Joyce, you are absolutely correct. Detailed requirements should have been created at the beginning, but I inherited the problem and need to find a resolution. Yes, our quantity is very low, <70 per year, and that makes it challenging too. Thank you for your reply!
Howard Watson
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Thursday, December 13, 2012 6:59 PM
To: [log in to unmask]
Subject: Re: [TN] Component issue - epoxy die attach
Well, if you need space grade, assume only few required, much better do it on your own: solder or preform die attach rather than epoxy. Or you can do something like some of the MIL house: build a environmental box around the COTS part, in order to keep it within its operational boundary (temp, RH, etc). Use parts outside its designed intend is risky. In space, risky is specially costly, if not deadly. (By the way, you can keep testing the same part around all the Labs, one of them might come out with the results you like :-). Logistically, your team might missing a dept that over see the design requirement and select the parts. The epoxy die attach should get caught at datasheet review stage. Something is iffy here. Follow Inge's advise and read ALL his papers might be a good start. :-)
--------------------------
Sent using BlackBerry
----- Original Message -----
From: Inge Hernefjord [mailto:[log in to unmask]]
Sent: Thursday, December 13, 2012 08:04 PM
To: [log in to unmask] <[log in to unmask]>
Subject: Re: [TN] Component issue - epoxy die attach
Hi Watson..where is Sherlock Holmes as you need contact TN?
There is a lot to comment. If we start with the second question, I
assume you did NOT send one and the same test objects to the two labs. If
you did, which is highly unusual, no wonder that you got so different
results, because during the first RGA, vacuum is used, which gives free
passage for the residual gas(es) to escape. If you then do the RGA -test
with an other lab, you will of course read a lower value. If you did NOT
use same test samples, it's still not remarkable that you got different
values, because you don't get perfect components even if they had <<5,000
ppm. The reason is this: The sealing is done in a 20 ppm O2 environment,
and do you perform RGA immediately after sealing, you measure on fresh
packages filled with dry Nitrogen (or an inert gas). If one or two of the
packages have a leak, there will be an exchange of the inner and outer gas
pressure i.e. water vapor will flow through the leak and worse it will be,
depending on how many days, weeks, pass between the two RGAs. Understood?
The other half of Q2 was, whether >> 5,000 ppm is negative for the
component functions. Depends on the chips own protection (oxides,
polymers.), but the answer is 'yes' in general terms. Examples: 1) NiCr
etching 2) Bond pad corrosion 3)Metal migration 4) whisker formation 5)
Dendrites 6) Lekage currents 7) parameter instability.
I have some experience from this area and i have lots of saved reports.
Before we continoue, I tell you that this is a labyrinth of opinions and
interpretations of test methods, discussions of gas molecule's free medium
way transportation through orifices, Hydrogen molecules that penetrates
right through metal cans
etc.
Will be back with more stuff..
Inge
On 14 December 2012 00:23, Watson, Howard A <[log in to unmask]> wrote:
> Hello 'netters,
>
> I debated posting this, as it seems to me to be an obscure problem, but
> then, I'm always amazed at your knowledge base. I have a JFET 2N6550
> component to be used for space application. Unfortunately, it is not
> manufactured at JANS space quality, so we buy the commercial grade, plus an
> option 2 screening, which upscreens the part to "like" JANTXV. I think this
> is called re-branding. Then, we send it to a lab for further upscreening to
> JANS. The problem is that the parts are failing the moisture test of the
> residual gas analysis (RGA). I found out that epoxy is used for the die
> attach, and likely the epoxy is outgasing during subsequent baking as part
> of the testing. My first question is who knows of a standard for die attach
> of this component type stating that epoxy is forbidden for military and
> space use? The epoxy used by the manufacturer is Ablestik p/n 84-1LMI;
> Material # 1119570. I just found out today that they do have the
> capability of eutectic die attach, and I'm pursing this option, expecting a
> huge expense and lead time.
>
> Secondly, I had two independent labs perform the RGA. The first lab had
> results averaging ~28,000 PPM. The second lab results averaged ~5600 PPM.
> The standard is no more than 5000 PPM. They both performed the testing
> to the same MIL-STD-750. I can't understand the wide range of results, but
> my second question is who knows of any studies related to the negative
> effects of excessive (>5000 PPM) moisture inside hermetically sealed
> devices used in space? By the way, they all passed the seal tests. Perhaps
> some of you are knowledgeable in this area. Thanks in advance for your
> help.
>
> Howard Watson
>
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