Miniature press-fit devices have evolved to the point where they now provide equivalent or better electrical performance than BGA connectors, while also offering superior manufacturability characteristics.
BY DAVE HELSTER & JAMES FEDDER
Among the recent advances in compliant-pin connector technology is a dramatic reduction in PCB through-hole diameter requirements compared to earlier press-fit designs. New-generation devices retain the traditional assembly and operational advantages of press-fit connectors while achieving breakthrough electrical performance that meets or exceeds surface-mount BGA-type attachment. The net result is that recent press-fit connector design improvements have, in essence, “trumped” the traditional advantage–electrical performance–of surface-mount (SMT) versus earlier compliant-pin attachment techniques.
Originally developed for multi-layer PCB applications in the 1970s, press-fit connectors have experienced steady miniaturization through the present day. The original devices, designed for Eurocard connectors, featured pins designed for 1.5-mm finished holes. The unique design of these compliant pin, press-fit contact tails was called the ACTION PIN contact. Recent advances in the technology, like the Micro ACTION PIN (MAP) connector from Tyco Electronics, have resulted in a new generation of connector that provides the same robust mechanical and electrical connection to circuit boards but with finished holes just 0.22 mm in diameter (Figure 1).
 FIGURE 1. The evolution of the ACTION PIN press-fit connector shows a steady miniaturization trend from finished-hole sizes of 1.5 mm in the 1970s to 0.22 mm today. |
This improved contact-pin topology now makes it possible to produce high-density connectors (Figure 2) suitable for very high frequency interconnections between motherboard and daughterboards and/or mezzanine boards, I/O boards, and even backplanes. The conventional wisdom that surface-mount attachment of connectors is required in some of these high-performance applications is no longer valid.
Signal integrity
Because ball-grid array (BGA)attached connectors are still considered the benchmark topology for electrical performance in many high-frequency applications, we conducted laboratory tests to measure them against comparable press-fit connections. The topology of the two test boards consisted of 0.30 mm drill, 0.56 mm pads and 0.81 mm antipads. The comparative electrical data shown in the figure “Differential impedance” indicates generally similar results in terms of insertion loss, but the press-fit connector shows a marked improvement over the surface-mount type in terms of impedance.
 FIGURE 2. An example of a high-density compliant-pin device, Tyco Electronics’ MAP connector configuration was used in the analysis cited in this article. |
One of the main performance advantages of surface-mount attachment methods has been the smaller diameter via that typically produces better electrical performance. But the compliant-pin contact’s reduced hole diameter, coupled with the elimination of the solder pad required by surface-mount methods, resulted in more consistent electrical performance. Again, in the figure “Differential impedance,” the differential impedance plot of the surface-mount connector displays a significant dip caused by capacitance of the solder pad. The relatively stable impedance of the press-fit connector, which requires no solder pad, more closely approximates the desired nominal value of 100 Ω. The empirical data calls into question the conventional wisdom that press-fit connectors cannot meet, much less exceed, the electrical performance of surface-mount attachment methods.
In high-density applications, the press-fit connector’s impedance-matched footprint results in maximum signal transmission and low insertion loss. For maximum power transfer, insertion loss–defined as the ratio of power received at the end of the line to the power transmitted into the line–should be as close to unity as possible, which in decibels translates to 0 dB. The figure “Differential insertion loss” compares insertion loss of a MAP connector with an equivalent BGA configuration. Note that both display relatively similar performance characteristics up to about 8 GHz, after which insertion loss increases for both, especially the BGA out through 20 GHz.
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A comparison of crosstalk, in terms of both near-end noise (NEN) and far-end noise (FEN) showed strikingly similar plots for both the Micro ACTION PIN contact and BGA test devices. The differential NEN chart showed relatively flat, stable performance in the –30 to –40 dB range from about3 to 20 GHz. Both crosstalk plots showed virtually identical performance across the entire test frequency range for both the MAP connectors and BGA devices. These tests demonstrate that, in terms of near-end and far-end crosstalk performance, the press-fit connector displays comparable electrical characteristics with the BGA configuration, though the latter is traditionally associated with superior performance.
Return loss is another parameter in which the MAP press-fit connector concept demonstrates similar or better performance versus BGA.Described as signal attenuation caused by impedance variations in the structure of the connector, return loss has minimal effect at low frequencies, but in the gigahertz ranges of today’s communications protocols, return loss can be critically important. The figure “Differential return loss” illustrates an approximately 10 dB or better return loss performance of the Micro ACTION PIN contact over the spread of test frequencies.
