The demand for additional integration and high density packaging in circuits for telecommunications, aviation and military applications continues to increase. The result is that components assembled on PCBs and into packages must be reduced in size and made suitable for new assembly techniques such as surface-mount technology (SMT). However, classic coaxial connectors and their mating interfaces have reached their limitations in some applications and have become a critical factor in high signal speed and broadband applications.
Faced with an evolving market with challenging demands Rosenberger took a close look at existing state-of-the-art coaxial connectors, considered their limitations and put forward possible solutions for high speed signal transmission in board-to-board applications, multi-chip mode (MCM) packages and on-board interconnection techniques. The objective being to develop an innovative interconnection system for high frequency board-to-board connections and the result is the Flexible Microstrip Connector (FMC) series.
Over the past 10 to 15 years coaxial connectors have been developed for use in board-to-board applications with frequencies of 50 GHz or higher, the most prominent being the SMP (shown in Figure 1) and Mini SMP connectors. They provide good overall frequency characteristics together with the ability for high density packaging. Since these connector interfaces are pin/socket configurations, strict rules for tolerances are required to avoid destructive mating.
The most limiting design factor for these small connectors, however, is the center contact. The nominal center contact diameters for the SMP and Mini SMP connectors are 0.85 mm and 0.75 mm, respectively. Now factor in a mating-pin diameter of 0.40 mm for SMP and 0.32 mm for Mini SMP and the potential for contact finger deflection and destructive mating is high, especially if tight positioning tolerances are not maintained. Furthermore, tight tolerances in piece part manufacturing and in PCB-board layouts can have significant cost implications.
Another consideration that the Rosenberger development engineers had to consider is the limited axial direction tolerance in board-to-board coaxial connectors. The standard configuration is a PCB-mount connector on both boards, which are connected via a coaxial bullet to equalize the displacement in the axial direction. Some designs accommodate for this displacement with a spring-loaded, floating coaxial connector on one of the PCBs but this can be expensive and requires additional geography on the PCB making it less desirable for high density packages.
The SMP and Mini SMP offer a simple and cost-effective solution to this problem. The principle is, that on one side there is a smooth bore PCB connector and on the adjacent board, a full detent or semi-detent style connector is mounted. This allows the coaxial bullet to accommodate the axial tolerances in the board-to-board configuration. However, the disadvantage is the change of impedance in the interface from the bullet to the PCB connector depending on the (distance) tolerances between the boards. With the SMP interface, the step in impedance due to axial displacement is a change from an almost 50 Ω characteristic impedance to 100 W. At higher frequencies this has a significant impact on the repeatability of the interconnection.
The next area that had to be addressed was the angular and radial displacement of the boards. In this situation, a high mechanical stress is loaded on the contact fingers of the socket center contact. Since there is only a maximum 0.20 mm wall thickness of these contact fingers too much radial and angular deflection may destroy the socket contact or bend the mating pin such that the characteristics of the connection can no longer be guaranteed.
Another consideration is that special care has to be taken at the connection of the PCB connector to the board. At this interface is a transition from the coaxial TEM mode of propagation into the mode of propagation of the individual planar transmission line structure. To reach the optimum transmission characteristics, a footprint has to be developed, especially for higher frequency ranges. Rosenberger therefore developed special connectors for PCB and planar structures to support high frequency applications whereby the transition from coaxial line structure to the planar structure is made inside the coaxial connector. Thus, the rear interface is a planar structure almost mirror imaging the footprint on the board, which provides a very reliable impedance controlled and repeatable transition from the coaxial connector to the PCB.
This technique is able to support every available assembly process, such as surface mount, pin in paste, etc. Also, the footprints take into consideration all of the parameters in relation to board materials, board thickness, transmission line structure and plating materials.
A New Concept
By analyzing all these parameters and the traditionally available options, Rosenberger has been able to develop a totally new concept in coaxial board-to-board connectors — the Flexible Microstrip Connector series. The basic idea behind this series was to avoid an expensive slotted contact with all its mechanical and electrical restrictions. Other significant design criteria included ensuring that the angular and radial displacement should not be limited by the center contact only and that a floating element must be used to compensate for the axial displacement.
Other major considerations were that the transition from planar to coaxial should be carried out in an impedance controlled environment and that the connectors’ mating surface should mechanically and electrically match the standard footprints of existing connector series, such as SMP, Mini SMP and others on the market.
The FMC in general follows the principle of the SMP board-to-board connectors. On one side there is a smooth bore outer contact to allow the axial displacement. On the adjacent board is a semi-detent outer contact designed to keep the coaxial bullet in place. The configuration is shown in Figure 2. The pin and socket center contact that is incorporated in traditional designs has also been eliminated. Here, the center contact of the PCB connector is a spring-loaded metal strip, which connects to the board’s stripline in an impedance-controlled manner, as shown in Figure 3.
The other end of this metal strip connects to the ball-shaped end of a ‘barrel’s’ center contact. This end of the metal strip incorporates a surface dimple to enable a multi-contact point, low resistance transition. As the spherical end of the center contact is ball-shaped it means that the barrel can accept displacement in angular and radial directions without damaging a pin or a contact finger.
In the axial direction, the longitudinal tolerances are equalized by the movement of the metal strip inside the board connector, while the center contact has some limited float relative to the dielectric isolation and support (see Figure 4). The net effect is that both sides of the barrel are constantly positioned with an equal force created by the pressure exerted from the spring-loaded metal striplines.
For improved impedance matching, the flexible metal strip is positioned inside a cylindrical metal outer contact. The electrical structure is a quasi-coplanar line and the electromagnetic fields are distributed between the metal strip and the ground surfaces of the cylindrical bore of the outer contact. This feature guarantees high repeatability, since there is no variation in the geometry of the center contacts and the metal strip’s mating plane.
The contact area is similar to the existing SMP structure and, with a properly designed footprint, will provide an optimum frequency response up to 20 GHz. The Flexible Microstrip Connector is designed for frequencies of up to 10 GHz, but still has good performance up to 20 GHz, while connectors featuring the FMC principle for higher frequencies are in development.
The technological development of the FMC is not only practical but the innovative design of the center contact mating structure also has a significant, positive impact on the manufacturing cost of the connectors. In particular, the flexible microstrip of the board connector is produced with a one step precision molding and stamping process, and the spherical ends on the barrel’s center contact are manufactured via a micro-forming process. Both of these processes guarantee a high level of precision and low manufacturing costs in high volume production.
The Rosenberger Flexible Microstrip Connector matches the very good performance of pin and socket type connectors, such as the SMP and Mini SMP, while also offering a cost-effective solution that eliminates some of the mechanical and electrical restrictions of other types of board-to-board coaxial connectors. The FMC easily matches the performance of other connectors up to 10 GHz frequency at a significant reduction in manufacturing and application costs.
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