Over the last two months, in November and December, we took a bit of a detour from our discussion of the various standard space product screenings performed at ADI. I would like to get back on track with that discussion this month. I’ve highlighted the steps we will focus on in this blog in green in the table excerpt below (recall back in my blog from September we had the full table). The steps we will look closely at are the fine leak and gross leak seal tests, external visual, and radiation latch-up tests. In case you would like to catch up on the early part of the discussion you can refer to my previous blog ADI Standard Space Products Screening: Group A Testing.
ADI Standard Space Level Flow (Focus on Steps 19-22)
Now let’s move along and dive into fine leak and gross leak testing. One might ask what exactly are these tests? Well, these tests are designed to test the hermeticity of the device package. If a device is going to be utilized in a space application, then it is extremely beneficial to have a hermetic package. An improperly sealed or leaky package could allow contaminants to enter the package and possibly reduce the reliability of the device. To ensure the seal of the hermetic package it must be tested properly to ensure that it is indeed hermetic.
In the fine leak test devices are subject to helium gas exposure in a pressurized chamber. A mass spectrometer-type leak detector is required with sufficient leak rate sensitivity to read helium leak rates of 10-9 atm cm3/s and greater. The chamber volume should be the absolute minimum practical amount such that the chamber volume does not have a detrimental effect on the sensitivity limits. The leak detector must be calibrated using a diffusion-type calibrated standard leak at least once during any given working shift.
In the gross leak test the device is placed into a low-density perfluorocarbon liquid bath and subsequently immersed into a high-density perfluorocarbon liquid bath. Typically, Freon is the perfluorocarbon that is utilized for this test. This test verifies the hermeticity of the device by showing if there has been any of the low-density perfluorocarbon that has entered the package and escaped during the bath in the high-density perfluorocarbon. According to MIL-STD-883 a definite stream of bubbles or two or more large bubbles that originate from the same point on the package is classified as a failure. I like what the Microwaves101 website has to say about these two tests: “Why not just use the fine leak test and skip the gross leak test? If you had a module with a giant hole in it (a gross leak), the helium that is bombed in would all leak out and fly away before the sniffer could detect any!” If you’d like to read more on what is there on fine and gross leaks you can visit the site here: hermeticity.
The external visual inspection is performed to verify the integrity of several factors with regards to the device. Areas inspected include package body/lid finish, leads, and glass seals if they exist. The device is inspected to make sure that part markings are correct, lead conditions are good, the dimensions are correct, and the surface quality is good. The inspection looks to find any evidence of a secondary coating. The markings must be appropriate and clearly legible. The leads must not be burred, bent, broken, or misaligned. The device dimensions must be within the specified tolerances. The surface quality must be in proper shape with no anomalies. There must not be any evidence of cracks, delamination, separation, of voiding of any package element. More details can be found in the MIL-STD-883 Method 2009.9 (the specification can be found at the link below).
The last item we will discuss here is the Radiation Latch-up. This is a condition where a parasitic transistor or other device may be induced and turned on to create a large surge of current when the device is exposed to radiation. This even could be a large current that could be cleared via a device reset or some other means such as a power cycle where the device is still operable afterwards. It could also be a destructive event where there is such a large amount of current that device damage occurs, and the device functionality is either limited or ceases after the event. Obviously, it is important to understand this behavior before a device is employed in a space application.
If you would like to continue reading for a deeper dive, and you have a lot of time on your hands, you can visit the DLA and NASA websites to read more on the MIL-PRF-38535 and MIL-STD-883 specifications here: PERFORMANCE SPECIFICATION INTEGRATED CIRCUITS (MICROCIRCUITS) MANUFACTURING, GENERAL SPECIFICATION FOR DoD and here: DEPARTMENT OF DEFENSE TEST METHOD STANDARD MICROCIRCUITS.
I hope this has been a fun journey of exploration (ha-ha, pun intended) into the different space screening steps for Analog Devices’ standard space products. If you have any questions, please feel free to drop a comment. I welcome any comments or questions. As we have learned it is important to perform the proper screening so that devices employed in space applications can be expected to be reliable and maintain a long lifetime in the harsh environment of space. As I have mentioned in previous blogs it is quite difficult to perform any type of maintenance on a system once it is deployed into space. It is possible in some cases, but in many cases it is simply not feasible to make any device repairs once a system is deployed.