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There are a number of techniques for reflowing solder.
Each method has its advantages and disadvantages. The surface tension of the molten solder helps keep the components in place, and if the solder pad geometries are correctly designed, surface tension automatically aligns the components on their pads. The boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the component leads to the pads on the circuit board.
Another technology which is becoming popular again is special fluorocarbon liquids with high boiling points which use a method called vapor phase reflow. With infrared reflow, the board designer must lay the board out so that short components do not fall into the shadows of tall components. Different terms describe the components, technique, and machines used in manufacturing. Another is to use a hot gas convection.
These terms are listed in the following table: [ 3 ]. Heat to about °C at a slower temperature rise rate (°C/sec), then the gradient rises to about ° within S, and then quickly rises to about ° at °/sec, Z Then quickly cool down at no more than 4°C/sec. After pasting, the boards proceed to the pick-and-place machines , where they are placed on a conveyor belt. Solder paste , a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel or nickel stencil using a screen printing process.
One is to use infrared lamps; this is called infrared reflow. Alternatively, SMT and through-hole components can be soldered on the same side of a board without adhesive if the SMT parts are first reflow-soldered, then a selective solder mask is used to prevent the solder holding those parts in place from reflowing and the parts floating away during wave soldering. At the end of , convection soldering was the most popular reflow technology using either standard air or nitrogen gas.
If the circuit board is double-sided then this printing, placement, reflow process may be repeated using either solder paste or glue to hold the components in place. Much of the pioneering work in this technology was done by IBM. Components became much smaller and component placement on both sides of a board became far more common with surface mounting than through-hole mounting, allowing much higher circuit densities and smaller circuit boards and, in turn, machines or subassemblies containing the boards.
If a wave soldering process is used, then the parts must be glued to the board prior to processing to prevent them from floating off when the solder paste holding them in place is melted. This is the current best reflow oven temperature curve. It may have short pins or leads of various styles, flat contacts, a matrix of solder balls BGAs , or terminations on the body of the component. Water-soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water.
Surface-mount technology SMT , originally called planar mounting , [ 1 ] is a method in which the electrical components are mounted directly onto the surface of a printed circuit board PCB. In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality.
Some large integrated circuits are delivered in static-free trays. However, most electronic assemblies are made using a "No-Clean" process where the flux residues are designed to be left on the circuit board, since they are considered harmless.
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Component location is less restricted if the designer knows that vapor phase reflow or convection soldering will be used in production. Om smöret stelnar igen efter att det har smält, ordnar sig fettsyrorna i andra kristaller och smöret blir kornigt och får inte samma konsistens som. Rosin flux is removed with fluorocarbon solvents, high flash point hydrocarbon solvents, or low flash solvents e. Conversely, SMT does not lend itself well to manual or low-automation fabrication, which is more economical and faster for one-off prototyping and small-scale production; this is one reason why many through-hole components are still manufactured.
Often the surface tension of the solder is enough to hold the parts to the board; in rare cases parts on the bottom or "second" side of the board may be secured with adhesive to keep components from dropping off inside reflow ovens. An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all. They first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised to prevent thermal shock.
However, it is generally suggested to wash the assembly, even when a "No-Clean" process is used, when the application uses very high frequency clock signals in excess of 1 GHz. Smör och hårda margariner har högre smältpunkter, ungefär ˚C, och är fasta i kylskåpstemperatur. Following reflow soldering, certain irregular or heat-sensitive components may be installed and soldered by hand, or in large-scale automation, by focused infrared beam FIB or localized convection equipment.
Numerical control pick-and-place machines remove the parts from the tapes, tubes or trays and place them on the PCB. The boards are then conveyed into the reflow soldering oven.
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Värme gör att smöret får en flytande konsistens och fettkristallerna löses upp. After soldering, the boards may be washed to remove flux residues and any stray solder balls that could short out closely spaced component leads. Some SMDs can be soldered with a temperature-controlled manual soldering iron, but those that are very small or have too fine a lead pitch are often almost impossible to manually solder without expensive equipment.
It can also be applied by a jet-printing mechanism, similar to an inkjet printer. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly increasing production rates. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.
Where components are to be placed, the printed circuit board normally has flat, usually tin -lead, silver, or gold plated copper pads without holes, called solder pads. Gradual Heating Mode. Due to environmental concerns, this method was falling out of favor until lead-free legislation was introduced which requires tighter controls on soldering. This saves the cost of cleaning, speeds up the manufacturing process, and reduces waste.
Surface-mount technology was developed in the s.