How to Solder

The key to good soldering is an understanding of what soldering actually is. Soldering relies on two hot, clean metallic surfaces being adhered together via an alloy that is formed on the surface of the metal. This minutely thin surface alloy adheres well to both the metallic surface and the solder As well as adherence it provides thermal and electrical conductivity between to two metallic surfaces. A well soldered joint is so strong that a fracture will only occur in the solder itself and rarely at the solder metal alloy interface. The process is dependant on a clean metallic surface and enough heat , hence the use of flux, the importance of cleanliness of the work and the correct tools.

A student usually has little choice in the soldering iron available but should ensure that at least the tip is appropriate for the work being done. Tips should not be pitted or heavily discoloured. The tip should have coating of solder (tinned) at all times, even when sitting idle. Never clean a tip with abrasive things. Wipe only on the damped sponge provided. Don't use the tip for burning into or melting substances like plastic (or flesh). The hot iron should only ever be in its holder or being used for soldering a connection.

Solder is a mixture of two metals that are not chemically combined. This mixture is chosen to create a melting point below that of either of the two metals. Many metals and mixtures are used as solder but for normal electronics work this mixture is 58% Lead (Pb) and 42% Tin (Sn). Normally called 60/40 resin cored solder it comes in a variety of gauges. The solder contains a core of resin "flux" which helps to remove the oxide layer from the metals being soldered. Often called "rosin" this useful material is actually refined resin collected from pine and fir trees and is the same stuff used on the strings of instruments like violins etc. and has many other uses. Fossilised resin becomes amber. (see Jurassic Park).

Like most heavy metals, the lead in solder is poisonous and you should never put it near your mouth. Wash your hands thoroughly with soap and hot water after soldering.

Commercially made PCB's have a solder coating on the component pads which makes for easy soldering. Students, however, rarely have the luxury of a commercially made board and often run into trouble attempting to solder heavily oxidised copper, especially months after the PCB has actually been made. To prevent this problem the board should be cleaned using a fine grit paper , steel wool or a scourer immediately after drilling. Rub the board in flowing water until all the drilling burs have been removed and the copper is bright and shiny. Wash clean in hot water and dry quickly. Do not touch the surface with your fingers. Lightly coat the board with a special spray-on clear enamel (lacquer). (N.B. Do this only in a fume cupboard or outside in the open air, read the instructions on the can) Allow to dry. This product is manufactured for PCB's and has the ability to protect the copper from oxidisation and helps with soldering by acting as a flux.

Component leads are often highly oxidized. They should be cleaned with fine grit paper or at least wiped with methylated spirits.

Any serious student of electronics will purchase high quality side cutters and needle nose pliers and never let them out of their sight. High quality tools are expensive but will last for decades if looked after. These tools are made for electronics work and should not be used for anything else. You can get good ones from most electronics hobby shops.

Components come in both through hole and surface mount. The technique used for hand soldering these components is different.

The soldered joint should look bright and have no granulation or greyness. This is a sign of a "cold" or "dry" joint which is caused by low temperature soldering or movement during cooling.

In industry S-M components are normally soldered by automated machines. Semi-manual machines are also available with component manipulators and a microscope. Students however do not normally have access to such expensive and sophisticated equipment.

Unfortunately some components often only come in surface mount packages or you need to use them for space considerations or for other technical reasons. Without the luxury of a pick and place machine the student is left with the often daunting task of hand soldering parts that are hard to see and seemingly impossible to place and solder.

For S-M you will need a pair of tweezers and some kind of magnifying apparatus like a "Maggy Lamp" , stereoscopic microscope or binocular glasses.

The solder used should be of a fine gauge, no bigger than 0.3mm. A fine gauge solder iron tip should also be used. Soldering speed is of the essence so these techniques should not be attempted until the through hole techniques have been perfected. The use of solder pastes and hot air gun soldering techniques are also useful and should be investigated. There are glues available to temporarily glue the component in place during soldering, although useful for certain operations their general use can be time consuming. By all means try this technique and use it if it suits you.

Sometimes it is desirable to clean flux and accumulated rubbish from a completed circuit board assembly. This should only be done when the circuit is working and the chance that it has to be reworked or modified is low. The whole board may be washed in a solvent such as methylated spirits. Acetone may be used but only by following relevant safety procedures and only inside a fume cupboard. You should also ensure that everything on the PCB assembly is tolerant to acetone. When in doubt use Methylated spirits.

Do not use petrol or other dangerous substances. Under no circumstances use industrial solvents like Trichloroethane which is used for cleaning PCB's in industry. This substance is carcinogenic and potentially deadly.

Using a toothbrush lightly scrub the board free of all flux and coating material. Wash thoroughly in solvent. Dry completely. Do not use water. Mask off all connector pins and socket pins with tape. Coat the board and all components in 2 or 3 light coatings of circuit board lacquer. Dry between coats. Place PCB on a horizontal surface. Leave lacquer to harden for a few hours.

The design of an "in-house" PCB should be in keeping with the method used in the circuit's construction. For student projects the following limitations and design guide lines must be kept in mind.

The following technical bulletin is from http://americanbeautytools.com/bulletins.php?cat=20

Topic: A general understanding of Fluxes and how they work.

Introduction:

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.