Electronic Design

Press-Fit Connectors Realize Breakthrough Performance

There is good news for high-density applications. The latest compliant-pin connectors demonstrate BGA-class (ball-grid array) electrical performance with excellent mechanical design attributes. Recent advances in compliant-pin connector technology have led to a dramatic reduction in printed-circuit board (PCB) through-hole diameter requirements compared to previous press-fit designs.

Next-generation devices retain the traditional assembly and operational advantages of press-fit connectors while achieving electrical performance that meets or exceeds surface-mount BGA-type attachment. The new generation of press-fit conectors represents a significant leap forward in terms of durability, manufacturability, and electrical performance. These devices surpass existing technologies while simultaneously proving more than comparable to existing high-performance, surfacemount techniques.

KEY ELECTRICAL FEATURES AND BENEFITS
One of the main performance advantages of surface-mount attachment methods is the smaller-diameter via that typically produces higher electrical performance. Because BGA-mount connectors are still the benchmark topology for electrical performance in many high-frequency applications, laboratory tests are necessary to measure them against comparable pressfit connections.

Coupled with the elimination of the solder pad required by surface-mount methods, the press-fit connections result in more consistent electrical performance. For example, Figure 1 and Figure 2 compare a BGA connector (red) to a micro action pin (MAP) press-fit connector (black). The differential insertion loss plot tracks close, but slightly better than the BGA (Fig. 1). The differential impedance plot of the surface-mount connector, though, displays a significant dip caused by capacitance of the solder pad (Fig. 2).

The relatively stable impedance of the press-fit connector with no solder pad more closely approximates the desired nominal value of 100 Ω. The data effectively dispels any preconceived notion that press-fit connectors cannot meet, much less exceed, the electrical performance of surface-mount attachment methods.

IMPROVED CIRCUIT DESIGN FLEXIBLITY
The press-fit connector contact's significantly smaller plated through- hole diameter versus earlier generation press-fit products increases flexibility for board designers by allowing for wider traces with lower tolerances. Also, where appropriate, the use of dual-trace pairs or quad routing in lieu of single-pair traces is possible.

Despite the high degree of precision inherent in quad-routing, manufacturability of board traces proves to be well within the fabrication capability of typical PCB manufacturers. As an alternative to quad-routing, these much smaller connector pins afford designers the option of employing wider-than-typical traces that can either boost overall system performance or allow the use of lower-cost PCB materials.

MINIMAL ELECTRICAL STUB
Compared to other press-fit connector technologies, the pin lengths in next-generation MAP connectors are much shorter (Fig. 3). This approach provides the opportunity to reduce the electrical stub in the through-hole to a minimum by increasing the depth of the counter-bore in the plated through-holes.

Other critical considerations with plated through-holes include radial-hole distortion and remaining wall thickness following pin insertion. To ensure electrical integrity, the 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, and IEC 60352-5 requires a greater than 8-µm remaining wall thickness.

REMOVAL AND REINSTALLATION
Press-fit connectors possess a number of distinct mechanical advantages over surface- mount connections, such as removal and reinsertion. A recent battery of lab tests involving two PCB vendors demonstrates that three such insertions and re-installations are viable without materially affecting the electrical or mechanical performance of the connector.

Comparing test results, insertion and retention forces averaged 4.06 and 2.68 lb per contact, respectively, showing that MAP connectors offer an easy and manageable repair process versus soldered BGA connectors, which are costly and problematic to remove and reattach.

TOOLING COMPATIBILITY
Two other critical parameters associated with all press-fit connections are the amount of force required to drive the pin into the hole and the surface area, or shoulder, of the pin required for contact with the driver housing. Higher insertion forces require more surface area, which typically results in a performance-reducing electrical stub. Large shouldered connectors also cause thinner plastic walls that are more difficult to mold and tend to crack easier.

MAP connectors also are compatible with simple flat-rock tooling, due to their low insertion force and minimal shoulder. Once the MAP connector pins are located over the array of holes on the PCB and the flat-rock tool positioned on the connector, uniform pressure applied by a simple press is all that is required. Many current connector styles, i.e., pin headers requiring intricate tooling, will benefit from the latter contact approach, which may lead to simpler designs, wider application of flatrock tooling, and easier implementation.

CONCLUSION
Recent developments in through-hole connector technology have seen a significant reduction in hole diameter over previous designs. This leads to an electrical performance breakthrough, while sacrificing none of the assembly and operational advantages of traditional press-fit connectors.

The new topology now makes it possible to produce high-density connectors suitable for very high-frequency interconnections between motherboards and daughterboards and/or mezzanine boards, I/O boards, and even backplanes. The standard presumption that surface-mount attachment of connectors is required for optimum electrical performance in many high-density applications is an outmoded concept.

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