What happens when connector technology outpaces military specifications? Methodologies and tests must be modified to fit—outside the box.
By David Koenig
As the name implies, the RC Series stackable compliant connector was designed for use in applications where parallel printed wiring boards (PWBs) are "stacked" on top of each other. Customers are often compelled by the RC Series' unique features to specify the connectors in military applications—for example, a missile application to stack nine printed circuit boards without any soldering. Unfortunately, the same special features of the RC connectors that customers require are too new to be addressed by current military specifications. These critical differences in the connector design determine the relevance of MIL-Spec requirements, as well as the type and sequence of tests to be performed.
The RC connectors have solderless, press-fit, compliant terminations that electrically connect the contacts to the PWB on which the connector is mounted. Extending beyond the press-fit solderless compliant portion of the termination is a solid, round pin intended to mate with the receptacle portion of another identical connector on the adjacent PWB. This allows the electrical signals to be transmitted directly from the first board to the second board without being routed to the edge of the PWB, through a backplane, and back to the center of the second PWB.
Thus, the RC Series connectors do not come in separate "plug" and "receptacle" versions. Instead, a single RC connector acts as both a plug and receptacle connector with a round pin "plug" contact at one end, a female socket "receptacle" contact at the other end, and a solderless, compliant PWB termination in the middle.
The contact used in this connector series has a dual-beam-style socket with an "eye of the needle" compliant section, and a round pin at the end opposite the receptacle end (see Fig. 1). A high-speed metal stamping press creates a "stamped and formed" contact and coins it to form a fully rounded tail during the stamping process.
To accommodate applications with different stacked board heights, the stamping tool is built to allow easy modification of the round tail to standard or custom lengths. Following stamping, the contacts are plated with hard gold over nickel.
Made from a special high-conductivity copper alloy, the contacts have low contact resistance. This copper alloy is ideal for its stress relaxation characteristics. As a result, it provides long-term stability at continuous operating temperatures up to 125°C. The contacts are plated in accordance with the gold thickness requirements of Mil-C-55302E (50 µ" minimum gold over nickel). The connectors have two versions with different contact pitch: 0.075" and 0.100". The male pin portion of the 0.075" contact has a nominal diameter of 0.0236" and is rated at 3 A while the pin portion of the 0.100" contact is 0.030" in diameter and rated at 5 A. The respective socket portions of the contacts are sized to mate with the appropriate pins.
Testing methodology
An independent connector-testing laboratory performed a series of product verification tests on the RC Series connectors. These tests were intended to closely parallel the qualification test sequence of Mil-C-55302, but some of the tests and requirements either did not apply or had to be modified to apply to these particular connectors. For example, because the pin section of some of the RC connectors is so much longer than a standard Mil-C-55302 dip solder terminal, the contact resistance of long-pin RC connectors can exceed the Mil-C-55302 requirements.
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Another example is oversize pin exclusion, a design requirement of Mil-C-55302 that does not to apply to the RC connectors. The socket contacts in most conventional Mil-C-55302 connectors would be severely damaged by insertion of a pin 0.005" larger in diameter than the intended mating pin. Thus, the Mil-C-55302 specification requires larger pins be prevented from entering the socket. The socket contacts of the RC connectors, however, are designed to withstand insertion of an oversized pin.
 Figure 2. Selectively loading contacts into an insulator allows for routing of signals through boards. |
The RC connector samples were subjected to—and passed—a battery of 19 evaluations (see Table). The tests included contact resistance, low-level contact resistance (LLCR), dielectric withstanding voltage at sea level, dielectric withstanding voltage at high altitude, insulation resistance, compliant-pin retention force (CPRF), and compliant-pin insertion force (Ins. Force).
Conditioning tests measured vibration, shock, salt spray, thermal shock, humidity, durability, and temperature life (T-life). Other miscellaneous tests included engagement force, separation force, mating force, unmating force, and plated-thru-hole integrity.
 Figure 3. Stackable RC complaint connectors offer the user a reliable solderless interconnection system for board-stacking applications. |
In addition to testing the connectors themselves, the press-fit, solderless compliant termination was individually subjected to a separate sequence of tests based on a combination of the qualification test requirements of Mil-C-55302 and those of Mil-Std-2166 ("Connectors, Electrical, Compliant Pin," which was cancelled in June 1999 and not replaced).
Shock and awe
The test program for the compliant pin consisted of four sequences of tests, identified as Groups A, B, C, and D. Test samples in all four groups consisted of individual contacts, which were pressed into appropriately sized plated-thru holes (PTHs) in FR4 (glass-enforced epoxy) PWBs.
The samples in Group A were exposed sequentially to vibration, shock, and salt spray. Low-level contact resistance was measured before and after each exposure, and compliant-pin retention force was measured at the end of the test. This sequence of tests is intended to parallel the "Subgroup 2" series of tests in the Mil-C-55302 qualification test plan (Mil-C-55302 Table V).
The samples in Group B were exposed sequentially to thermal shock and humidity. Low-level contact resistance was measured before and after each exposure, and CPRF was measured at the end of the test. This sequence of tests is intended to parallel the "Subgroup 3" series of tests in the Mil-C-55302-qualification test plan (Mil-C-55302 Table V). For these two groups, the resulting measurements showed no damage from exposure to vibration, shock, thermal shock, humidity, or salt spray. The LLCR consistently returned values well under the maximum of 1.0 mΩ set by the Mil-C-55302.
The samples in Group C were exposed to the T-life test. Low-level contact resistance was again measured before and after high-temperature exposure, and CPRF was measured at the end of the test. The T-life test is not part of the qualification inspection test sequence in either Mil-C-55302 or Mil-Std-2166. This test was added to the test sequence because Mil-C-55302 rates qualified connectors for continuous operation at 125°C, but nowhere in the qualification test program are the connectors ever exposed to 125°C for any significant period of time. The test showed no damage after a period of heat exposure. The LLCR was within the 1.0-mΩ maximum, with a measured resistance of <0.39 mΩ for the 0.100" Series, and <0.46 mΩ for the 0.075" Series.
The samples in Group D were exposed to the plated-thru-hole integrity test (hole conditioning). Low-level contact resistance and CPRF were measured after each insertion of a new pin into the PTH. The cross-sections of the plated-thru holes (with contacts installed) were examined for damage using a microscope following the insertion of the third pin. This test is very similar to the Subgroup 1 test sequence in the "Qualification Testing Requirements" as shown in Table III in Mil-Std-2166. Results for Group D also met requirements for this test sequence, and no damage was sustained to the sample.
All tests performed on the RC contacts and connectors were compiled in two reports by the independent testing laboratory, with a "summary report" document that is intended for connector end-users. The integrity of verification testing on critical features of a new connector is of value particularly when current military specifications are not available. Verification tests should be designed with the end-use environment in mind even if it means surpassing the requirements of any applicable MIL-Spec.
DAVID KOENIG is R-series product manager at AirBorn, 215 Royal Drive, Georgetown TX, 78626. Tel: (512) 863-5585; email: koenigd@airborn.com.
