Analysis of the Causes of Poor Connector Detection

I. Introduction

Whether it is a high-frequency electrical connector or a low-frequency electrical connector, the insulation resistance, dielectric withstand voltage (also known as electrical strength), and contact resistance are the most basic electrical parameters to ensure the normal and reliable operation of electrical connectors. Usually, in the quality conformance inspection of the product technical conditions of electrical connectors, there are clear technical index requirements and test methods listed in the routine acceptance inspection items of Group A and Group B. These three inspection items are also important bases for users to judge the quality and reliability of electrical connectors. However, according to the author's practice in electrical connector inspection over the years, it has been found that there are still many inconsistencies and differences among various manufacturers and between manufacturers and users in the specific implementation of relevant technical conditions. Often, due to differences in factors such as the instruments used, test fixtures, operation methods, sample processing, and environmental conditions, the accuracy and consistency of inspections are directly affected. Therefore, the author believes that it is very beneficial to conduct some special discussions on the current three routine electrical performance inspection items and the problems existing in actual operations to improve the reliability of electrical connector inspections.

In addition, with the rapid development of electronic information technology, a new generation of multi-functional automatic inspection instruments is gradually replacing the original single-parameter test instruments. The application of these new test instruments will surely greatly improve the detection speed, efficiency, accuracy, and reliability of electrical performance.

II. Insulation Resistance Inspection

Working Principle
Insulation resistance refers to the resistance value presented when a voltage is applied to the insulating part of a connector, causing leakage current to be generated on the surface or inside of the insulating part. That is, insulation resistance (MΩ) = voltage applied to the insulator (V) / leakage current (μA). Through insulation resistance inspection, it is determined whether the insulation performance of the connector can meet the requirements of circuit design or whether its insulation resistance complies with the provisions of relevant technical conditions when subjected to environmental stresses such as high temperature and humidity.
Insulation resistance is a limiting factor in the design of high-impedance circuits. A low insulation resistance means a large leakage current, which will disrupt the circuit and normal operation. For example, if a feedback loop is formed, the heat generated by excessive leakage current and direct current electrolysis will damage the insulation or degrade the electrical performance of the connector.
Influencing Factors

It is mainly affected by factors such as insulating materials, temperature, humidity, contamination, test voltage, and the duration of continuous application of the test voltage.

Insulating Materials

It is very important to select which insulating materials to use when designing electrical connectors, as it often affects whether the insulation resistance of the product can be stably qualified. For example, a factory originally used phenolic glass fiber plastic and reinforced nylon materials to make insulators. These materials contain polar groups and have high hygroscopicity. Their insulation performance can meet the product requirements at room temperature but is unqualified under high temperature and humidity conditions. Later, special engineering plastic PES (polyphenylene ether sulfone) materials were adopted. After the products underwent 200 °C, 1000h, and 240h humidity tests, the change in insulation resistance was relatively small and remained above 105 MΩ without abnormal changes.

Temperature

High temperatures can damage insulating materials and cause a reduction in insulation resistance and withstand voltage performance. For metal shells, high temperatures can cause contact parts to lose elasticity, accelerate oxidation, and cause plating deterioration. For example, for the environmentally resistant quick-disconnect electrical connector series II products produced according to GJB598, the insulation resistance is specified to be not less than 5000 MΩ at 25 °C, but it decreases to not less than 500 MΩ at 200 °C.

Humidity

A humid environment causes the attraction and diffusion of water vapor on the surface of insulators, easily reducing the insulation resistance to below the MΩ level. Long-term exposure to a high-temperature environment can cause physical deformation, decomposition, escape of products, generation of breathing effects, electrolytic corrosion, and cracks in insulators. For example, for the ribbon cable electrical connectors produced according to GJB2281, the insulation resistance value under standard atmospheric conditions should be not less than 5000 MΩ, but after a 90% - 95% relative humidity, 40 ± 2 °C, 96h damp heat test, the insulation resistance drops to not less than 1000 MΩ.

Contamination

The cleanliness of the inside and surface of insulators has a great impact on the insulation resistance. Because impurities are mixed in the powder used for injection-molded insulators or in the adhesives for bonding the upper and lower insulating mounting plates, or because metal scraps remaining from multiple insertions and withdrawals and fluxes remaining from soldering terminations penetrate the surface of insulators, the insulation resistance will be significantly reduced. For example, during the finished product acceptance test of a circular electrical connector produced by a factory, it was found that the insulation resistance between the contact parts of one product was very low, only 20 MΩ, which was unqualified. Later, through anatomical analysis, it was found that this was caused by impurities mixed in the powder used for injection-molded insulators. As a result, the entire batch of products had to be scrapped.

