“A Technical Review of the Problems Encountered in Leak Testing Small Cavity Ordnance Devices”

George R. Neff* and Jimmie K. Neff**

IsoVac Engineering, Inc., Glendale, California

Abstract

Military Ordnance Devices such as Detonators, Fuzes, Squibs, etc., are usually intended to be hermetic. They are tested for hermeticity, using the same tests we have always used, and then assumed to be sealed. The Military Standards and various “Derived-Procedures” used for these leak tests are widely “Misunderstood” and “Misapplied”, and as a result are not rejecting all of the non-hermetic devices tested. This is largely due to the lack of understanding the seal testing technology used. It is also due to the use of procedures that have not been changed in over twenty years. This paper specifically addresses the issues that are responsible for improper seal testing of ordnance as well as electronic devices. It is also found that many failed ordnance devices are ‘discarded’, ‘replaced’, and go unreported, leaving us with very poor statistical data. Although very commonly used for testing ordnance devices, MIL-STDs 883 (T/M 1014)(1) & 750 (T/M 1071)(2) are for “Microelectronics” & “Semiconductors” respectively. They are not written for the leak testing of ordnance devices. The Defense Supply Center recently investigated their MIL-STD-750 seal testing standards, which are used as one of the criteria to guarantee an “assumed 15 year life” for the devices they purchase. That investigation clearly showed that the leak testing specifications were not guaranteeing internal device environments for even one year for many small cavity devices. As a result of these findings, MIL-STD-750E T/M 1071.8 has just been tightened to 1 x 10-9 atm cc/sec (air) for small cavity devices, and many of those small-cavity electronic devices have cavities ‘orders of magnitude’ larger than many compressed-charge ordnance devices. This paper addresses the many difficulties encountered in leak testing small cavity ordnance devices. It explains the practices that are allowing those escapes, as well as the state-of-the-art technology and procedures that are now being used to prevent those non-hermetic escapes.

I. Introduction

We are commonly encountering the conflicts involved in the leak testing data that is produced from the leak testing of common ordnance devices. It has been seen that data from two or more facilities running the same devices, is widely spread, especially when individual test methodologies have been employed. The most difficult problems are due to the wide variation in the procedures applied, the optional modifications in those procedures, and the total lack of specificity by the procuring body in calling out precise test methods to be followed. This, unfortunately, is the result of the ‘chain-of-people’ involved not being adequately informed of the technology involved in leak testing, and the weaknesses of the test procedures that have been allowed. Devices such as semiconductor devices, hybrid-electronic devices, and ordnance devices such as detonators, fuzes, and squibs, are always procured with the assumption they have some degree of hermeticity. Most procurement specifications are very weak in the callouts for hermeticity requirements. They are rarely specific as to the MIL-STD, Test Method, and Procedure to be used, and many do not call out the exact leak rate limit required. The electronics industry has a well developed pattern for the level of hermeticity that they request, (although it has recently been recognized as far inadequate for the life expectancy they procured for). Steps have just been taken to rectify that problem by tightening the MIL-STDs used for the procurement and qualification of electronic devices. _______________________ * President, 614 Justin Avenue, radiflo@aol.com **Vice President Technical Director, 614 Justin Avenue, radiflo@aol.com

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44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit21 - 23 July 2008, Hartford, CT

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Copyright © 2008 by IsoVac Engineering, Inc. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

II. Background

The ordnance industry has been allowed to use procedures that they have used for decades to leak test devices. It is realized that in many applications, the hermeticity of the ordnance device is not a critical issue. However, that has in many cases caused the hermeticity concerns to be relaxed on devices that should be hermetic to a rather high sensitivity. Ordnance devices are found to be stored in a variety of environments, some of which can be very deleterious to the materials used in the device. The internal cavity size relative to the leak rate of the device is critical to the maintenance of the environment the internal materials will be exposed to. The DSCC has recently reviewed and been shocked at the very rapid rate of exchange of atmosphere within a device due to the inadequate level of hermeticity they required(3). That review has shown that the leak test sensitivity being used was allowing internal environments to be at equilibrium with the atmosphere in less than one year in many cases, when they were procuring for a fifteen year life. There is about 20% oxygen in the atmosphere and,(depending on the location), around 3% moisture in the air. Considering that a very large percentage of the ordnance devices we build have internal “free-volumes” of less than a 1.0 cm3 cavity, and that most of our leak test procedures are only catching leaks in the gross-leak rate range, the exchange with atmosphere is quite rapid. Internal Volume Leak Rate % Atmosphere Exposure Time Interval Cm3 atm cm3/s Exchanged 0.5 1 x 10-3 100 1 hour 1 x 10-4 51% 1 hour 1 x 10-5 7% 1 hour “ 82% 24 hours

