints. The use of fluxes does not necessarily eliminate the need for pre-cleaning the surfaces you are joining, especially if heavy oxidation or large amounts of grease, oil or dirt are present.

Clean: Thoroughly clean all surfaces to be joined, removing any dirt, grease, oil, oxidation, paint, coatings or other impurities that may exist before attempting to solder. Proper wetting can only occur when the intended solder joint area has been properly cleaned. Soldering should be performed as soon as possible after cleaning to eliminate the possibility of re oxidation or contamination of the items being soldered.
Flux: Apply flux sparingly to each of the intended joint surfaces (this step may not be required when using rosin or acid core solder, or when performing flux free applications like ultrasonic soldering). Flux is primarily used for the removal of light oxidation and to protect against re-oxidation during the actual soldering process. Make sure you have the right flux for the application being performed.
Heat: Apply heat directly to the intended joint area. The correct application of heat is important and should be consistent with the operating requirements determined by the type of equipment being used. Fast and accurate heating will minimize the risk of thermal damage to heat sensitive components. Heat sinks are sometimes used as an added precaution against thermal damage to heat sensitive components that need soldered.
Solder: Add solder to the heated surfaces you are joining (do not apply solder directly to the tip, or other heat source being used). The solder should flow uniformly over all of the surfaces that are being connected. Stop feeding solder as soon as you have applied an adequate amount and then remove the heat source. The amount of solder is important because too much will create unnecessary waste and can cause bridging on a PCB, while too little can affect the mechanical strength and conductivity of the finished solder joint.
Cool: Allow the finished solder joint to remain undisturbed until it has completely cooled. You should never attempt to speed up the cooling process by blowing on the solder joint. Even minor vibrations or disturbances during cooling, can cause micro fractures or other types of damage that may severely weaken the solder joint.
Inspect: Check all finished joints for proper wetting, the right amount of solder, a good physical appearance, the required mechanical strength and the necessary current carrying capacity.
A quality solder joint is not achieved solely by the equipment and techniques being used, but also by the operator being trained to use them properly. An operator should know how the physical appearance of a finished solder joint helps to determine possible flaws that may exist. A quality solder joint appears bright, shiny and smooth with all components appearing well soldered. The surface of a finished connection should never look rough, grainy, dull, or flaky (these are signs of what is commonly referred to as a cold solder joint). Problems with proper wetting (solder balling up and not adhering to the components surface) are sometimes associated with too much heat, but are more often related to cleanliness issues.

It is important to know that quality soldering is rarely achieved by using inappropriate (or inadequate) tools, materials, or equipment.

3

Pretty good article. Keep it clean, best advice. Thanks for posting.

4

The heatsink absorbs all the heat and it is nearly imposible to solder a wore on it.
Any suggestions on that?
Useful post.

5

folks…Im not the author by no means. There’s a few more sections im adding and displays the source, American Beauty.
Im in no way condoning brand X…Just the free flow of good info that can help others…

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Flux: A General Overview
Flux is a key contributor to most soldering applications. It is a compound that is used to lift tarnish films from a metals surface, keep the surface clean during the soldering process, and aid in the wetting and spreading action of the solder. There are many different types and brands of flux available on the market; check with the manufacturer or reseller of your flux to ensure that it is appropriate for your application, taking into consideration both the solder being used and the two metals involved in the process. Although there are many types of flux available, each will include two basic parts, chemicals and solvents.

The chemical part includes the active portion, while the solvent is the carrying agent. The flux does not become a part of the soldered joint, but retains the captured oxides and lies inert on the joints finished surface until properly removed. It is usually the solvent that determines the cleaning method required to remove the remaining residue after the soldering is completed. It should be noted that while flux is used to remove the tarnish film from a metals surface, it will not (and should not be expected to) remove paint, grease, varnish, dirt or other types of inert matter. A thorough cleaning of the metals surface is necessary to remove these types of contaminates. This will greatly improve the fluxing efficiency and also aid in the soldering methods and techniques being used.

Detailed Examination

All common untreated metals and metal alloys (including solders) are subject to an environmental attack in which their bare surfaces become covered with a non-metallic film, commonly referred to as tarnish. This tarnish layer consists of oxides, sulfides, carbonates, or other corrosion products and is an effective insulating barrier that will prevent any direct contact with the clean metal surface which lies beneath. When metal parts are joined together by soldering, a metallic continuity is established as a result of the interface between the solder and the surfaces of the two metals. As long as the tarnish layer remains, the solder and metal interface cannot take place, because without being able to make direct contact it is impossible to effectively wet the metals surface with solder.

The surface tarnishes that form on metal are generally not soluble in (and cannot be removed by) most conventional cleaning solvents. They must, therefore be reacted upon chemically in order to be removed. This required chemical reaction is most often accomplished by the use of soldering fluxes. These soldering fluxes will displace the atmospheric gas layer on the metals surface and upon heating will chemically react to remove the tarnish layer from the fluxed metals and maintain the clean metal surface throughout the soldering process.

The chemical reaction that is required will usually be one of two basic types. It can be a reaction where the tarnish and flux combine forming a third compound that is soluble in either the flux or its carrier. An example of this type of reaction takes place between water-white rosin and copper oxides. Water-white rosin, when used as a flux is usually in an isopropyl alcohol carrier and consists mainly of abietic acid and other isomeric diterpene acids that are soluble in several organic solvents. When applied to an oxidized copper surface and heated, the copper oxides will combine with the abietic acid forming a copper abiet (which mixes easily with the unreacted rosin) leaving a clean metallic surface for solder wetting. The hot molten solder displaces the rosin flux and the copper abiet, which can then be removed by conventional cleaning methods.

Another type of reaction is one that causes the tarnish film, or oxidized layer to return to its original metallic state restoring the metals clean surface. An example of this type of reaction takes place when soldering under a blanket of heated hydrogen. At elevated temperatures (the temperature that is required for the intended reaction to take place is unique to each type of base metal) the hydrogen removes the oxides from the surface, forming water and restoring the metallic surface, which the solder will then wet. There are several other variations and combinations that are based on these two types of reactions.

Once the desired chemical reaction has taken place (lifting or dissolving the tarnish layer) the fluxing agent must provide a protective coating on the cleaned metal surface until it is displaced by the molten solder. This is due to the elevated temperatures required for soldering causing the increased likelihood that the metal’s surface may rapidly re-oxidize if not properly coated. Any compound that can be used to create one of the required types of chemical reactions, under the operating conditions necessary for soldering, might be considered for use as a fluxing material. However most organic and inorganic compounds will not hold up under the high temperature conditions that are required for proper soldering. That is why one of the more important considerations is a compounds thermal stability, or its ability to withstand the high temperatures that are required for soldering without burning, breaking down, or evaporating.

When evaluating all of the requirements necessary for a compound to be considered as a fluxing agent, it is important to consider the various soldering methods, techniques and processes available and the wide range of materials and temperatures they may require. A certain flux may perform well on a specific surface using one method of soldering and yet not be at all suitable for that same surface using a different soldering method. When in doubt it never hurts to check with the flux, or solder manufacturer for recommendations.