B2b4. stencil dispensing of solder paste - has become the process of overwhelming choice. It is similar to screen dispensing except that a mask made of sheet metal is used instead of a screen, but occasionally the mask openings are combined with a stainless steel mesh for reinforcement. As in screen printing, a rubber-like squeegee forces the solder paste through the stencil openings and onto the desired locations on the circuit board. The thickness of the sheet metal determines the thickness of the solder paste deposit. Typical thickness are in the range of 0.005 to 0.012 in (130 to 300 ì m). Stencils are either cut from sheet metal or formed by electroforming (See 2L2.). Cutting stencils from sheet metal involves either photochemical blanking (3S4), or laser cutting (3O). The metals commonly used are stainless steel for cut stencils and nickel for electroformed stencils. Photochemical blanking normally involves the use of two resist coatings, one on each side of the stencil sheet, exposed and developed with opposite-side images in alignment with each other. The chemical etching takes place from both surfaces. The etching operation is followed by electropolishing (See 8B3.) to remove a sharp edge, if it exists where the etching for each side intersects, and to smooth the whole surface of the stencil. Laser-cut stencils may also be electropolished. Electroformed stencils are made by first coating, imaging, and developing a photo-resist on a substrate. Then electroforming takes place in the openings of the photoresist to create a stencil that is removed from the substrate. As with screen dispensing, stencil dispensing is limited to flat circuit boards, and stencils are less forgiving than silk screens of minor variations in the board surfaces. However, stencil printing is usually more consistent, and alignment with the board is easier to maintain. Stencils are better adapted than screens to fine pitch spacings, especially the laser-cut and electroformed stencils. Metal stencils also have a considerably longer life though they are more costly than screens.  

B2b5. submerged disk dispensing of solder paste - is used for dispensing solid or dashed lines of paste on a board. A rotating disk is positioned so that its edge is immersed in solder paste. As it rotates, the disk carries an amount of paste which, in turn, is deposited on a board that passes above it. The principle is shown in Fig. 13B2b5. A thickness control knife removes excess solder paste before the disk contacts the workpiece. The disk is usually made of nonmetallic material and can be of whatever width is needed for the solder paste deposit.  

Fig. 13B2b5 The principle of submerged-disk dispensing of solder paste to a printed circuit board.

B2b6. dip coating of solder paste - is used occasionally in applying solder paste to the leads of components such as capacitors. The lead is immersed in solder paste in a tray or other container. Reflow follows assembly of the component to the board.  

B2b7. roller coating of solder paste - to leads of capacitors or other devices is sometimes employed. A nap-covered roller, similar to a paint roller, is used. One common application is the coating of “nail head” ends of capacitor leads by manufacturers of such devices. The capacitors are arranged in a carrier so that they are all have the same orientation. A roller, coated with solder paste from a reservoir, is rolled over the row of “nail heads”. The capacitors can then be assembled to circuit boards or other devices and the solder paste reflowed. 4  

B2c. using solder preforms - Solder preforms are solid shapes of solder manufactured by various methods of metal forming and cutting. Typical shapes are washers, disks, rectangular or square shims, rings, and other wire forms, spheres, sleeves and other special shapes. Typical preformmaking operations are extruding, either with or without a flux core, rolling, ring forming and blanking. (These operations are described in general terms in Chapter 3.) Flux coating of preforms, when used, usually involves a spraying operation. Fluxcored preforms are usually made from flux-cored wire solder which may be flattened by being rolled into a ribbon and then blanked to the shape desired. The advantage of a preform is that it can supply the exact amount of solder needed for the joint and, if there is a flux core or coating, the exact amount of flux required. The amount of solder and flux supplied is also constant from assembly to assembly. High-quality, very uniform solder joints are possible. In many instances, the preforms can be placed with automatic equipment. Further economies result from the fact that all joints on a board or on a device or groups of them can be heated simultaneously.

B3. cleaning prior to soldering - Cleanliness of the surfaces to be joined is an important prerequisite of good solderability. Most cleaning of circuit boards and other electronic components is undertaken to remove foreign matter that may accumulate on the components from prior operations, handling and storage. The foreign matter includes dust, oils, wax, chips from machining the circuit board, and tarnish. Two methods are in predominant use for in-line cleaning, prior to fluxing and soldering: 1) vapor degreasing and 2) water washing. With both methods, care must be taken not to dislodge components assembled to the boards. Vapor degreasing using conventional equipment (See Chapter 8, section A2a3.), is quite satisfactory because it leaves the boards dry as well as clean and does not involve forces that may dislodge devices assembled to the board. An airknife may be used before the next operation (fluxing), to remove any trapped solvent and to cool the board to facilitate foam fluxing. Vapor degreasing has disadvantages from health and environmental standpoints, however. Water washing is more difficult because the force of water agitation or spray can dislodge components on the board. However, satisfactory results can be obtained. Saponifiers and other additives are included in the washing solution. Ultrasonic agitation may be utilized in both kinds of cleaning if the board is immersed in a liquid. Drying after washing is essential. Baking may be required if the soldering operation immediately follows cleaning. Components that require more aggressive cleaning because of chemical contamination, corrosion, or to remove protective coatings, may undergo vapor blasting, acid treatment, brushing or scouring. Great care must be taken in such operations to avoid damage to softer materials or dislodging assembled components.  

B4. prebaking before soldering - is an operation that often is not required, but is used when it is necessary to remove volatile materials from the circuit board. These materials include trapped solvent or moisture from a cleaning operation, the volatile materials in the laminated board itself, and moisture that may be absorbed in the board or assembled components during storage and assembly. Baking usually takes place with temperatures ranging from 180 to 250 ° F (80 to 120 ° C) for periods of 1.5 to 16 hours. Lower temperatures and shorter times can be used with vacuum ovens (with approximately 1 torr of vacuum). assembled components during storage and assembly. Baking usually takes place with temperatures ranging from 180 to 250 ° F (80 to 120 ° C) for periods of 1.5 to 16 hours. Lower temperatures and shorter times can be used with vacuum ovens (with approximately 1 torr of vacuum).