Soldering and Re-working techniques

1. Soldering and Re-working techniques Eng. Mohammed Alsumady

2. What is Soldering? • Soldering is a process for joining metal parts by making use of any of the various fusible alloys (solder), whose melting temperature is lower than that of the material to be joined, and whereby the surface of the parts create an intermolecular bond, without becoming molten. • Soldering can be classified as soft soldering (soldering), which takes place at temperatures below 450 °C and hard soldering (brazing), which is done at temperatures above 450 °C. Hard soldering is commonly employed on such metals as silver, gold, steel and bronze where in it makes a much stronger joint than soft soldering, the shearing strength being 20 to 30 times higher. However, both thermal joining processes are generally termed as soldering, because in both cases, the molten metal (solder) is drawn into the capillary gap between two closely fitting clean solid metal surfaces.

3. A soldered connection ensures metal continuity. On the other hand, when two metals are joined to behave like a single solid metal by bolting, or physically attaching to each other, the connection could be discontinuous. Sometimes, if there is an insulating film of oxides on the surfaces of the metals, they may not be even in physical contact. The disadvantages of mechanical joints versus soldering are that oxidation will continually occur on the surface and will increase the electrical resistance. Moreover, vibration and other mechanical shocks may later make the joint loose. A soldered connection does away with both of these problems. There is no movement in the joint and no interfacing surfaces to oxidize, so that a continuous, conductive path can be maintained. Soldering is an alloying process between two metals. In its molten state, solder dissolves some of the metal with which it comes in contact. The metals to be soldered are more often than not covered with a thin film of oxide that the solder cannot dissolve. A flux is used to remove this oxide film from the area to be soldered.

4. The soldering process involves: • Melting of the flux which, in turn, removes the oxide film on the metal to be soldered; • Melting of the solder which makes the lighter flux and brings the impurities suspended in it to the surface; • Partial dissolution of some of the metal in the connection by the solder; and • Cooling and fusing of the solder with the metal. • often, for locating a problem in the functioning of the circuit, it is necessary to remove a component from the printed circuit board and carry out the requisite tests on it. The process of repair usually involves: • Disassembly of a particular component; • Testing of the component; • Replacement of the component found defective; • Testing the circuit for performance check.

5. The reliable operation and success of equipment in space, defense, medical electronics, traffic control systems, communication systems or monitoring and control systems all depend on proper soldered connections. • Under harsh and hostile environmental conditions like changing of temperature, humidity, vibration etc., even a single incorrect joint can cause the system to either fail completely or partially.

6. Theory of Soldering • Soldering is not a simple physical attachment of one metal to another, but chemically form an intermolecular bond. • The Wetting Action: an important variable in soldering process, which need to be controlled for proper soldering results. • the hot liquid solder dissolves and penetrates the metal surface to be soldered, it is referred to as “wets the metal” or “the metal is wetted”. • Wetting can only occur if the surface of the copper is free from contaminations and from any oxide film that forms when the metal is exposed to air. Also, the solder and work surface need to have reached the proper temperature.

7. Soldering Variables • The important variables of soldering are: temperature, time, tarnish-free surface, right flux and right solder. These variables are important to all the soldering techniques. • For achieving good results the golden rule is “Applying the right temperature to the solder as well as to the lands/terminations to be soldered for the correct time on a clean surface by using the right flux and proper solder will provide excellent joint looking bright and shiny”. • Tarnish-free Surface: The solder will wet the metal only when the metal to be soldered is free from any tarnish. Although the surfaces to be soldered may look clean, there is always a thin film of oxide covering it. • For a good solder bond, all dirt, grease and surface oxides must be removed before and with the help of flux during the soldering process.

8. Application of Right Flux and Proper Solder: • Fluxes should remove the tarnish from base metals and prevent them from reforming oxides while soldering. A highly active flux will remove oxides. • The selection of the flux depends on the soldering process chosen, the metal being soldered and on the cleanness of the metal.

9. The soldering process basically includes an understanding of: • Soldering material (solder and flux); • Soldering tools; and • Soldering procedure. • Solder: The soldering material or solder usually employed for the purpose of joining together two or more metals at temperatures below their melting point is a fusible alloy consisting essentially of lead (37 %) and tin (63 %). It may sometimes contain varying quantities of antimony, bismuth, silver or cadmium which are added to vary the physical properties of the alloy. • Flux: In order to aid the soldering process, a substance knows as ‘flux’ is used. Flux is needed to remove the microscopic film of oxides on the surfaces of metals to be soldered and it forms a protective film that prevents re-oxidation while the connection is heated to the point at which the solder melts. Flux is helpful on a stubborn joint that would not accept solder. Most metals tend to form compounds with atmospheric oxygen which leaves a coating of oxide even at room temperature. The oxides are removed by fluxes which remain liquid at soldering temperature, react chemically with the oxides and disperse the reaction products. Fluxes are applied before or during soldering. Thus a good solder flux must simultaneously perform a number of important functions such as promoting thermal transfer to the area of the solder joint, enhancing wetting of the solder on the base metal and preventing oxidation of the metal surface at soldering temperatures

10. Soldering Tools • Soldering Iron: A soldering iron is the basic tool for hand soldering. It generates the heat required to heat the surfaces to be soldered and to melt the solder. • Soldering irons used for soldering electronic components consist of the following three main parts: (i) a handle, (ii) a heating element, and (iii) a bit/tip. • Handle: This is made of a good electrical and thermal insulator having an ergonomic shape so that it is comfortable for the operator. • Heating Element: This must have sufficient thermal capacity so that the set and working temperatures are the same. The heating element responds immediately to heat loss at tip while soldering and returns lost heat to the tip. The heating element must be properly insulated so that there is no electrical leakage appearing on the tip of the bit to cause damage to the components. • Bit/Tip: This is made of copper to provide good heat transfer. It is plated to prevent the solder dissolving them. Iron on the other hand is not attacked by solder, so iron plated copper bits are normally used. Unfortunately, iron is not readily wetted by solder, so the bit is further covered by nickel or chromium in order to provide a hard outer surface that will wet properly by the solder. With the passage of time and usage, the outer plated coat (nickel or chrome) will dissolve away. The bit then must be replaced. Since a bit is the tip of the hand soldering iron, it is often called ‘tip’.

