Press-fit connector

At optimized operating conditions, uncoated and deactivated restriction capillaries are installed between injector and precolumn by means of simple press-fit connectors to reduce carrier gas velocity within the precolumn. By means of such a column combination suitable pre- and main columns may be easily exchanged and adopted to optimal efficiency [28]. 

Use an inlet adapter for the column injection port that is an easily replaced disposable liner. See Figure 20-26. Press Fit Connectors... 

Figure 3.18. Thermal modulators for comprehensive two-dimensional gas chromatography. The slotted moving heater (left) and longitudinally modulated cryogenic trap (right). Components 1 = injector 2 = first dimension column 3 = slotted heater 4 = stepper motor 5 = press fit connector 6 = thick film modulator capillary 7 = second dimension column 8 = detector. (Adapted from ref. [168 ] and [225] Elsevier).

Press-fit assemblies are extensively used in high-performance applications. PAI press-fit suspension bushings are used in race cars [20]. Drill bushings are press-fit assembled [21] solderless connectors are press-fit into printed circuitboards [22] compliant press-fit connectors are assembled with bi-spring press-fit contacts [23] security magnet sensors with crush-proof plastic casings are press-fit installed on doors, windows, and cabinets [24] and General Electric Home Security sensors are press-fit assembled [25]. 

Connector bodies may or may not be compatible with the thermal rigors of reflow or wave, so press-fit connectors are typically applied after all mass soldering of the PCA is completed. Further, press-pin connectors are not generally soldered into place since rework would be rendered impractical. Rework of press-fit connectors will be discussed later in this chapter. 

The load necessary to apply a press-fit connector depends on a variety of factors, including pin design, pin material, pin size, pin surface finish (plating), PTH surface finish, plating thickness, pin and hole size, and the number of pins per connector. The connector manufacturer should provide the required loading per pin for each connector for proper insertion and connector seating. 

Since pressing force generaUy ranges from some 10s of grams to kilograms per lead, the mechanical advantage of a reproducible press machine is required for press-fit connector apphcation. 

For purposes of affordabihty, mechanical integrity, electrical properties, and materials conservation, all electrical contacts, regardless of assembly methodology, are made from inexpensive base metals and plated with a minimum thickness of a more precious or more practical material. Many press-fit connectors have been made of berylhum-copper alloy, but due to beryllium dust toxicity, a factor in press-fit lead preparation, the switch to safer phosphor-bronze or other copper aUoys has occurred throughout the industry. 

In the case of the press-fit connector pin, there is neither a physical wetting of material nor metallic encapsulation to protect the intercoimection. Additionally, no chemical fluxing agents are used. 

Surface finish of the PWB s PTH has a significant effect on the force required to seat a press-fit connector. Changes in surface finish and PTH finish quality can have a profound effect on pressing force and reliability. 

As previously mentioned, recently the European Union exempted press-fit connectors from its RoHS legislation, allowing the use of Sn-Pb coatings on comphant tails. The Pb adds a small amount of lubricity, allowing for easier pressing into notably difficult surface finishes such as organic solderabihty preservative (OSP)-copper. 

The Pb of Sn-Pb solder-coated barrels is known to reduce the force required for press-fit connector insertion and seating. Although Sn-Pb coating is one of the best surface finishes for pressing, if it is thick, then the press-fit connector pin may skive a thin shver of solder and push it out the secondary side of the PWB (see Fig. 49.8). Very often these slivers... 

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. 

The press-fit operation is inherently high yielding, but problems with the process or a bad press-fit connector may necessitate the removal and replacement of a connector. As a result of the press cycle, the compliant pins of a press-fit connector are plastically deformed.Therefore, once used, the same connector cannot be removed and reinserted in a circuit board. However, most press-fit connectors are designed to be reworked—that is, repaired or replaced. In some cases, individual leads or contacts can be replaced. In other cases, banks of leads or contacts can be substituted. Some press-fit connectors require complete removal of the damaged connector for replacement with a new one. Many different press-fit connectors are on the market, and each has its own manufacturer-recommended repair strategy. 

Press-fit assemblies are generally designed to accommodate at least two connector replacement cycles. The two cycles are predicated on (1) the normal force exerted by the press-pin s compliant section on the PWB s plated through-hole barrel wall, and (2) the quality and condition of the PWB s plated through-hole barrel surface after subsequent press-fit connector removals and replacements. 

Follow the press-fit connector manufacturer s recommendation for PTH finished hole size, tolerances, and acceptable or tested surface finishes. Use cross-section sample boards with high-aspect-ratio (depthidiameter) holes to quaUfy the PWB vendor and determine typical PTH barrel profile. Ensure that wall thickness of plated through-holes are uniform throughout their depth and meet connector specifications. Make certain that the surface finish on the PTH annular ring does not interfere with insertion of the comphant press-fit pin. 

Prior to insertion into the board, check press-fit connector pins for straightness and integrity. An inexpensive go/no-go gauge (see Fig. 49.14) can be machined to check that connector pins are not bent. It should be constructed so that it neither compresses the compliant section of the press-fit pin nor damages its surface coating. This is especially helpful for fine-pitch press-fit connectors (1 mm centers) and is better than relying on visual inspection for pin field integrity. 

FIGURE 49.14 Gauge for checking connector pin position prior to pressing. This is best used for high lead count, fine-pitch pin press-fit connectors. 

Apply press-fit connectors after all SMT and wave-soldering operations. Do not try to solder press-pin connectors in place. Press-fit connector bodies may not be reflow-compatible. Soldering a press-fit connector will also make rework difficult or impossible. 

Parenti, D., and Mitchell, I, Validating Press-Fit Connector Installation, Circuits Assembly, Apii 2003, pp. 26-29. 

The impact of different surface finishes on press fit connectors is another factor to be considered when selecting PWB surface finishes for products such as backplanes. The plastic deformation of the Sn plating, the hardness of the surface finishes, along with the dimensional tolerances of the pin and through-holes, the design of the connector pins (and their mechanical compliance), and the mechanical properties of the PWB, all affect the insertion force required (and consequently the retention force) for the press fit connectors. Typically, the difference in the insertion force among the different surface finishes (Ni/Au, TAg, I-Sn) is not significant (<10%). 

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