Date: February 25, 2003
Here is the ninth in a series of OLD News articles.
Recently, failures in digital systems have been attributed to heat sinks of integrated circuit packages causing shorts to circuitry located under the parts. Care must be taken in the physical realization of the system to ensure that the systems are reliable and that ample margins are present for long-term reliability. While this is not a new issue, it is an issue that is easily overlooked.
This issue applies to standard military, radiation-tolerant, radiation-hardened, and commercial-grade, "COTS" devices. Care must be taken when assembling parts. In particular, new builds and upgrades must be carefully inspected and analyzed.
- Different lots of parts may have a different mechanical configurations
- Different vendors' packages for similar parts (perhaps the same die) may be radically different.
- Different parts from the same vendor in the "same package" -- same suffix in the part number -- can differ substantially
Below are some images illustrating these concepts. This is a generic issue and these images are from a number of devices laying around the laboratory; this is not specific to any vendor.
Note: All of the images of devices are showing the bottom of the package.
Figure 1. Representative printed board top layer. The layout technician for
this board has vias and traces under the part, a common technique.
Figure 2. Two "1280" devices. The device on the left is a Loral RH1280/203A666-11
without a heat sink. The device on the right is a SEI 1280ARP with a conductive
radiation shield on the bottom surface. Flight Actel A1280As (not shown) have
been seen both with and without heat sinks.
Figure 3. Four devices in the same package, as designated by the part suffix from a
single vendor. Note for devices with heat sinks, the heat sinks can be different sizes.
These devices are either similar or identical technologies, in the same part family.
Figure 4. Two commercial PQ208 devices from a single vendor.Lastly, one organization reported that the solder on some vias formed a small dome when it was reflowed, and this dome was high enough off the board that it made contact through the "thermal goop" in between the package bottom and the PCB.
In my new OLD (Office of Logic Design) position, I am now making some of my informal e-mail lists semi-formal. These mailings will have pointers to technical tips that can [hopefully] proactively prevent errors from getting into flight designs or make things go faster and smoother. I have included an array of people from a number of number of organizations; different NASA Centers, ESA, etc., as you all may distribute to people in your own organizations and other colleagues. Please let me know if you are on this list in error or if someone should be added to it. This list is targeted towards those that either will design or review space flight digital electronics. Feel free to suggest topics for discussion and research or to contribute news items. [Note for this web-based release: to become a recipient on this mailing list, please send e-mail to: richard.b.katz@nasa.gov.]
All application notes are uploaded onto my www site. New additions are noted on the what's new page. I will give these mailings from time to time; too much and they will be filtered and ignored - too little and not enough information flows. So I'll try and hit a good balance.
Best regards,
-- rk
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