11. Traditional soldering tips which are made of copper, conduct heat well and are inexpensive. However, they have the disadvantage that the tip oxidizes heavily when heated. • New soldering iron tips have been designed which are galvanically plated with an iron coating and is then shielded against oxidation and corrosion by a layer of chrome. The heating element of the soldering iron is protected against over-heating and premature wear due to quick heat transfer. • The selection of a soldering iron is made with regard to its tip size, shape, operating voltage and wattage. Soldering iron temperature is selected and controlled according to the work to be performed. The temperature is normally controlled through the use of a variable power supply and occasionally by tip selection.

12. Soldering irons are available in various forms. • Soldering Pencils: Soldering pencils are lightweight soldering tools which can generate as little as 12 watts or as much as 50 watts of heat. A 25 watt unit is well suited for light duty work such as soldering on printed circuit boards. Modular soldering irons use interchangeable heating elements and tips which mate to a main pencil body. Such elements screw into a threaded receptacle at the end of the pencil. A variety of tips are available to handle most soldering tasks. Very fine, almost needle-like tips are used on printed circuit boards with IC component foil pads which are closely spaced.

13. A pencil type soldering iron takes a few minutes to attain working temperature and it is better to keep it continuously powered even for interrupted type of soldering work. This would need to keep the iron secured in a safe place at working temperature. One method is to keep it in special soldering iron holder which may be a coiled steel form into which the hot soldering iron can be inserted. Most stands of this type also include a sponge which is kept moistened and used periodically to clean the soldering tip.

14. Soldering Gun: A gun is usually heavier and generates more heat than the average pencil. Soldering of heavy duty conductors or connectors requires the use of a gun because it can generate enough heat to quickly bring a heavy metal joint up to the proper soldering temperature. These soldering tools are called guns simply because they resemble pistols. The gun’s trigger is actually a switch that controls application of ac power to the heating element. The working temperature is reached instantaneously. Some guns provide for selection of different heat levels through multi-position trigger switch.

15. Soldering Stations: Soldering stations contain an iron and a control console that offers switch selectable temperatures, marked low, medium and high. Obviously, this is more convenient than waiting for a modular pencils’ heating element to cool, unscrewing it from the holder and then replacing it with another heater tip combination. The tip temperature is controlled by using a heat sensor and closed-loop feedback control to gate power to heating element. Obviously, soldering stations are expensive compared to basic soldering pencils.

16. Battery-operated Irons: Sometimes, it is inconvenient to depend on the mains power supply for operating a soldering iron. Battery-operated soldering irons are available which depend upon rechargeable batteries as a power source. Recharging is done automatically when the iron is placed in its charger, which is built on the stand, and is connected to an ac power source. In these soldering irons, the tips attain working temperature in 5-8 seconds and cool off to ambient temperature in about one second. Typically, about 125 connections can be made on one charge. For a standard iron, a typical charging interval of approximately fourteen hours is required to return the cells to full strength.

17. Heat Sinks • Some components such as semiconductor devices, and meter movements are highly heat-sensitive. They must be protected from damage due to heat while soldering. Devices such as a set of alligator clips, nosepliers, felt-tipped tweezers, and similar such devices are usually placed or clamped at the site of soldering so that they prevent the heat from reaching the components.

18. Hand Soldering • Even though mass soldering techniques have become popular due to economic reasons, hand soldering still has a great relevance. In small and medium scale manufacturing facilities, hand soldering is still practiced. Also when joints are faulty and require re-work. • Good soldering skill is essential for operators undertaking manual soldering as well as for field service technicians who undertake repairs at customer sites. • Hand Soldering Requirements: The most important requirement for hand soldering, besides a skilled trained operator working with a proper Electro Static Discharge (ESD)controlled, clean workstation, is a good quality and temperature-controlled soldering iron with a suitable, clean and fine bit which is properly earthed. • Making good soldered joints is a skilled task, which requires proper tools, a high level of cleanliness and an ESD-controlled workplace. • Quality solder joints can only be achieved under clean conditions. Boards, component leads, and soldering equipment must be carefully cleaned both before and after the soldering operation.

19. Also, two main parameters must be controlled to achieve repeatable quality joints : • Temperature: The metal to be joined and solder need a temperature range from 215 °C to 250 °C. The setting temperature depends upon the equipment used as well as of the shape of the tip and is approximately 50 °C higher. • Time of heat application: There is a minimum and maximum time limit, ranging between 2–4 seconds. • During the soldering operation, tin and copper forms an intermolecular bond. The thickness of this intermolecular bond should be at least 0.5 mm and should not extend 1mm. The temperature/time applied should not be excessive, otherwise if the thickness of molecular bond is more than 1 mm the mechanical strength of joint deteriorates. Above 300 °C, there is a rapid deterioration in mechanical strength. Furthermore, all components being soldered have a maximum temperature/time profile, and if an excessively high soldering temperature is applied for a lesser temperature and applied for too long time, the component is damaged.

20. End of Chapter 4