Connector Testing EIA 364D Test Group 3 –
The house is only as strong as the foundation
Test group 3 of the EIA 364D test sequence is intended to directly assess the stability of the polymer used in the manufacture of the connector housing. The development of so-called engineering polymers over the past few decades has resulted in families of materials that have electrical properties far in excess of those needed in typical connector applications. In that sense, this test group can be looked upon as a check to ensure that those outstanding properties have not been significantly degraded during the molding of the connector housing.
The main functions of a connector housing are to:
· Electrically insulate the individual contacts from one another
· Maintain the desired contact spacing
· Provide mechanical support for the contact
· Provide environmental shielding of the contacts from the operating environment
Test group 3 is directed to the first function, electrical insulation. Before addressing this issue, a few general comments on the other functions are appropriate.
Maintaining the contact spacing is clearly an important function and, arguably, increases in importance as the contact spacing decreases. This function is influenced by the basic dimensional stability of the polymer as a function of stress and temperature, and by the ability to control the shrinkage of the polymer during molding.
Mechanical support for the contact may be important in at least two ways. The contacts may be latched into the connector housing in a variety of means, e.g. press-fit or retention latches, most of which will rely on the deformation and elastic recovery of the polymer. The mechanical characteristics of a polymer can be degraded during the molding process.
Environmental shielding is a purely geometric effect independent of polymer characteristics. It is well known that the shielding of the contact interface by the connector housing is a major factor in reducing corrosion-related degradation of connectors in harsh environments. This issue is worth mentioning as a reminder to connector designers while developing new connectors, and connector users in selecting an appropriate connector for harsh environments. The effects of harsh environments on connector performance will be addressed in more detail when EIA 364D Test Group 4 is discussed.
The sequence for Test Group 3 is shown at left. The full EIA 364D test sequence is included in the first article in this testing series, Testing Programs. The two measurements included in the sequence are insulation resistance (IR, TP21) and dielectric withstanding voltage (DWV,TP20). The two exposures are thermal Shock (TS, TP32) and humidity (TP31).
Insulation resistance is a composite quantity because it depends on both the surface and volume resistivities of the polymer. These properties are intrinsic to the polymer and are generally verified in specification or qualification testing of the base polymer resin. ASTM D257 covers testing procedures for these measurements. The tests differ in the electrode geometry, but are both generally tested under a 500 volt load applied for 60 seconds. Dry as molded surface and volume resistivities for typical connector housing materials exceed 1013 ohms and ohm-cm, respectively. While the voltage and duration of application are open to specification in TP21, 500 volts applied for 60 seconds is a default condition.
Dielectric withstanding voltage is a hybrid measurement and is dependent on an intrinsic property of the polymer, the dielectric strength, and the spacing of the contacts in the subject connector. The dielectric strength of a polymer is given by the magnitude of the electric field, in volts/mm that causes an electric arc to pass through the sample. Dielectric strength measurement procedures are covered in ASTM D149. Dielectric strength values for typical connector housing materials are expressed in Kv/mm. It should be noted that the value of dielectric strength for a given polymer increases as the sample thickness decreases. Because of this thickness dependence, the DWV of a connector will vary with the contact spacing.
Dielectric strength testing is generally done under DC conditions. DWD testing per TP20 allows both AC and DC voltages. The specified voltage is applied between adjacent contacts, or contact and shield, and held for 60 seconds. The connector passes the test if no arcing occurs under the specified conditions.
The two exposures, thermal shock (TP32) and humidity (TP31), are legacy exposures carried over from earlier test programs. Thermal shock serves as a monitor for degradation of mechanical properties of the polymer due to residual molding stresses, or polymer degradation. Humidity exposures allow for moisture absorption by the polymer and, thus, moisture-related degradation.
As with Test Group 2, discussed in a previous article, these exposures, while they simulate application conditions, cannot be straightforwardly related to a connector’s lifetime in the field. Test Group 3, therefore, is also a comparative test against the field history performance of connectors of similar materials and design.
In the next article in this series, Max Peel, Fellow of Contech Research, will provide his practical perspective on Test Group 3 of EIA 364D.
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