Electronics Assembly: Soldering and Wire Connection Techniques, Exercises of Fundamentals of Design

Download Electronics Assembly: Soldering and Wire Connection Techniques and more Exercises Fundamentals of Design in PDF only on Docsity! 1 Electronics Assembly Electronic devices for the purpose of this document are components or collections of components that require and/or modify electricity to perform their function. Motors and solenoids turn electric power into mechanical motion. Electronic components like resistors, capacitors and inductors modify the voltage or the current of electricity as it passes through them to enable sensors, control systems and other devices to function. But in order to combines components into devices and devices into more complex systems (like robots), the components and devices must share both a mechanical connection and an electrical connection. The mechanical connection is necessary to keep components in place and in contact. This connection should be strong and prevent the device or system from damage in case of accidental bumping, shaking, tipping, or dropping. The electrical connection is necessary to produce a path of very low resistance, so the electricity may pass through all of the components unimpeded and with tolerable losses. It is sometimes possible to create the mechanical and electrical connection using the same mechanism, but the mechanical strength of the bond is often very low in these cases and the devices could be very fragile. Whenever possible, use a separate method to create the two connections to increase strength and robustness of the design. There are a number of methods that have been developed to allow the assembly of electronic components including soldering, crimping, and off-the-shelf electrical quick connect devices. 1.1 Wire Although many electronic components are designed to allow them to interface directly with other components, many still require wire to carry the electricity between devices. 1.1.1 Wire Electrical wire is just flexible thread or slender rod made from a metal or metal alloy with very low electrical resistivity (usually copper). docsity.com 2 1.1.1.1 Stranded Vs. Solid Core Wire made up of many strands of thread twisted around each other is called "stranded" wire, whereas wire made up of a solid rod of metal is referred to as "solid core" wire. For most applications, it doesn't matter which type of wire you choose. Stranded wire is more flexible and much more resistant to fatigue than solid core but the ends tend to fray if not taped or soldered together. Stranded wire should be used whenever the wire will be moved around a lot and especially when the components will be subjected to vibration or cyclical loading. Solid core will have a slightly lower resistivity over long distances and does not fray so it can be more convenient to use for bread board work but can be more expensive than stranded wire. 1.1.1.2 Wire Gauge The size of wire is specified based on "gauge" as opposed to diameter. In general, the larger the gauge, the smaller the wire diameter. To calculate the exact diameter for a given gauge of wire, use the following equation: 0.3248*exp(0.1159*gauge)=wire diameter(inch) This formula works for any gauge from (0 to 40) or larger. For 00 gauge enter -1, for 000 enter -2 and 0000 -3. Smaller gauge (larger diameter) wires have less resistance per unit length than smaller gauge wires. Since they dissipate less energy, they generate less heat per unit length and can carry more power. However, they are larger, heavier, less flexible and more expensive than larger gauge (smaller diameter) wires. For electronics applications (as opposed to power lines over miles) it is best to use the smallest wire that can carry the electrical load you require. docsity.com 5 1.1.3 Hazards and Safety Information There are relatively few hazards associated with cutting and stripping wires. It is possible to accidentally cut yourself with the wire cutters or on sharp portions of exposed wire, to bang you hand while pulling off wire insulation with wire strippers, and to hit yourself or others (especially in the eye or face) with flying bits of wire or insulation. As a result, eye protection (i.e. safety glasses or goggles) is required when working with wires. 1.2 Soldering Soldering is a manufacturing process by which two metal surfaces are joined by applying a third molten metal at the interface. (The metal of the two original surfaces does not melt.) Soldering is similar to brazing but is done at much lower temperatures. Although the term soldering can be applied to other applications, including joining and sealing pipes, for this application we will only discuss soldering for electrical components. 1.2.1 Soldering Tools 1.2.1.1 Solder Solder is a metal alloy that has a low melting point and good electric conductivity. It often contains a combination of one or more of the following metals: lead, silver, tin and antimony. It also often contains trace amounts of other metals. The melting point of solder can range from 180 C to 450 C depending on the alloy used. The electrical resistivity is usually on the order of 1e-5 ohms - cm. (In comparison, the electrical resistivity of copper is on the order of 2e-6 ohms - cm and the electrical resistivity of plastic is on the order of 1e15 ohms - cm.) The tensile strength of solder is generally on the order of 20 - 60 MPa, which is comparable to the strength of plastics and an order of magnitude lower than the strength of other metals like steel. However, there are high strength solders that are up to 100x stronger for special applications. Lead free solders are available for use and contain tin, copper, silver and other metals in varying amounts. Lead free solders have higher melting points than their counterparts and may produce mechanically weaker solder joints depending. docsity.com 6 1.2.1.2 Flux Soldering flux is a gooey substance that contains an acid that cleans the surfaces to be soldered by dissolving oxides that have formed on those surfaces. This allows the molten solder to better wet the surfaces and create a stronger solder joint. Flux also dissolves oxide off of the liquid solder, making it less crusty and more shiny. Since the flux cannot remove significant corrosion, you should pre-clean any badly oxidized surfaces with an abrasive pad or clean steel wool before applying the flux. For convenience, solder is often manufactured as a hollow tube and filled with flux but it can also be purchased separately. There are three main types of flux: rosin