TM 1014 Update and Spec Change
Wed, 08/22/2012 - 12:07 — tgreen@tjgreenllc
As many of you know Mil-Std-750 TM 1071 was recently changed and the hermeticity specification was tightened by two orders of magnitude. This had a significant impact on Mil-Prf-19500 component suppliers and forced them to evaluate and purchase new leak test equipment. This was because most conventional helium mass spectrometry (HMS) leak detectors lack the precision and sensitivity to measure leak rates to the new spec. Of course you can achieve the desired sensitivity if you're willing to "helium bomb" the parts for some ridiculous amount of time like 100 hours or so. A two order magnitude reduction in the Lair spec limit from 1E-06 cc/sec to 1E-08 cc/sec requires equivalent HMS readings to be in the low 1E-10 or high 1E-11 cc/sec Helium, for typical bomb time and pressure conditions.
Mil-Std-883 TM 1014 is currently under revision. At a recent government coordination meeting there are proposed changes to the specification that may require component suppliers to once again evaluate and purchase new leak test equipment.TM 1014 is the defacto leak test hermeticity standard and is used and referenced by many more companies in the defense, aerospace, telecom and medical device community. A spec change to this important test method will have profound implications for many companies within and beyond the military/aerospace industry.
There are essentially three test procedures and associated equipment that can meet the proposed new criteria. They are: Optical Leak Test (OLT), Cumulative Helium Leak Detection (CHLD) and Radioisotope Kr-85. The test conditions for each of these test methods are totally different. The Radioisotope method forces a radioactive tracer gas Kr-85 mixture into a package and then counts the number of radioactive molecules that remain inside at ambient temperature and pressure conditions. CHLD extends the sensitivity of conventional HMS. Like HMS, CHLD forces a tracer gas into the package and then measures a leak rate, or immediately identifies a gross leak by detecting a signal burst. In either case the escaping helium is measured while the part is under vacuum. OLT on the other hand, precisely measures the lid or package deflection while the part is under pressure, and there is no injection of a tracer gas. OLT thus eliminates the concern of residual tracer gas on the surface, which tends to confound the results for both helium and Kr-85 testing. OLT also performs a gross and fine leak test in the same pass and is “fail safe” for gross leakers.
If the new version of TM 1014 causes you to have to purchase new equipment then my advice would be to evaluate all three methods and pick the one that best suits the leak rate requirement for your particular package type, package volume, expected throughput ect. Be especially careful that any new equipment is able to detect and fail for any possible "gross leakers". Gross leaks are a major cause for concern because they can easily lead to early field failures. Moisture induced failure mechanisms have been well chronicled in the literature over the years and a part with a "gross leak" is prone to these types of failures. In my opinion there isn't a big difference in a part leaking at 2E-09 Lair vs. 9E-10 Lair, but there surely is a problem in "gross leak" escapes that evade detection and get built up into our military systems.
Remember to convert everything back to “air equivalent” leak rates for data used in comparison studies and the new specification levels. This can be tricky and keep in mind for the reasons stated above precise “correlation” amongst the three methods may not be achievable. However, based on my experience all three methods are capable of meeting the new specs and are scientifically sound. They also agree reasonable well with each other when viewed as “go/no-go” attributes data. In my mind that is what the leak test is all about. It’s a one time check on the seal process done on the factory floor under idealized conditions. To use the leak test data to make reliability predictions or project expected lifetimes in the field is fraught with problems. The first of which is relating the measured leak rate to an actual theoretical model to predict moisture ingress into a sealed enclosure, then factor in the moisture susceptibility of the components inside. Not for the lighthearted.
Each one of these major methods has advantages and disadvantages so buyer beware!
Tom Green is the principle at TJ Green Associates LLC, a company specializing in consulting and teaching hermetic seal and leak testing processes.
Blog post by Thomas Green