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Micro SMT Connector Combines Old and New Technology
December 31, 1969 |Estimated reading time: 4 minutes
By Mark Leach and Ann Cibelli, Advanced Interconnections Corp.
In this case study, a 1.0-mm-pitch SMT connector is replaced by a new connector with the same footprint that surpassed shock/vibration, thermal shock, cyclic humidity, high-temperature life, and MFG tests. The connector had to be low-profile and lead-free for a telecom application.
The case study follows a telecommunications/data market assembly that needed a high reliability micro surface mount connector. The assembler was using a 1.0-mm-pitch SMT connector, as specified by the Optical Internetworking Forum, to provide power to a tunable transmitter assembly through a flex-circuit cable. The specified connector did not pass the 20-day mixed flowing gas (MFG) test, however, which this OEM required.
In addition to the criterion of the connector passing the MFG test, the connector interface had to match the existing application in fit, form, and function. The industry specification called for a lead-free, 14-position, dual row surface mount design on 1.0-mm pitch. Heavy gold plating was specified for telecommunication hardware metallurgical compliance. Presenting further challenges were a mated height restriction of approximately 4.0 mm as well as X and Y dimensions of 7.36 and 3.81 mm, respectively.
Based on screw-machine technology and proprietary manufacturing processes already field-proven, a new, small surface mount connector was designed and produced in a 12-week timeframe (Figure 1).The Building BlocksDue to the success of lead-free 1.0-mm-pitch BGA socketing systems, the design team selected an existing screw-machined terminal assembly (pin-in-socket) featuring an integral multi-finger beryllium/copper contact. The connector has screw-machined terminals for both the header and receptacle (socket) at the contact interface to provide superior reliability. Screw-machined from brass (copper alloy), the terminals exceed stamped-and-formed alternatives in durability and long-term performance. The terminals are plated with 30 µinches of gold over a nickel underplate to meet severe environmental test requirements.
The connectors were put through a battery of mechanical and environmental tests, including shock/vibration, thermal shock, cyclic humidity, high-temperature life, and MFG. The MFG test subjects connectors to a severe environment for 20 days, after which the parts are checked for low-level contact resistance. The gas mixture used in the test is based on actual field data simulating typical severe environments. Electrical resistance is checked using low-level current and voltage to ensure that the electrical test current does not artificially break through any oxidation layers that have formed on contact surfaces due to the test environment. This test is one of the most severe in the connector industry, targeting the most reliable connector end products.
Thanks to the durability of screw-machined terminals with multi-finger contacts and heavy gold plating, the SMT connector design passed the 20-day MFG test without concerns, making it suitable for high-reliability applications such as telecommunications, data storage, and medical use.
Flexible PCB InterfaceThe next consideration was the board interface. The connector system had to match existing board layouts. To do so with a robust solder joint at the board level, each terminal was mechanically attached to a leadframe using a cloverleaf interference joint (Figure 2). For added reliability, the cloverleaf joint was soldered using a convection reflow soldering oven.
While the screw-machined terminals provide a rigid, robust interface, the lead-frame solder tails provide flexibility to absorb any movement during PCB solder attachment due to coefficient of thermal expansion (CTE) differences between the connector and the board. The leadframe terminals are plated with matte tin, conforming to RoHS and other lead-free manufacturing regulations.
Heat ResistanceTo meet the higher processing temperatures required in lead-free solder reflow profiles, which typically peak at up to 260°C and have continuous operating temperatures of up to 125°C, the design team selected a prequalified insulator material used successfully in BGA sockets and board-to-board connectors. The greatest challenge was the connector's small size, so a new mold design, using precision injection molding equipment, was used to create a low-profile insulator from liquid crystal polymer (LCP). Under lean manufacturing and vertical integration structure, the manufacturing was performed in America to meet process and quality control requirements.
Because of the connector's small size and fine pitch, the leadframes are overmolded with high-temperature plastic to eliminate stresses due to mechanical insertion and to improve finished part strength. In addition to durability, the overmolded lead frame seals the surface mount pins, preventing solder wicking and ensuring a secure solder joint at the PCB level. Close attention must be paid to the design of the mold and its runner system to ensure that all the intricate features of each part are filled adequately. Each overmolded part requires only about one pellet of molding material. Process control is critical and the molding press selected was designed specifically for insert molding.
Tape-and-reel Packaging Whether providing power to an optical transceiver or tunable laser or connecting a mezzanine board to a motherboard, the low-profile SMT connector system devised for this OEM is suitable for any high-reliability application. For protection during storage and transit, and to facilitate automated pick-and-place assembly, the connectors are packaged in tape-and-reel packaging. By combining both old and new components and manufacturing process technologies, the high-reliability connector system was designed and produced on a new micro scale.
Mark Leach, engineering manager and Ann Cibelli, director of communications, Advanced Interconnections Corp., may be contacted at acibelli@advanced.com.