“ 97% 48 hours 1 x 10-6 16% 24 hours “ 82% 10 days . 1.0 1 x 10-3 97% 1 hour 1 x 10-4 97% 10 hours 1 x 10-5 97% 4.2 days 1 x 10-6 97% 42 days

Table 1 Table 1 presents a sample of the short time required for devices to come to equilibrium with the environment around it. It is obvious that most devices are intended for longer lifetimes than a few hours to 42 days, but the exchange rates give us a feel for how quickly the typical ordnance package can be at total exposure to as deleterious an environment as it may be placed in. The rates of degradation obviously vary with the types of ordnance and internal materials. It is recognized that there are many technical factors to be considered relative to the survival of the ordnance materials as well as the other internal components, and the life for which they must survive. As you can see, most ordnance specifications are not guaranteeing a long life if the devices are subjected to a challenging environment. The authors are performing leak tests and failure analyses on ordnance devices that are being evaluated due to failures or anomalies in tests being run on the devices(4). Devices that fail in less than two years are seen with causes attributable to corrosion, resulting from poor hermeticity on devices that were improperly leak tested. The leak rate ranges that are called out for ordnance devices are almost always in the “Gross-Leak Range”, (those are leaks that are greater or larger than 1 x 10-6 atm cc/s). Such leaks are in the “Viscous-Flow” regime, very sensitive to pressure differential, and, having very small cavities, they can lose their helium tracer gas instantly when they are evacuated in a HMS test chamber to be measured. Note that the average HMS requires 30 to 45 seconds to evacuate the test chamber before the part can be measured.

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> 98% Grey Zone

Figure 1 Bell Distribution of device Leak Rates(1)

The “Bell Curve”(5) in Figure 1 was developed from the measurement of over a million leaking devices during the past few decades. It shows that most leakers when measured quantitatively are greater than 5 x 10-6. These large leak rates coupled with the very small cavities within the device, and the external organic materials used on so many ordnance devices, seriously complicates the leak test process. It is difficult to clearly establish true cause for failure in many failed ordnance devices. There are poor statistics on the failure of ordnance devices for the same reason being found in the electronics industry. This is the result of devices that fail:

• Either being thrown away and replaced with another unit without reports or any failure analysis • Devices failed and/or returned are usually re-tested with the same leak test procedure used when they were

first manufactured, (and that test procedure is flawed) • The parts have been subjected to fluorocarbon bubble testing which prevents the accurate measurement of

the true leak rate of the device • The devices are not subjected to any type of RGA gas analysis to determine internal constituents

III. Commonly Used Leak Test Specifications

There are several Military Standards and procedures commonly used for the leak testing of hermetic devices. The most common ones are:

MIL-STD-883G, Test Method 1014.12 Revised 2006* MIL-STD-750E, Test Method 1071.8 Revised 2006* MIL-STD-202G, Test Method 112E Revised 1988 MIL-STD-331C, Test C8 “Seal” Revised 1989 MIL-STD-1576, Test Method 1111 Revised 1984 MIL-D-21625E 1974, Rev 1993 MIL-C-83124 1969 MIL-C-83125 1969

* Currently undergoing further revisions. The Military Standards need to be reviewed to put them into the categories for which they are designed to be applied, and then consider their revision status based on current technology.