 FIGURE 4. The MAP connector’s impedance-matched footprint results in maximum signal transmission and low insertion loss comparable to or better than BGA performance |
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Improved flexibility
The new-generation press-fit connector’s greatly reduced plated-through-hole (PTH) diameter, versus typical press-fit products, increases flexibility for board designers by allowing use of wider traces with lower tolerances and, where appropriate, dual-trace pairs, or quad routing, in lieu of single-pair traces. Despite the high degree of precision inherent in quad-routing, manufacturability of board traces employing the connector has been demonstrated to be well within the fabrication capability of typical PCB manufacturers. As an alternative to quad-routing, new-generation press-fit connectors afford designers the option of employing wider-than-typical traces that can either boost overall system performance or allow use of lower-cost PCB materials.
Compared to other press-fit connector technologies, the shorter pin length of the MAP connector allows the electrical stub in the through-hole to be reduced to a minimum by increasing the depth of the counter-bore in the plated through-holes.
EIA 364-96 and IEC 60352-5 standards require radial-hole-distortion measurements of less than 37.5 µm and 70 µm, respectively. These standards likewise specify allowable remaining copper plated wall thickness. EIA364-96 permits no breakthrough, whereas IEC 60352-5 requires greater than 8 µm remaining wall thickness. As shown in the press-fit connector cross-sections in Figure 6, both the radial-hole distortion and the remaining wall thickness are shown to be well within the more stringent standard.
As the data in the table “PCB performance test results” indicates, with the application of a uniform unmating force, it is possible to remove a MAP connector from the printed circuit board. Laboratory tests have demonstrated that three such insertions and re-installations can be performed without materially affecting the electrical or mechanical performance of the connector. In this way, press-fit connectors offer an extremely easy and manageable repair process, if needed, as opposed to soldered BGA connectors that are extremely costly and problematic to remove and reattach.
Compatible with “flat rock”
Two critical parameters associated with press-fit connectors are the amount of force required to drive the pin into the hole and the surface area (shoulder) of the pin required for contact with the driver housing. Higher insertion forces logically require more shoulder surface area, which typically results in a performance-reducing electrical stub. Another problem caused by large shouldered connectors is thinner plastic walls that are more difficult to mold and tend to crack easier.
 FIGURE 5. Differential return loss performance of the MAP connector shows marked improvement over BGA. |
Alternatively, MAP connectors are compatible with simple “flat rock” tooling, due to their low insertion force and minimal shoulder. Once the connector pins are located over the array of holes on the printed circuit board and the flat-rock tool positioned on the connector, uniform pressure applied by a simple press is all that is required. Many connector styles today, like typical pin headers that require intricate tooling, will benefit from the Micro ACTION PIN contact approach, leading to simpler connector designs, wider application of flat rock tooling methods, and easier implementation.
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Recognized standard test sequences were conducted to evaluate the characteristics of the press-fit connector alone (unmated) and also when plugged into its mating receptacle. The test devices met all requirements for low-level circuit resistance (LLCR), insulation resistance (IR), dielectric withstanding voltage (DWV), as well as other characteristics shown in the table “MAP connector performance testing.”
Performance results
The greatly reduced plated-through-hole diameter of the new generation of press-fit products increases flexibility for board designers by allowing use of wider traces with lower tolerances and, where appropriate, dual-trace pairs, or quad routing, in lieu of single-pair traces. Despite the precision required for quad-routing, manufacturability of board traces employing the new, smaller connector has been demonstrated to be well within the fabrication capability of typical PCB manufacturers. As an alternative to quad-routing, these connectors afford designers the option of employing wider-than-typical traces that can either boost overall system performance or allow use of lower-cost PCB materials.
The conventional wisdom that press-fit connectors cannot offer the same degree of electrical performance as surface-mount connectors has been successfully challenged by Tyco Electronics’ Micro ACTION PIN contact technique and other new-generation devices that have consistently demonstrated equivalent or better electrical performance without the mechanical problems associated with surface-mount attachment methods. The technology represents a quantum technical leap forward, providing durability, ease of manufacturability, and electrical performance that is superior to traditional press-fit connector technology, and more than comparable to surface-mount techniques. CS
DAVID HELSTER, an 18-year industry veteran, is director of the Circuits & Design Group at Tyco Electronics Communications and Industrial Solutions Business Unit in Middletown, PA. He may be reached at dave.helster@tycoelectronics.com. A 30-year veteran, JAMES FEDDER is manager of Development Engineering at Tyco Electronics’ Communications and Industrial Solutions Business Unit . He may be reached at jlfedder@tycoelectronics.com.
