Mehmet Özer, 030070050, 10th Week

1. Twin Sheet Forming (Manufacturing method)

There is no previous entry about Twin Sheet Forming

New Answer

Dual sheet forming (twin sheet forming) is a method for producing hollow objects from two plastic sheets. Fig. 4D13 illustrates one method for achieving such a result. Two sheets are fed to the machine together, slightly spaced apart. Both are clamped and heated and moved between two halves of a mold. The process requires longer heating time than that required for a single sheet. This is sometimes compensated for by using a rotary table system that includes two heating stations so that each sheet goes through two heating cycles before it is formed. After the sheets are placed for forming, an inflation pin enters the space between them and the mold closes. Air pressure is introduced between the sheets and vacuums are drawn from the two opposing mold cavities. Pressure on one side and vacuum on the other causes the sheets to press against the walls of the two mold cavities. The pressure of the mold closure also bonds the softened plastic sheets together. The formed sheets and the joint between them cool and harden. The inflation pin is withdrawn, the mold opens, and a hollow part is ejected or removed.

Sometimes, the bottom sheet is formed first and an insert is placed on it before the other half of the part is formed and assembled. Another variation of the process introduces urethane foam instead of air pressure between the sheets. The foam adheres to both sheets, to make a strong sandwich construction. Foam-filled boat hulls are produced with this method.

Dual sheet forming is used extensively in Europe to produce such items as phone booth roofs and gaming table tops from PVC or ABS sheet. Shipping pallets and other dunnage products are produced from high-density polyethylene heated to above its melt temperature.

(Bralla, J. G., Handbook Manufacturing Processes, How Products, Components and Materials Are Made, pp. 171-172)

2. Continuous Strip Molding (Manufacturing method)

There is no previous entry about Twin Sheet Forming

New Answer

Reel-to-reel molding (continuous strip molding) is an automatic insert molding process. Prior to the molding operation, the insert is blanked and formed from metal strip but is not severed from the strip, which is wound on a reel. The strip is then fed from the reel into the injection molding machine, which molds plastic material around the insert. The metal strip is precisely indexed in the injection molding machine. The individual molded parts are not separated from the strip during or immediately after molding. Instead, the strip of molded parts is wound onto a take-up reel. The major use of the process is for small electronic components which are not separated from the strip until they are automatically assembled to circuit boards or other components. The process requires high volume production to amortize the costs of the equipment required. Connectors, dip switches, shunts and other devices are the most common applications but medical devices, toys, and other products may also make use of the process. The molded part may include several stamped components. Wire may be used instead of strip in some components.

(Bralla, J. G., Handbook Manufacturing Processes, How Products, Components and Materials Are Made, pp. 207)

3. Wave Soldering (Manufacturing method)

Previous Answer

Wave soldering is a mechanical technique in which printed circuit is boards containing inserted components are moved by conveyor over a standing wave of molten solder.The position of the conveyor is such that only the underside of the board ,with component leads projecting through the holes, is in contact with the solder.The combination of the capillary action and the upward force of the wave cause the liquid solder to flow into the clearances between leads and through-holes to obtain a good solder joint.The tremendous advantage of wave soldering is that all of the solder joints on a board are made in a single pass through the process.

(Fundamentals of Modern Manufacturing.Materials,processes and system 3rd edition, Mikell Groover, p. 844)

New Answer (better)

Wave soldering is a variation of dip soldering in that the workpiece, a printed circuit board, held in a conveyor, momentarily contacts liquid solder that both heats and feeds molten solder to the joints to be connected. Wave soldering differs from dip soldering in that the liquid solder is lifted as a standing wave, instead of remaining as a level surface in the container. This simplifies the immersion process by limiting the area of contact with molten solder to only part of the circuit board at one time, reducing the chance of overheating electronic components. The solder contacts only the underside of the board. The molten solder is pumped from an intake below the surface so that the wave is essentially free of dross. An oil coating on the molten solder, or an inert atmosphere blanket, may be used to control dross formation. The operation heats and wets (coats) the joints of both devices connected in through-holes and those that are surface mounted. It coats the wire paths on the board and fills plated through-holes in the board. The wave solder operation is illustrated schematically in Fig. 13C5.

The sequence of operations in wave soldering a circuit board in a typical wave soldering machine is as follows:

1) The circuit board, with electronic devices attached but not solder-connected, is fluxed by spraying, contact with flux foam, or by contact with a wave of liquid flux, generated with a method similar to that used in wave soldering. The step applies a coating of flux to the underside of the board.

2) The board is preheated to a temperature of 180 to 270" (80 to 130") to drive volatile ingredients from the flux, to start the activation of the flux and to reduce thermal shock when the board contacts the molten solder.

3) Wave soldering then takes place while the board is still warm. Each portion of the circuit board contacts the molten solder for about 5 sec. Exposed metal surfaces, including leads or contacts of the attached devices, are wetted with solder. It also fills plated through-holes in the board. A hot airknife (air temperature above the solder melting point) may be used to remove excess solder in the form of solder bridges, solder balls, or other excess deposits.

4) The board is cooled at a controlled rate consistent with the sensitivity of the components to temperature changes. Blower-driven air may be used.

5) Board cleaning, for high-reliability applications, where no-clean flux is not used, is an important operation and immediately follows circuit board soldering. If no-clean flux is used, the cleaning operation is not required.

(Bralla, J. G., Handbook Manufacturing Processes, How Products, Components and Materials Are Made, pp. 537)

4. Solder Paste (Material)

Previous Answer

Solder paste is a suspension of solder powders in a flux binder.It has three functions 1)It is solder -typically %80 to %90 of total paste volume 2)It is the flux and 3)It is the adhesive that secures the components to the surface of the board.Methods of applying the solder paste to the board surface include screen printing and syringe dispensing.Properties of the paste must be compatible with these application methods, the paste must flow yet not be so liquid that it spreads beyond the localized area where it is applied.

After solder paste application components are placed on the board by the same type of onsertion machines used with the adhesive bonding assembly method.A low temperature baking operation is performed to dry the flux binder;this reduces gas escape during soldering.Finally the solder reflow process heats the solder paste sufficiently that the solder particles melt to form a high-quality mechanical and electrical joint between the components leads and the circuit lands on the board.

(Fundamentals of Modern Manufacturing.Materials,processes and system 3rd edition, Mikell Groover, p. 847-848)

New Answer (better)

Solder paste is a homogeneous, stable mixture of flux, pre-alloyed solder powder and other ingredients (thickeners, tackifiers, plasticizers, thinners) into a single material. Solder paste is used extensively in the manufacture of printed circuit boards that include surface- mounted devices. Metal content for electronics applications is typically in the vicinity of 85 to 90 percent metal content, by weight. Solder paste is tacky enough to hold electronic devices in place until the solder is melted ("reflowed"). The paste can be deposited before devices are assembled to the board, to control the position of the devices and the size and shape of the solder fillet on each joint. Formulations with various properties are available. The necessary ingredients are mixed on a batch basis, because the usual order quantities and the many varieties produced make batch-type production most. A key to satisfactory solder paste is the use of solder powder with particles of the proper shapes, and distribution of sizes. Several methods are in current use for producing solder powder with the necessary characteristics:

1)      making solder powder by gas atomizaltion

2)      spinning disk powder making

3)      ultrasonic method of powder making

4)      screen classification of powder

5)      air classification of powder

6)      inspection of powder

7)      mixing solder paste

8)      inspection of paste

(Bralla, J. G., Handbook Manufacturing Processes, How Products, Components and Materials Are Made, pp. 544)