flux, acid flux and water soluble flux. Rosin based fluxes contain organic acids in a rosin matrix. Additional compounds which decompose at high temperature to form additional acids (like ammonia or hydrochloric acid) may also be added. The acids are strong enough to remove oxides from metals like copper, tin, lead and silver but not from steel, iron, chrome or aluminum. Rosin based fluxes leave behind small amounts of plastic-like solids after use which are not very corrosive and can be removed with solvents. Acid fluxes contain a stronger acid than the acid present in rosin fluxes so it is able to remove oxides from more difficult metals like steel, however the residue is also corrosive so it should be removed immediately and should not be used on electronic components. Water soluble flux is a type of flux whose residue is water soluble. docsity.com 7 1.2.1.3 Soldering Irons A soldering iron is basically a heating element contained inside of an insulated handle connected to a hot metal tip. The tip is used to heat the solder and the surfaces to be joined. The tips are interchangeable, allowing different shaped tips to be used for different applications and damaged tips to be replaced. Soldering irons are typically electrically powered, but they can also be cordless or butane powered. Once plugged in or turned on, a soldering iron is always hot. Soldering irons often come with a stand to hold the hot iron. This prevents the iron from rolling around or burning the work surface. The stand may have a sponge for cleaning the tip. Some soldering irons for professional use come as part of a soldering station, which allows the temperature of the tip to be adjusted. A cheap soldering iron with a low quality tip won't solder very well for very long. Consider investing in a good quality iron or soldering station. docsity.com 10 • Strip both wires more than 0.5". • Cross them in an "x" shape. • Wrap them around each other. Wrap the top wire down and the bottom wire up, and continue wrapping until the splice is complete. Two methods for wrapping are shown below. • Solder the wrapped wires being careful not to melt too much of the insulation and not to burn your fingers. • When they are cool, wrap with electrical tape, shrink wrap, or other form of insulation. Exercise 3: Soldering a Wire to Copper Clad Soldering a wire to a piece of copper clad plate is one of the most difficult and least common soldering operations that you will perform. This is because the copper clad plate will conduct the heat from the iron away from the solder joint very quickly and it is difficult to get the solder to flow. If you can perform this soldering operation, you should be able to solder anything together. To solder a wire to a piece of copper clad: • Cut a small piece copper clad plate. • Strip the wire (stranded will work best). • Tin the tip of the iron. • Place the iron tip on top of the stripped wire and press hard, spreading out the strands a bit if necessary. • Feed the solder into the wire / iron interface and wait for the solder to flow. • If the solder will not flow after a few moment, clean and re-tin the iron and repeat. The solder on the tip of the iron will oxidize after a few moments and will not adhere properly to the copper after this occurs. You should not be able to pull the wire off of the copper clad plate once the solder joint cools. docsity.com 11 1.2.3 Hazards and Safety Information Solder and the surface it touches is very hot and can remain very hot for a few moments after a solder joint is complete. Touching the solder, the iron or the heated surfaces can cause serious burns. When in doubt, use a tool (like a pair of pliers) instead of your fingers to manipulate recently heated surfaces. Solder contains lead which is hazardous to your health and can cause problems including nerve, muscle, brain and kidney damage. During and after soldering, keep your hands away from food and out of your mouth until after you've washed them thoroughly. The fumes from the solder flux can also be very hazardous to your health. Try to minimize exposure to fumes during soldering by soldering in a well ventilated and minimizing time spent soldering. 1.3 Desoldering Occasionally, soldered joints must be taken apart. This may be necessary to replace a damaged component, to repair a bad solder joint or for another reason. Before attempting to desolder a joint, determine what components are critical to the circuit and what components can be sacrificed. For example, wire can often be removed and replaced with another piece of wire, but the lead on a motor cannot be replaced and must be protected at all costs. After determining which components must be protected, use a pair of wire cutters to remove excess solder and wire from the system. You may then use either a desoldering pump or desoldering braid to remove the remained of the excess solder from the important surfaces. docsity.com 12 1.3.1 Desoldering Pump A desoldering pump is to used when large amounts of solder are present in the electronic system. Desoldering pumps are hollow cylinders with a plastic nozzle at one end, a button on the side, and a spring-loaded piston at the other end. When the piston is depressed fully, it clicks into place and creates a vacuum within the central chamber. When the button is pressed, the piston is released, sucking any material near the nozzle into the chamber. To use the desoldering pump: • Depress the piston until it clicks into place. • Use a soldering iron to heat the solder on the joint until it melts. • Quickly remove the soldering iron and place the nozzle of the desoldering pump on or near the solder and press the button. The liquid solder should be sucked into the central chamber where it will cool and fall out again. Do this step very quickly, or the solder may solidify again before the pump can be put into place. • The pump removes the bulk of the solder, but a thin coating will remain. • Be careful not to melt the tip of the desoldering pump when it is near the sol- dering iron. While the desoldering pump removes most of the excess solder, wires and other electronic components often remain joined by a small amount of solder (such as wire still wrapped around a motor To remove wire from a motor lead: • Use a soldering iron the heat the joint until the solder is melted. • Using a pair of needle nose pliers, gently tug on the wire until it comes loose. You may need to tug on the wire while still in contact with the soldering iron or to reapply the soldering iron after the solder solidifies. docsity.com