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IV. The Standards & Specifications Since all of our ordnance devices are required to have some degree of hermeticity, and since the above MIL-STDs are most commonly used, we must look at the shortcomings of these Specifications and Standards. We find many conflicts between the ‘Intent’ of the standards and the ‘Application’ of the standards. In practice we find most of these standards to be: unclear; lacking in adequate detail; in conflict with related or referent specifications; using inaccurate data for the resultant test result, etc. Looking at each of the referent specifications will point out some of the technical shortcomings that are why we are finding many defective ordnance devices. MIL-STD-883G, Test Method 1014.12 This specification is probably the most “Misapplied” of all the specs. We find it called out for every type of device throughout the world. It is written for “Microelectronics”. Much of the theory is current and many of the technical guidance is accurate. It needs to be tightened for the electronic devices it is intended to test, (but that is currently being reviewed). It however, is not intended for ordnance devices, and it does not allow bubble testing before a dry gas leak test. This spec also addresses the problems of “Surface Sorption”, a major interference with reliable leak testing of ordnance devices. It includes Helium Mass Spectrometer, (HMS), for Fine-Leak only, and three Krypton85 procedures for Gross & Fine Leak testing. MIL-STD-750E, Test Method 1071.8 This specification is written for semiconductor devices, many of which have small cavities similar to many ordnance devices. The technology is current, including the tightening of sensitivities to 1 x 10-9 atm cm3/s. This specification is recommending the elimination of the common bubble testing due to its masking of leaks that are causing non-hermetic devices to pass a leak test. This spec also addresses the subject of “Surface Sorption”. It includes HMS leak testing and five Krypton85 test procedures, covering Gross, Fine, Kr85 Dye-Penetrant, and Kr85 Thermal Testing. MIL-STD-202, Test Method 112E This specification needs revision. It states an accept limit of 1 x 10-6 atm cc/s (He). It uses a fixed table of bomb time and pressure for the package volume and dwell times that are in error for the sensitivities of most ordnance devices. If you follow the Table bombing conditions, all devices will likely fail. It does not clearly define the “Leak-Rate” callout for the package, which is the “L” value, (or leak rate in atm cm3/s (air)), but it does provide the definitions for “the standard leak rate”. It does not clarify that the reading on the HMS, (R1), is not the leak rate of the part, but the reading on the HMS as a result of whatever amount of helium you were able to get into the part. It also does not provide the necessary procedure to accommodate “Surface-Sorption”, and its effects on leak measurements. MIL-STD-331 This test method is very widely “misapplied” for ordnance devices. It needs to be updated to state-of-the-art procedures. It still includes the old “Halogen” gas procedure, which is “Environmentally Unfriendly”, and not considered for the fine leak range, and not very accurate. It still contains a procedure called a “Volume-Sharing” method, which has extreme limitation, dependent on accurate knowledge of the devices internal volumes, very accurate knowledge of the chamber characteristics, as well as readout accuracy of the instrumentation. Some specific needs to update this specification:

a) Procedures to establish the “Wait-Time” for testing devices with external organic materials b) Time limits for leak rate vs cavity size c) Precautions and guidance for testing devices with very small cavities, d) Effects of liquids masking leak test results e) Specific precautions limiting the performing of dry-gas only, before liquid bubble testing f) Include the Krypton85 Gross & Fine leak test procedures

MIL-STD-1576, Test Method 1111 This test method is based on MIL-STD-202, T/M 112. The conditions listed in Table 1111-1 for helium bombing are Flawed and in conflict with the referent Table I in 202. The same complexities described above, (for 202), apply to this specification.

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MIL- MIL-D-21625D This ordnance spec was written in the 1974,revised 1993. This specification requires hermeticity to a sensitivity of 1 x 10-5 atm cc/s (air). It references MIL-STD-331, (which calls for 1 x 10-6 atm cc/s). It calls for a 5 year life, (unless longer life is called for). A “…longer than 5 year service life will be supported by analysis or test”. If that life is based on hermeticity requirements, a much , much tighter leak rate sensitivity would be required to assure that the device is not at equilibrium with the outside environment during that life. There is no inclusion in this procedure for the “Surface-Sorption” and resultant “Wait-Time” following any of the dry-gas leak test pressurizations. MIL-C-83124 This specification was written in 1969. The specification calls for hermeticity of 1 x 10-5 atm cc/s, before and after environmental testing, using a dry gas method. It does not state the requirements for re-bombing the device in a tracer gas after environmental test. There are no definitive parameters for any device, such as cavity free volume, dwell-time or wait-time after bombing before leak measurements must be taken. No pressure/time conditions are spelled out, and no references for those parameters are given. The specification needs to be updated to modern technology and test methodology. MIL-C-83125 This specification was written in 1969. It calls for a 99.9% reliability. The hermeticity requirement is 10 cc/s at one atmosphere differential. The internal cavity volume range for these devices is not provided, which makes it impossible to predict the rate of equilibrium of the internal cavity with the atmosphere. It is not clear as to the predicted life of the device. A device with a 10 cc/s leak rate is a part with a hole visible to the naked eye, putting the device at equilibrium with the environment around it in a few seconds.

