Soldering is among the first things you learn while entering the world of electronics, yet one of those skills that very few conquer. V.Ramavallabhan, Management & Technical Consultant, JVR Consultants, speaks to Dilin Anand from EFY.
Q. How should an engineer go about selecting the right soldering method?
A. Select the soldering method best suited to your application by considering the pros and cons of each soldering method, as well as the heat resistance of the surface-mount device (SMD). Soldering methods are broadly divided into the partial heating method and total heating method. Heat is applied to the package leads and printed wiring boards (PWB) in a localised manner in the former, while the latter sees heat being applied to the entire package. This means that partial heating Partial heating involves less heat stress on the device and printed wiring board, but is unsuitable for large volume production. Therefore, this method is mainly used to correct soldering or for devices with a low heat resistance. The total heating method is excellent in terms of productivity and running cost, but it can place considerable heat stress on the semiconductor device and board.
Q. How do you select the right soldering tip and iron for a particular job?
A. This is something that needs to be practically validated according to the nature of the job and components used, but I can give you guidelines. You need to first figure out the job dimension, and then select a tip dimension that is about 0.5 to 5.0 mm larger with different type of shapes like for example ‘C’ Type (round chamfer shape) or ‘D’ Type (Flat Screw driver type). You also need to consider the proximity of other components that are in proximity to take this further. The below illustration would also help you understand this.
Q. What would be your advice to maintain these soldering tips?
A. The optimum temperature at which proper solder takes place to be set, so that over heating is avoided. When the tip is cleaned on the sponge, the sponge should have only moisture. This means, water should not drop from a sponge while squeezing it — you should only feel the moistness. Hence it should be moistened to that level periodically. Solder tips should not be rubbed or tried for sharpening with a hard surface nor should it be tapped on the table. Always keep the handle set with the tip in the stand when not in use, preferably with the tip covered in solder. Every day or every alternate day the tip should be removed from the element, followed by having the element cleaned with a dry soft cloth and once again fitted with the tip.
Q. Could you elaborate about what engineers need to know about thermal effects?
A. Controlling the soldering iron tip temperature is not the key element in soldering. The key element is in controlling the heat cycle of the work — how fast the work gets hot, how hot it gets, and how long it stays hot is the element to control for reliable solder connections. The engineers should also learn about Process Validation of Soldering process, with respect to the job dimension, tip dimension, solder wire dimension and temperature. Based on the validation results, and operator’s speed of doing soldering, the temperature should be set between the minimum to maximum range of validated temperature for a particular job.
Q. How can we reduce the occurrence of solder balls or soldering defects?
A. Never touch the hot solder tip on the solder wire, while soldering on the job. Keep the solder wire on one side of the job and the tip on the other side or on the different location on the job but as close as possible. This should be practiced and this will yield better result of avoiding solder balls.
What are the precautions to be taken while soldering sensitive components?
Some components, such as transistors, can be damaged by heat when soldering. It is wise to use a heat sink clipped to the lead between the joint and the component body, as shown in the picture. You can buy a special tool, but a standard crocodile clip works just as well and is cheaper!
Q. How can we ensure reliable solder connections?
A. Reliable solder connections can only be accomplished with truly cleaned surfaces of copper pad and the component leads. But generally the pads and the component leads may be contaminated due to the film of oxide layer formed over the surfaces. This will be the cause of poor soldering. Hence before performing a soldering process, one needs to ensure the cleanliness of the surfaces. For this purpose fluxes are used. Fluxes consist of natural or synthetic rosins as well as chemical additives called activators. It is the function of the flux to remove oxides and keep them removed during the soldering operation. This is accomplished by the flux action, which is very corrosive at solder melt temperatures and accounts for flux’s ability to rapidly remove metal oxides. In its unheated state, however, rosin flux is non-corrosive and non-conductive and thus will not affect the circuitry. It is the fluxing action of removing oxides and carrying them away, as well as preventing the reformation of new oxides that allows the solder to form the desired inter-metallic bond.
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