19-03-2014, 04:25 PM
[b]Surface Mount Technology.[/b]
[attachment=60268]
ABSTRACT
Wave Soldering is a large-scale soldering process by which pin-through-hole components are soldered to an electronic assembly. The name is derived from the fact that the process uses a tank to hold a quanitity of molten solder; the components are placed on the board and the component side (back side) of the board is run across the surface of the pool of solder. The solder sticks to the areas of the board not protected with solder mask via surface tension effects, creating a reliable mechanical and electrical connection. The process is much faster and can create a higher quality job compared to manual soldering of components. As pin-through-hole components have been largely replaced by surface mount components, wave soldering has been supplanted by reflow soldering methods in large-scale electronics applications.
Wave Solder Process
There are many types of wave solder machines, however the basic components and principles of these machines are the same.The wave solder machine consists of three zones. They are the fluxing zone, the preheating zone and the soldering zone.
Fixtures
The printed circuit board with pin through hole components, and/or surface mount technology (SMT) components glued on the bottom side are placed on a fixture. The fixture is a fiberglass, or titanium frame with openings exposing the components to be soldered. The fixture is then placed on a conveyor which will carry the PCB through the machine. The conveyor consists of titanium fingers. Titanium is used because solder will not bond to this metal.
Fluxing
The PCB will then enter the fluxing zone. Two types of fluxers are used. They are the spray fluxer and foam fluxer. The spray fluxer consists of a robotic arm which travels from side to side while spraying a fine mist of no-clean flux onto the bottom side of the board. The second type of fluxer used is the foam fluxer. The foam fluxer consists of a tank of water soluble flux into which a plastic cylinder with tiny holes are immersed. The plastic cylinder is covered with a metal chimney. Air is passed through this cylinder which causes flux foam to rise up the chimney. As the PCB passes over the foam head flux is applied to the PCB
Preheating
The PCB will then enter the preheating zone. The preheating zone consists of convection heaters which blow hot air onto the PCB increase its temperature. For PCB which are thicker, or densely populated an upper preheater might be used. The upper preheater is usually an infra red preheater. Preheating is necessary to activate the flux. Preheating is also necessary to prevent thermal shock. Thermal shock occurs when a PCB is exposed to the 187 degrees Celsius temperature of the molten solder wave from the 25 degree ambient room temperature.
Reflow
is the process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. The reflow method is now used in SMT in preference to wave soldering. Typical exam questions
(1) What is Surface Mount Technology (2) Describe, with the aid of a sketch, how a component is soldered on a board in SMT. (3) Explain 5benefits and 5 limitations of Surface Mount Technology (4) Give 2 reasons why the reflow process is preferred to wave soldering.
[attachment=60268]
ABSTRACT
Wave Soldering is a large-scale soldering process by which pin-through-hole components are soldered to an electronic assembly. The name is derived from the fact that the process uses a tank to hold a quanitity of molten solder; the components are placed on the board and the component side (back side) of the board is run across the surface of the pool of solder. The solder sticks to the areas of the board not protected with solder mask via surface tension effects, creating a reliable mechanical and electrical connection. The process is much faster and can create a higher quality job compared to manual soldering of components. As pin-through-hole components have been largely replaced by surface mount components, wave soldering has been supplanted by reflow soldering methods in large-scale electronics applications.
Wave Solder Process
There are many types of wave solder machines, however the basic components and principles of these machines are the same.The wave solder machine consists of three zones. They are the fluxing zone, the preheating zone and the soldering zone.
Fixtures
The printed circuit board with pin through hole components, and/or surface mount technology (SMT) components glued on the bottom side are placed on a fixture. The fixture is a fiberglass, or titanium frame with openings exposing the components to be soldered. The fixture is then placed on a conveyor which will carry the PCB through the machine. The conveyor consists of titanium fingers. Titanium is used because solder will not bond to this metal.
Fluxing
The PCB will then enter the fluxing zone. Two types of fluxers are used. They are the spray fluxer and foam fluxer. The spray fluxer consists of a robotic arm which travels from side to side while spraying a fine mist of no-clean flux onto the bottom side of the board. The second type of fluxer used is the foam fluxer. The foam fluxer consists of a tank of water soluble flux into which a plastic cylinder with tiny holes are immersed. The plastic cylinder is covered with a metal chimney. Air is passed through this cylinder which causes flux foam to rise up the chimney. As the PCB passes over the foam head flux is applied to the PCB
Preheating
The PCB will then enter the preheating zone. The preheating zone consists of convection heaters which blow hot air onto the PCB increase its temperature. For PCB which are thicker, or densely populated an upper preheater might be used. The upper preheater is usually an infra red preheater. Preheating is necessary to activate the flux. Preheating is also necessary to prevent thermal shock. Thermal shock occurs when a PCB is exposed to the 187 degrees Celsius temperature of the molten solder wave from the 25 degree ambient room temperature.
Reflow
is the process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. The reflow method is now used in SMT in preference to wave soldering. Typical exam questions
(1) What is Surface Mount Technology (2) Describe, with the aid of a sketch, how a component is soldered on a board in SMT. (3) Explain 5benefits and 5 limitations of Surface Mount Technology (4) Give 2 reasons why the reflow process is preferred to wave soldering.