Test Voltage

The test voltage applied during the insulation resistance inspection has a great relationship with the test results. Because when the test voltage increases, the increase in leakage current is not in a linear relationship, and the rate of current increase is greater than the rate of voltage increase. Therefore, when the test voltage increases, the measured insulation resistance value will decrease. In the test methods cited in the technical conditions of electrical connector products, there are clear regulations on the test voltage, which is usually specified as 500V. Therefore, general ohmmeters, direct current bridges, and other resistance-measuring instruments cannot be used to measure insulation resistance.

Since there is a certain capacitance between the measuring electrodes of the tested electrical connector, the power supply needs to charge the capacitor first at the beginning of the measurement. Therefore, during the test, there is often a tendency for the resistance value indicated on the insulation resistance tester to gradually increase, which is a normal phenomenon. In many electrical connector test methods, it is clearly specified that the reading on the insulation resistance tester must be taken 1 minute after the voltage is applied.

Problem Discussions

Influence of Inspection Environmental Temperature and Humidity

The technical conditions of electrical connectors usually specify the operating environmental temperature and humidity of the products, such as a temperature range of -55 to 125 °C and a humidity of 40 ± 2 °C, 95% ± 3%. The author believes that there is a difference between inspection environmental conditions and operating environmental conditions. The fact that the technical conditions stipulate that the product can work under the above temperature and humidity operating environments does not mean that the manufacturer should meet the assessment indicators under normal atmospheric pressure when testing the insulation resistance under the above operating environmental conditions. If the insulation resistance is measured under the environmental conditions of an upper operating temperature of 125 °C and a 40 ± 2 °C, 93% ± 3% damp heat environment, it should be assessed according to the assessment indicators of the high-temperature and damp heat environment tests specified in the technical conditions, rather than the assessment indicators under normal atmospheric pressure.

The author has found in actual inspections many times that the same batch of products had an insulation resistance greater than 1000 MΩ during factory inspections under the relatively dry climate conditions in the north (humidity < 50%) and was qualified. However, when the products were shipped to the user in the south and reinspected under the relatively humid environment (humidity > 80%), the insulation resistance was only 100 MΩ - 200 MΩ, which was unqualified. In such cases, sometimes after cleaning with alcohol and drying, the inspection was qualified immediately after taking them out, but when retested the next day, they were unqualified again. Therefore, it is recommended that manufacturers control the insulation resistance at an appropriate level above the specified value during product acceptance tests and maintain a certain margin. Do not judge products that barely reach the specified value under a dry environment as qualified for shipment to avoid disputes caused by inconsistent inspection results due to different inspection climates and environmental conditions between the supplier and the demander.

To clarify the requirements for inspection of environmental temperature and humidity, some test methods now specify both the environmental temperature and humidity for testing (a relatively wide range) and the temperature and humidity requirements for arbitration in case of differences (a relatively narrow range in the middle). For example, GJB1217-91 "Test Methods for Electrical Connectors" stipulates that the standard atmospheric conditions for testing are a temperature of 15 - 35 °C, a humidity of 20% - 80%, and an air pressure of 73 - 103 kPa. The standard atmospheric conditions for arbitration testing are a temperature of 25 ± 1 °C, a humidity of 50% ± 2 °C, and an air pressure of 86 - 106 kPa.

Influence of Inspection Fixtures

The technical conditions of electrical connectors stipulate that the insulation resistance between all contact parts and between all contact parts and the housing of electrical connectors should meet the specified values. It is also stipulated that the duration of the applied voltage should be greater than 1 minute. Therefore, many electrical connector manufacturers have 2 - 3 inspection fixtures with different connection arrangements (pin-hole mating socket-pin fixtures or pin-socket mating hole fixtures) for each model and specification of products they produce. By applying the test voltage in parallel between the contact points, between the rows, and between all the contacts and the housing, it is inspected whether the insulation resistance is qualified. Applying the voltage in parallel with the inspection fixtures is a more stringent condition than applying the voltage between individual contacts. Therefore, if it is found that the insulation resistance is unqualified when tested with the inspection fixtures, it is allowed to directly use the test probes connected to the insulation resistance tester to apply the voltage between individual points for retesting. However, some existing manufacturers and the vast majority of users do not use inspection fixtures but directly use two test probes connected to the insulation resistance tester to bridge between each contact part or between the contact part and the housing to inspect whether the insulation resistance is qualified. This method of using different inspection fixtures has the following disadvantages: First, it has great randomness and is very likely to cause missed inspections. Second, each contact point can't read the value after staying for 1 minute as with the inspection fixtures, so it may cause misjudgment and the reliability of the inspection is poor.

Of course, even when using inspection fixtures, it is necessary to ensure that the fixtures are qualified before inspection. It is necessary to ensure that the fixtures are clean and dry, and their own insulation resistance must be qualified and with sufficient margin.