Mixed Flowing Gas (MFG)—A Corrosion Oriented Test
MFG—A Corrosion Oriented Test
Dr. Bob outlined a harsh environment test called mixed flowing gas (MFG) in his last article. Here’s my take on that subject.
The MFG test was a sponsored program initiated by the Battelle Institute and funded by over 70 companies from the private sector. Interest in this topic was high. This was due to the total dissatisfaction with the singular gas (H2S or SO2) test and the dual gas tests (H2S in combination with SO2), which resulted in connectors failing in the field, even though they had passed these two tests. And yet, connectors which failed these tests were operating in the field successfully. Thus, a total loss of confidence in the singular or dual mixture test resulted, as well as a confidence in corrosion testing in general.
For most private sector companies, salt spray was considered an inadequate test, except for marine applications. There were no accelerators for the salt spray test or the singular gas or dual gas system. Therefore, the demand for a meaningful corrosion test was becoming a high priority.
The evolution of the MFG test and accelerator was based on data generated allowing the evaluation of the following:
- Determination of concentration of key elements in the atmosphere
- Evaluation of samples using the base metal and finish systems commonly being used in the field
- Evaluation of the film and oxide formulation on the samples used in field sites
- Oxide and film growth rates
- Number of field sites—over 100 located in the U.S., Europe, and Far East
The test was established based on the above inter-relating factors, and most importantly, the establishment of accelerators. This program was performed from the early ‘70s to the late ‘80s. A key number of papers were presented discussing technical aspects of the work during this period of time, eventually leading to the establishment of ASTM test procedures, as well as the EIA 364, Test Procedure 65.
The telecom industry was the first major industry to adopt this test in the qualification testing. It has also been adopted by the computer (all variations) segment, and is being gradually adopted by the automotive industry as well.
Validation protocol of the tests is determined by the weight gain technique using control coupons, which are placed in the chamber at the same time as the samples under test. The corrosion rates established for validation of the test are as follows:
Class IIA : 12-18 micrograms/cm3
- Class IIIA: 36-46 micrograms/cm3
The common test procedures require two control coupons per test. However, five coupons are recommended, rather than two, which allows one to assure that the gases are being distributed as equally as possible throughout the test chamber, including the stabilization aspects.
Another key factor is to control humidity. Past studies have shown that the corrosion rates are significantly affected within the specified humidity tolerance range when all other elements are being held constant (as much as 2X within Class IIA and 4X in Class IIIA). This has resulted in the establishment for controlling humidity to ±1 percent, rather than the allowed ±2 percent. If the corrosion rates are not being maintained, then the flow rates have to be adjusted until the control corrosion rates are within the limits established.
This also dictates that understanding the interactions of the equipment being used becomes a key factor for a successful test to occur. Thus, dedicated personnel are important for consistency between tests to be created.
Another important feature is that the test chambers have to be placed in its own operating area to minimize the impact of room ambient conditions. For example, simply turning the lights “on” and “off” impacts the humidity, which in turn impacts the weight gain, that establishes the validation of the test.
Exposure to this environment in qualification testing is performed in its own sequence. This, in combination with durability levels of the application, thickness, and plating coverage/integrity will establish the susceptibility to potential electrical stability issues. The specific purpose of this test is as follows:
MFG tests are environmental test procedures whose primary purpose is to evaluate product performance under simulated storage or operating (field) conditions. For parts involving plated contact surfaces, such tests are also used to measure the effect of plating degradation (due to the environment) on the electrical and durability properties of a contact or connector system. The specific test conditions are usually chosen to simulate, in the test laboratory, the effects of certain representative field environments or environmental severity levels on standard metallic surfaces. It is specifically used to determine the susceptibility of contact interfaces to the corrosion process. It will establish, if the following faults exist, within the contact area:
Pore corrosion (porosity)
Edge effects (gold and nickel platings tend to thin-cut at the edges of the contacts)
Tendency for edge creep (Class IIIA predominate)
There are a number of variants used when MFG tests are specified.
If durability is a factor prior to use in the application, the applicable number of mating/unmating cycles are performed prior to exposure.
If the application can result in connectors being unmated for “long” periods of time (such as Option Card Capability), then a specific unmated exposure interval is also established.
Disturbance tests (mechanical or thermal) are normally performed at the end of the sequence.
If a connector is expected to be mated initially, and never unmated during its anticipated field life, then the initial durability and unmated interval may be deleted.
Special Note: If durability is a factor, tin (alloy) plated materials have to be exposed to the MFG test. This is to assure that base metal or underplates have not been compromised to the extent that corrosion will occur, which is detrimental to electrical stability or field life.
The variable measurement used for evaluation purposes during this test is low-level circuit resistance (LLCR), namely change in resistance, which determines the stability of the system. The “sheltering” effect can also have an impact. If one can visualize that the MFG can flow through a connector (or mated pair), then the interface can, or will be, exposed to the mixture. If air flow is prevented, then it will not likely be exposed. Receptacle connectors—where contacts are recessed in their individual cavities within the housings—are not as prone to exposure of the corrosive elements. Header or plug connectors, where contacts are unprotected, can be directly exposed to the MFG atmosphere. Experience has shown that unmated header and plug-style connectors are where corrosion occurs, if faults do exist.
For a 10-day or 20-day exposure, it is normal to perform variable measurements every five days. Experience has shown that corrosion products start to be visible after four or five days. Very rarely will corrosion products be visible less than four days. Also, after every measurement interval, new test coupons are placed into the chamber for revalidation purposes.
Plating thickness, coverage, and integrity are also key factors for passing this type of sequence. Gold flash contacts generally have gross porosity and have a high probability of failing this type of test, particularly when pre-exposure durability and an unmated interval are specified.
Also, since location of pore sites are important in establishing the probability of passing or failing, a basic nitric vapor porosity test can be performed on material to be submitted for test. If there is gross porosity, or porosity located in or at the end of an anticipated wear track, this is a good indicator of a high probability for failure. This is particularly important for those who may not have any experience with this test. This is a good sanity check.
One final point, there have been several instances where this test has correlated failures to the actual time when field failures occur, thus this tends to confirm the accelerator factors as established.
This test is gaining in popularity. This is particularly true as applications are increasing where corrosion is of a particular concern. This can also be a valuable tool to determine if the magnitude of porosity and its location will be problematic, or not. This test, if used properly, can be viewed as a performance-oriented porosity test.
By Max Peel, Senior Fellow, Contech Research
- Nanocrystalline Silver Alloy Contact Finishes in Electronic Applications - April 6, 2015
- Nanocrystalline Silver Contact Platings - March 16, 2015
- Dr. Bob on Gold Flash Contact Finishes (and Max Peel) - September 22, 2014