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and fluxes; it does not work as well with solder paste. A pin is dipped into a reservoir of the adhesive or flux. The length and diameter of the pin determine the quantity of material that is picked up upon its withdrawal from the reservoir. The pin is then lowered to a position just above the surface of the printed circuit board that allows the flux or adhesive to contact the circuit board. Surface tension causes a portion of the adhesive or flux to be deposited on the printed circuit board. It is very important that the pin does not touch the board because this will cause inconsistent dot sizes and shapes. This system requires the substrate to be relatively flat and free from distortion. The nature of the pin array can also allow for adhesive to be applied to the board even after through-hole parts have been put into place. 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. An important consideration about the pin transfer technique is that it requires an open bath of the adhesive or flux. Adhesives readily absorb water from the air. Fluxes lose vehicle (water or alcohol) and possibly other constituents through evaporation. By either mechanism, the material properties change, which affects the quantity of fluid retained on the pin and deposited at the site (including the flip-chip process previously described). Adhesives must have sufficient wet strength, and the fluxes must have enough tack to hold the component in place for the duration of component placement activity as well as subsequent handling of the circuit board on its way to the curing oven or reflow oven. Screen or stencil printing can be used for adhesives as well as solder paste. The low viscosity of most flux solutions precludes their successful dispensing by this technique. The adhesive or solder paste is deposited through openings in the stencil or screen called apertures. The apertures are located over the locations on the circuit board where the adhesive or solder paste is required. The placement of material is performed by a squeegee pushing a quantity of adhesive or paste ahead of it, over the screen or stencil, as illustrated by Fig. 40.16a.
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FIGURE 40.16 Screen and stencil printing: (a) the squeegee movement used to print adhesive or paste through a screen or stencil; the construction of a screen (b) and stencil (c) used for printing.
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The difference between a screen and a stencil is in their respective structures, as shown in Fig. 40.16b, c. The screen is comprised of two layers: the emulsion layer and the actual screen that supports the emulsion layer. The apertures through which the adhesive or solder paste is deposited are created in the emulsion by photoimaging techniques. The adhesive or solder paste simply flows past the screen cross-hatched wires. The stencil is simply a sheet of metal or alloy (commonly Mo, Ni, brass, or stainless steel) having the apertures formed in them. The apertures can be created by one or a combination of the following techniques:
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Photoimaging (photoresist definition) combined with etching by wet chemistry Laser cutting Build-up technology by electroplating processes
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The choice of fabrication technique depends on the required sizes and densities of the apertures. The stencil has replaced the screen in most surface-mount printing applications, not only because of its simpler construction, but also because it can accommodate finer, denser circuit board features. The thickness of the screen or stencil and the size of the individual aperture openings are the parameters that control the quantity of adhesive or solder paste deposited on the circuit board. Secondary factors are the aperture wall quality, the material viscosity, the hardness of the squeegee, and the speed of the squeegee. In the case of solder paste, the quantity that is actually deposited is usually less than the volume of the aperture, which is the product of the width, length, and stencil thickness. The degree of that discrepancy is called the transfer factor or transfer coefficient. Values can range from 60 percent for very small apertures to nearly 100 percent for larger aperture openings. Screen or stencil printing is the most widely used means for depositing solder paste on a surface-mount circuit board. The preferred paste viscosity for screen printing is 250 to 550 kcps (kilo-centipoises) for an 80 mesh screen. In the case of stencil printing, the desired viscosity is 400 800 kcps. The ability to print consistently upward of tens of thousands of paste deposits per circuit board has been critical to the realization of high-volume electronics production. This process is being developed for through-hole circuit boards and is referred to as paste-in-hole or pin-in-paste technology. Solder paste printing technology includes stepped stencils. Stepped stencils are made with two different thicknesses and are used when a circuit board has such a wide range of device pitches and joint configurations that a single stencil cannot yield the optimum paste deposits for all the components. The thinner sections are used for the very-fine-pitch packages, whereas the thicker sections deposit paste for the larger-pitch components. These stencils are more expensive to fabricate than the single-thickness products. The printing of adhesive or solder paste with a screen or stencil has several limitations. It can only be performed in a single pass; if there is a fault during that pass, the board must be removed and cleaned prior to a second attempt. Second, the circuit board surface must be flat and have no obstructions that will interfere with the stencil or screen surface sealing against the board surface as the adhesive or paste is pushed into the apertures by the squeegee. As such, it is important that the screen or stencil be thoroughly cleaned of paste residues prior to use in order to minimize printing defects, which can subsequently become solder joint defects if not caught prior to reflow. Third, stencils and screens wear with time, resulting in an increased number of printing defects. The harder the metal or alloy, the longer is the lifetime. For example, brass stencils, which are relatively inexpensive, have a short service life. Stainless steel stencils have a longer lifetime, but are also considerably more expensive. Referring to the solder paste printing process with a stencil, Pb-free solder pastes behave very similar to the Sn-Pb pastes for leaded and area-array pitches of greater than 0.5 mm. At the smaller pitches that are characterized by smaller apertures, it has been observed that the Pb-free solders have a slightly reduced transfer coefficient. The likely
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