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1. Single-sided (top side), surface-mount only: a. Print paste b. Place components c. Reflow solder paste 2. Double-sided, surface-mount only: a. Bottom side, print paste b. Bottom side, dispense adhesive for larger components, if necessary c. Bottom side, place components d. Bottom side, reflow solder paste and cure adhesive Flip the board over to the top side: e. Top side, print paste f. Top side, place components g. Top side, reflow solder paste 3. Double-sided, mixed technology (with bottomside wave soldering): a. Bottom side, dispense adhesive for surface-mount components b. Bottom side, place surface-mount components c. Bottom side, cure adhesive Flip the board over to the top side: d. Top side, print paste f. Top side, place components g. Top side, reflow solder paste h. Top side, insert through-hole components Keep the top side facing up: i. Wave-solder the through-hole and the bottom-side surface-mount components In case (3), for mixed-technology products, there is some flexibility regarding the sequence of staking (gluing) of bottomside components versus the topside soldering step, based largely upon the capabilities of the pick-and-place equipment. In the case of area-array packaging technology having solder balls (BGA, CSP, and DCA), there is the option of not adding solder paste to the pads. Instead, only the flux is deposited on the pads, which serves to tack the package in place during transport of the board to the reflow oven. The solder balls provide the solder to complete the joint; however, the penalty is a reduced gap thickness that may affect self-alignment during soldering (as well as solder-joint reliability later on). Omitting the solder paste is most often considered for the DCA/FC assembly process because the solder balls, and the corresponding pads, are so small that it is not possible to print the acceptable quantity and dimensions of the paste consistently. There is also the use of step-solder processes for double-sided, surface-mount-only circuit boards, that is, case (2). A high-temperature solder, often the 96.5 Sn-3.5 Ag (Sn-Ag) alloy (Teutectic = 221 C), is used to attach the bottomside components. These solder joints do not remelt when the topside joints are made using the Sn-Pb solder (Teutectic = 183 C). Somewhat tighter process control is required, since the 221 C Sn-Ag eutectic temperature is very near the optimum, peak process temperature for the Sn-Pb process (210 220 C). This approach provides the option of placing larger passive and active components on both sides of the circuit board without the need for an adhesive dispensing and curing step. Unfortunately, the use of the higher melting temperature, Pb-free Sn-Ag-Cu solders all but eliminates this step-soldering approach without identifying a higher melting temperature solder for the first step. The following sections examine each of the steps in a surface-mount assembly process, beginning with an examination of component types. The different dispensing technologies are reviewed, including those for solder paste and adhesives. Component placement machines are examined, followed by a description of the use of the various heat sources such as convection/radiation reflow ovens, wave-soldering equipment, condensation reflow apparatus, hand soldering, and conduction soldering. Lastly, cleaning processes are described.
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40.3.3.1 Components. A wide variety of components are available for surface-mount printed circuit boards. The form, size, and materials used for these components change constantly as new offerings are made by suppliers to meet miniaturization, functionality, and reliability requirements. The most common surface-mount components are the passive or chip devices the resistors, capacitors, and inductors. Chip capacitors and resistors are often referred to by a four-digit number, such as 1825, 1210, or 0804.The first two digits refer to the component body length, which is the distance between the terminations, and are in hundredths-of-inches (0.xx). The second two digits refer to the body width in hundreds-of-inches (0.xx). (For passive devices, a similar numeric designation is given that is based upon the metric system [mm]. The actual values are very close to the English values and can be a source of confusion, especially when partnering with overseas companies.) Thus, an 1825 capacitor has a length of 0.18 in. and a width of 0.25 in. A stereo photograph appears in Fig. 40.12 that shows examples of passive components. Chip resistors tend to be very robust and thus relatively immune to damage during the assembly process. Multilayer chip capacitors are temperature-sensitive and therefore more prone to cracking during assembly, particularly under fast temperature ramp rates.
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FIGURE 40.12 (a) Stereo photograph showing surface-mount passive (chip) devices: inductor, capacitor, and resistor; (b d) cross sections of their respective solder joints. (Courtesy of Sandia National Laboratories.)
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The chip resistor is constructed of a piece of alumina ceramic on top of which is deposited a thin film that serves as the resistive element. Attached to the resistive element at the top, on the ends, and partially at the bottom are the termination structures to which the solder joint is made. The termination structures are comprised of a fired, Ag-based thick film; a Ni or Cu barrier layer; and lastly an electroplated finish of Sn, Sn-Pb, or Au. Chip capacitors are constructed from a special oxide-based ceramic that is built up of alternating layers of ceramics and thin film layers that provide the device capacitance value. This capacitor is the multilayer thin film (MLTF) type.The second capacitor type has electrodes on the top and bottom surfaces of a homogenous block of ceramic. The ceramics used to make
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