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From:
Steven Creswick <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steven Creswick <[log in to unmask]>
Date:
Thu, 13 Dec 2012 20:58:20 -0500
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Howard,

Sorry, this will be a bit long.

I am glad you stated that the packages passed hermeticity testing.  That
would have been the first item to check.

I assume this is TO-5 or smaller style package.  The smaller volume packages
can be problematic if there is a leak during puncture of the package because
the available tested volume is so small compared to a potential leak.

Different labs used to test for RGA in different manners, with different
style equipment.  

On one style of test equipment, the sample package would be placed into the
evacuated test chamber and punctured/broken.  The sample gas would then flow
into the chamber and subsequently be taken into the mass spec.  

Other equipment relied upon placing the package up against the sampling
port, sealing it via an o-ring [of sorts].  Once fixed against the evacuated
sampling port, a needle would protrude through the center of the o-ring and
puncture the package lid, allowing the test gas to enter the mass spec. 

Both methods have pro and cons.  The chamber method is most convenient for
all-ceramic style packages where there is no metal lid to puncture.  A
problem with it is that the entire exterior of the package must be
thoroughly cleaned, but can still carry ad/absorbed species into the test
chamber.  Plated surfaces also can contain a great deal of trapped hydrogen
put down during plating.  Some systems would see Hydrogen, and log it as
H2O.

With the puncture method, if the lid is too robust, the act of puncturing
could displace the sample from the seal and allow atmosphere to be
introduced, thereby squirreling up the data.  Most hybrids had 0.010-0.020"
thick lids and what WE often did was to take a small end mill [~0.050" dia]
and mill a small recess in the lid, leaving only about 0.005" of metal
thickness.  The lab would than center this in the seal.  Generally, this
gave us more consistent data.

Variance of data - either approach is prone to variation if everything is
not absolutely perfectly cleaned and performed.  You did not say how many
samples you ran at each lab.  Hopefully, you ran 4-6 at each.  As a minimum,
3, so you could throw out the low and high, and keep the middle.  I am
sorry, but to test 1 or 2 is almost fruitless due to variance.

To the adhesive - Two major potential issues come to mind.  

First - if you take a great adhesive and improperly process it, you end up
with garbage.  The 84-1LMI is a very good adhesive which has been used by
many firms in Space, Mil, and Implantable medical applications.  It can meet
the requirements of Mil Std 883, TM 5011 when properly processed.  That is a
very good material to use - if properly processed!!  Depending upon the date
of assembly, it was THE material to use. [I am not in any way associated
with Ablestik/Henkel, but I have indeed used this material on many hybrids,
including many space applications, one of which is still in the Saturnian
system]

Second - No matter how well you process the adhesive, if the pre-seal
conditioning [pre-seal bakes, vac bakes, package [and lid] cleanliness [and
bakes] is not adequate, one will end up trapping trash inside a hermetic
package.  

Yes the limit is 5000PPM.  The real issue is [if the RGA testing is
accurate] what other ionics and corruption do you have inside the package to
combine with the water??

Whether adhesive is allowed or dis-allowed is generally addressed by the
detail specification for the device.  I do indeed know of quite a few Space
level parts that have adhesive inside.... properly processed adhesives...

No one can really give you a definitive answer, at arms length, such as
this, but here are my free two bits ...


I would have to go back and review my dew point knowledge, but I strongly
suspect the 28000 PPM is bogus [unless device assembly is really sloppy -
see below].  You should be able to achieve well under 2000 PPM without too
much grief if it is a TO-5 or smaller package [properly processed].

My fear is that you are buying a commercial grade device, that was assembled
and processed as a commercial grade device, having no RGA requirements.
Therefore, the adhesives were likely not processed in a manner which would
give you good RGA results.  To make a poor analogy, it is like you are
buying a standard 75W incandescent bulb and banging it around as though it
were a Rough Service bulb.  Yes, some will survive the abuse, but most will
have the filament destroyed almost immediately because they were not
constructed in a manner which would offer a much greater likelihood that
they would pass testing.  That may be the situation you are in.  Sorry.

Hopefully Inge is eyes-open and he can add a few comments as well.


Steve Creswick
Sr Associate - Balanced Enterprise Solutions
http://www.linkedin.com/in/stevencreswick
                         



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Watson, Howard A
Sent: Thursday, December 13, 2012 6:24 PM
To: [log in to unmask]
Subject: [TN] Component issue - epoxy die attach

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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