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49.7 EQUIPMENT
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Several types of mechanical presses are used for applying press-fit connectors to PWBs. The lever-activated arbor press is the most primitive but suffices for simple, low-lead-count, coarsepitch connectors. One step up is the pneumatically driven press. Although it is easier to operate than the lever-activated arbor press, it does little for process control or assembly repeatability. Higher on the evolutionary scale is the pneumatic/hydraulic combination (the so-called airover-oil press), which offers a somewhat more controllable process. High-precision pressing cycles are mandated by the latest press-pin connectors with their delicate, molded body features, fragile electrical shielding, and fine pin pitch, so it is advisable to use a machine that is computercontrolled.Aside from force and speed precision for process reproducibility, press cycle data can be stored along with process recipes for ram force, speed of compression, and component location if equipped with programmable axes. Data logging is critical for statistical process control and is useful for press cycle root-cause defect analysis or machine troubleshooting. The state-of-the-art connector press is electromechanically actuated, relying on a motordriven coarse-pitch lead screw with tachometer speed control, z-axis positional encoders, and load cell feedback for an accurate pressing cycle (see Fig. 49.9).
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Connector press-fit tool
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Force transducer Press-fit connector PWB with plated throughholes for press pins
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Press-fit connector compliant pin
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Press-fit support tooling
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FIGURE 49.9 An electromechanical connector press.
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Some commercially available machines are automated even to the point that they are able to shuttle boards into the press, pick and place connectors, select a connector-appropriate pressing tool and supporting tool anvil, rotate the tools to the proper orientation, and press multiple connectors sequentially on a single board or multiple nested boards. There are several ways to control the press-fit process. Methods and merits of each are discussed in the remainder of this chapter, but first a more detailed review of the mechanics of the press-fit process is in order.
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ASSEMBLY PROCESS
Press tool and support plate are fabricated to match connector requirements and PCA layout. The PCA is placed on the press-fit tooling fixture support plate. The operator manually places the connector into the appropriate plated through-holes in the PWB. The ram and tool are aligned to the connector to be pressed. When press-fit connector leads are forced into PWB plated through-holes during a pressing cycle, there is a characteristic fingerprint of force versus vertical distance traveled for each connector and PWB type.The various events and slopes that comprise the fingerprint are shown in Fig. 49.10. As pressing progresses, connector leads are forced into the plated through-holes and the compliant section of its pins deform elastically, then plastically. Continued resistance is encountered due to the friction of the collapsed pins sliding along the PTH barrel walls. Another inflection in the plot occurs where the connector fully seats against the PWB. Further force affects little or no change in distance and the press cycle ceases. Were the cycle to continue, damage to the PWB would result.
FIGURE 49.10 Force versus distance plot of a press-fit cycle. Inflection points and regions are as follows: (a) initial force is applied to the press-fit connector; (b) the compliant section of the pin collapses; (c) the pin collapse is complete; (d) the pin slides along PTH barrel wall, creating friction; (e) the connector is fully seated; (f) continued pressing makes little change in press distance. The pressing cycle is complete.
PRINTED CIRCUITS HANDBOOK
PRESS ROUTINES
For best results, a method that allows real-time measurement and control of the pressing process is preferred and is available on today s commercial presses. There are four commonly used pressing routines, but only three take advantage of force-sensing, distance-sensing, or real-time feedback control of the connector press. Connector complexity and equipment availability dictate press-fit assembly methodology.
Uncontrolled Pressing This was the most prevalent technique for a long time, but it rapidly lost ground to more sophisticated pressing methods, as described later in this section. It commonly relies on an arbor press and human muscle to force the press-fit connector into the PWB. There is neither force sensing nor speed control. It is the least reliable way to press connectors into PWBs and is not recommended for complex connectors or fragile PWBs. If too high a ram speed is used on a delicate connector, its pins may buckle. This method may not provide enough force for some high-lead count connectors. Although this type of pressing is discouraged, it may be adequate for low lead-count, coarse-pitch connectors in noncritical assemblies.As with any press routine, a suitable support fixture must be used beneath the board to keep the assembly from breaking or flexing. It should incorporate support to counter the forces of the ram, and relieved areas to accept protrusion of press-pins below the bottom side of the board and clearances for bottomside components as shown in Fig. 49.11.
FIGURE 49.11 The supporting fixture is critical for good press-fit application. It must have clearances for press-pin protrusion and any bottom-side components.
Height-Limited Pressing As previously described, connector presses have matured into sophisticated machinery; improved greatly over the arbor press. Some are capable of precisely determining ram height above a datum such as the top surface of the PWB. If board-to-board thickness is consistent, then the connector can be forced into the board to a predetermined standoff
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