Coating reflow

Electrolytic Tinplate. Much of the tin mill product is made into electrolytic tinplate (ETP). A schematic of an ETP cross section is given in Figure 1. The steel strip is cleaned electrolytically in an alkaline bath to remove rolling lubricants and dirt, pickled in dilute mineral acid, usually with electric current applied to remove oxides, and plated with tin. It is then passed through a melting tower to melt and reflow the tin coating to form the shiny tin surface and the tin-iron alloy layer, chemically treated to stabilize the surface to prevent growth of tin oxide, and lubricated with a thin layer of synthetic oil. 

Complex substrate modifications involving intermediate layers and palladium alloy deposition methods are often required for superior membrane performance. Modification of a membrane support surface before palladium deposition by sintering on smaller particles can create a smoother surface with smaller pores, facilitating the deposition of a defect-free palladium layer. Nickel microparticles have been sintered together to form a porous support that was sputter-coated with palladium and then copper [118]. Thermal treatment at 700 °C for 1 h promoted reflow to create a durable, pinhole-free membrane with a Pd-Cu-Ni alloy film. In another case, starting with commercially available PSS with a 0.5 pm particle filtration cut-ofF, submicron nickel particles were dispersed on the surface, vacnium sintered for 5 h at 800 °C, and then sputtered with UN [159]. The nickel particles created a smoother surface with smaller pores, so a thinner palladium alloy layer... 

Solder powders coated with a thin layer of Parylene exhibit a high degree of resistance to oxidation and to reaction with the flux contained in the solder paste without substantially interfering with the reflow characteristics of the solder. The powders are used as such or as solder paste. The preferred Parylene type is Parylene E, made from 4-ethyl[2.2]paracylophane since it melts below the melting point of the solder, i.e., about 180°C." The formulation of a typical solder pastes are shown in Table 2.4. 

The metal object to be coated is preheated in an oven and dipped into the fluid bed. The powder particles stick to the hot surface of the metal and flow into a continuous coat. After the object is withdrawn from the bed, it may be heated further in an oven to reflow the coating more uniformly and smoothly and to complete thermosetting cure. 

Certain thermoplastic coating films will soften and flow to become smooth and glossy at elevated temperatures. This technique, called reflow, is used on acrylic lacquers by the automotive industry to eliminate buffing. 

Before the widespread use of solder mask, a primary method for protecting copper circuitry was to electroplate a coating of tin or tin-lead on the copper. After component insertion, the tin plating could be liquefied with oven or vapor-phase reflow to form solderjoints.The use of solder mask over bare copper to take advantage of surface mount, wave soldering, and mixed assembly restricted the use of tin plate finishing. For simple technology product, tin or tin-lead plate and reflow remains a viable fabrication and assembly method. 

A similar principle to pin transfer is used for the application of flux on the solder ball of FC components for DCA (See Fig. 40.14). The die is immersed into a thin film bath of flux. The flux depth allows only the balls to be coated so that, in effect, the solder balls become the pin, taking up the flux. The flux on the solder balls is transferred with the die to the circuit board, where it provides the tack function as well as fluxing action for the solder balls during the reflow step. 

Early ovens utilized focused and unfocused IR lamps, mounted in the reflow oven tunnel. These bathe the solder paste-coated circuit boards and associated components with a broad spectrum of photonic energy heavily weighted to the IR end of the electromagnetic spectrum. The radiant energy absorbed by the printed wiring assembly (PWA) and related materials brings about the thermal increase needed to reflow solder. As the board travels beyond the last reflow heaters at the exit end of the oven, the absorbed heat is lost to the environment or the board is actively cooled by fans. The cooling results in resolidification of the molten solder and solder-joint formation. 

Solid solder coatings such as hot air-leveled pads, solder-plated boards, or stenciled solder paste reflowed prior to hot-bar bonding are preferred for this bonding method. 

Solder can be applied to the board as a paste, solid pre-form, or solder-coated pad. In all cases, the component must be held down during the soldering process to ensure contact of component leads with bonding pads. Proper gas pressure, temperature, nozzle translation speed, and flux are required to effect joint formation. Otherwise, the same reflow considerations are required for this technique as for any other. Heating ramp rate, solder paste preheating, peak temperature, liquidus duration, etc., must be observed for successful joint formation and joint reliability considerations. 

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