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Assembly Process The through-hole printed circuit board can be a cost-effective technology for many applications. One determining factor is the level of automation used to make the product, which can range from hand assembly to fully automated processes (inline or batch). The specific assembly steps include component insertion (also referred to as board stuffing ), lead trimming, soldering, and post-assembly cleaning. Labor costs, capital expenditures, board design, and production volumes are contributing factors toward determining the details of these steps. There are two general formats for assembly processes: the cell or batch process and the line process. Both methods are discussed in the following subsections. 40.2.3.1 Cell (Batch) Process. The cell process routes the circuit boards in batches between the different steps. The cells or workstations are not always in immediate proximity to each another and can be entirely manual, semiautomated, or fully automated in terms of the actual process step. For example, component placement may be fully automated, but require several machines for inserting the different component types. Circuit boards are typically loaded and unloaded by hand between machines. Table 40.1 lists the advantages and disadvantages of the cell process. The cell or batch process is best suited for a facility that assembles a high mix of low production volume products (e.g., prototype development or high-reliability circuit boards) where flexibility is necessary on the factory floor.
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TABLE 40.1 Advantages and Disadvantages of the Cell Process Advantages Single machine outage does not immediately stop the entire process line. Adds greater flexibility to the process by creating alternative workflow routes. Attractive for high-mix, low volume applications that require frequent equipment change-outs and re-tooling. Disadvantages Higher product flow time due to the transfer of parts between cells is unattractive for high volume applications. Part transfer between cells increases the likelihood of handling damage Difficult to predict product line throughput in a multi-cell assembly process.
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40.2.3.2 Line Process. The second approach is the line process, where the different insertion machines, as well as the soldering process in some cases, are linked together with automatic board-handling equipment. Table 40.2 lists the advantages and disadvantages of the line process line. The line process is best suited for a factory floor where high production volumes (e.g., consumer electronics) and a low mix of product types are typical of the manufacturing operations. Less versatility is required by assembly processes, which justifies the capital equipment expense.
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TABLE 40.2 Advantages and Disadvantages of the Line Process Line Advantages Improved manageability of material inventory, product flow, and operator resources is attractive for high volume, low mix applications. Shorter process times by eliminating the transfer of parts between machines. Reduce the likelihood of handling damage to the assembled product. Disadvantages Failure or maintenance outage of a singe machine failure can potentially halt an entire assembly line. Reduced equipment flexibility is unattractive for high mix assembly. Capital equipment costs and fixed floor space requirements are critical considerations.
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The configurations of most through-hole components fall into one of three geometries: axial leaded, radial leaded, and dual inline pin (DIP) packages.These traditional configurations are used for resistors, capacitors, transistors, crystals, and, in the case of active devices, the DIP package. There are also odd-form packages for devices such as transformers, switches, and relays. New package configurations such as the pin grid array (PGA) are being developed to accommodate the increased functionality and further miniaturization of active devices. Besides the actual component body size, shape, and lead configuration, the other factor that can impact the through-hole assembly process is the lead finish. First of all, the finish can potentially add significantly to the diameter of the lead, which must be taken into account when considering the tolerance budget for the hole in the board design. Secondly, for hot solder dipped leads, the potential accumulation of solder at the end of the lead can interfere with part insertion. The Pb-free alloys can impact the hand-soldering assembly process. First, the higher melting temperature of these alloys requires a slightly longer soldering time. In the case of hand soldering, tip temperatures designated for Sn-Pb process can be used for the Pb-free soldering of traditional through-hole designs. However, hotter tips and/or irons with a higher power rating may be required for borderline designs such as those having bigger component leads or thicker circuit boards. Secondly, the Pb-free solders have a higher surface tension that tends to slow wetting and spreading on surfaces as well as capillary flow into holes. For example, the Pb-free solder may not fully coat pad surfaces on the opposite side of the circuit board. Third, the high Sn content of the Pb-free solder increases the rate of erosion of the soldering iron tip, wave-soldering machine parts, and the Cu features on the circuit board. 40.2.4 Hand-Soldering Process The sequence of steps used to hand solder a through-hole circuit board varies somewhat between different applications. First, there is part insertion. If the insertion operation is fully manual, the parts will be kitted into specific groups: The first group is inserted and then soldered; the second group is inserted and soldered; and so on.The order of the groups is determined so as to maximize throughput as well as to take into account human factors in order to minimize part error, lead damage, and operator fatigue or inattention. In semiautomated processes, the operator may receive the circuit board for soldering that was partially or fully populated by machine. Next, there is the soldering step. The location of the joint where soldering is performed depends on the architecture of the circuit board. For single-sided boards without a plated through-hole, soldering must be performed on the component side. On the other hand, in the case of double-sided and multilayer circuit boards with plated-through holes, soldering is typically performed on the bottom side in order to avoid potential heat damage to components by the soldering iron, particularly on densely populated circuit boards. The hand-soldering process proceeds as follows: 1. The operator applies flux to the joint. 2. The soldering iron tip is contacted to one side of the component lead (see Fig. 40.2). The tip should not contact the circuit board pad, if possible. It may be necessary to contact the pad of thicker circuit boards.
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FIGURE 40.2 Diagram showing placement of the soldering iron tip for heating the lead and placement of the solder wire to complete the joint.
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