V. The Leak Testing Problems There is what is commonly called the “Grey Zone” as seen in Figure 1 above. That is the area between the finest bubble testing ranges limit of detection, and the point of the largest leak reliably detectable by the HMS process. That Zone of questionable detection was established by DESC after evaluating each of the bubble testing process demonstrated sensitivities, as well as the HMS equipment sensitivities. Some carefully applied HMS testing can cover that zone, but only for devices with adequate cavity and minimal surface sorption problems. The Kr85 test methods have always been accepted to cover that zone. The use of external organic sealants and o-rings must be handled by performing a ‘characterization test’ on the materials. The military standards for electronic devices requires this step to establish the surface-sorption of the tracer gas in those materials, and the ‘wait-time’ needed to allow the tracer gas to desorb from the outside surfaces before a pass/fail judgment can be made. With small cavities that wait time frequently allows the tracer gas to be lost before it can be measured, so the surface materials or exposed organics must be carefully chosen and characterized before the devices are subjected to a leak test. The use of fluorocarbon bubble test fluids interferes with a subsequent dry gas leak test. This is especially seen when attempting to perform a valid test using HMS. The radioisotope leak test method is found to be a usable test on bubble tested parts, since it only requires a very small amount of the Kr85 tracer gas to dissolve through the bubble fluid that plugs the leak site, and the radiation from that tracer gas is then measured right through the wall of the part. Recent Kr85 leak testing of devices that had previously been through bubble test has shown that the data obtained is skewed to indicate leak rates as much as two decades below their true value. The Defense Supply Center Columbus in charge of MIL-STD-750 has recently suggested the deletion of F/C fluid bubble testing from their specs due to this problem of leak testing devices unknowingly having been through F/C testing. It essentially places the “User” or “Purchaser” of the device in a difficult position when he tries to perform receiving inspection leak tests of such devices. Many studies have shown that those bubble tet fluids are impossible to be completely removed from the leak sites.

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The problem of leak testing devices with essentially no internal cavity, (Zero-Cavity Devices), is that there is no internal chamber in which to store the tracer gas being used to pressurize the device. This is found in many ordnance devices which have a highly compressed ignition charge as well as a loose packed output chamber. Too often the loose pack chamber gets tested for hermeticity of the output seal, but the ignition charge compressed onto a bridge wire, has no cavity to entrap any tracer gas, and leakage through defective glass to metal seals onto which the charge is compressed, goes undetected. Most of these devices are utilizing highly compressed ordnance materials which have no solubility for the tracer gases and have no interstitial cavities to entrap the tracer gases. Tens of millions of ordnance devices are constructed yearly without a cavity, (< 10-5 cm3). A very successful to overcome this lack of cavity utilizes a few milligrams of charcoal blended with the ordnance material. The charcoal adsorbs Kr85 gas and holds it for long periods of time allowing the part to easily be detected as a non-hermetic reject.(x) One such device is shown in Figure 2. Just the can with the compressed charge containing charcoal, can easily be detected as non-hermetic with just a 36 second Kr85 pressurization.

Figure 2

This photomicrograph(2) shows a highly compressed ordnance material. The header is then pressed so tightly onto the ordnance material when welded, that the bridgewire makes a distinct impression

into the ordnance material, resulting in a Zero Cavity existing within the device. The random charcoal particles seen on that surface are adequate to assure the detection of Kr85 and the rejection

of the device.

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The following photomicrographs demonstrate some unusual findings as a result of non-hermetic ordnance devices that escaped detection using HMS testing at manufacture. They were confirmed as leakers with Kr85 and subjected to failure analysis which demonstrated internal corrosion of bridge-wires and header surfaces, and re-crystallization of ZPP producing chlorate crystals,

Figure 3

The above device was a field returned device. It was the result of a gross leak which allowed moisture into the device and the resultant corrosion of the bridge-wire

is evident. The wire was weakened until it broke. The dissimilar wire bond was also corroded, as was the surface of the header.

Figure 3 This photomicrograph shows an unusual formation of Chlorate crystals on the surface

of ZPP in an initiator that was detected as non-functional, with a large gross leak.

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References

1Semiconductor Devices Test Method, MIL-STD-750E, Test Method 1071.8, Defense Supply Center Columbus, November 12006 2Seal Testing Microelectronic Devices, MIL-STD-883G, Test Method 1014.12, Defense Supply Center Columbus, February 2006 3Neff, G. IsoVac Engineering, “Shock Tables”, Presented at JEDEC Conference, January 2007 4 Neff, G., Neff, J., Peterson, L., and Rink, K., “Non-Hermetic Impulse Cartridge Failures, A Case Study”, 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Sacramento, California, 2006 5 Neff, G., Neff, G., Neyer, B.T., K.K Rink, “Inadequacy of Traditional Test Methods for Detection of Non-Hermetic Energetic Components”, 50th Annual Fuse Conference, National Defense Industry Association, Norfolk, Virginia, May 2006