NetTechs-
Greetings!
My first week as a TechNet subscriber and I'm impressed. Good stuff!
A couple observations that maybe someone could clear up for me?...
First, regarding MIL-STD-2000A -- A few months ago, I was involved
in attempting to reconcile a concept in J-STD-001 with that in 2000A.
As I wasn't making the progress I had hoped, I called Mike Buetow (at IPC),
who informed me that 2000A had been canceled!
Sure enough, MIL-STD-2000A, Notice 1, dated June 7,1995 is a Notice of
Cancellation.
So- it's none of my business, but - doesn't that raise some questions
about that 'critical DoD application', Kenny? Contract review time, maybe?
Or do those older commitments hang on 'forever', like so many
federal programs, 'cause canceled doesn't REALLY mean canceled?
Then there's this SOT-23 (low profile, I'm assuming).
Again, I'm a new guy...and just getting my feet wet with these concepts,
but...
My perusal of the various IPC standards at my desk reveals that, though
solder in the upper lead bend or in contact with the package was once
NOT an acceptable condition, it appears that now IT IS.
(Compare IPC-SM-780, March 1988 (Figure 9-6) with J-STD-001A,
Jan. 1995 (section 9.2.6.2) & IPC-A-610B, December 1994 (Figure 10-48)).
Someone MUST have gathered data in support of this change. Or was
this change made because its validity wasn't provable, or because the
disallowed condition didn't result in actual failures in use?
We've just recently changed our internal assembly and inspection criteria
to allow and accept this condition, but not without some heated discussion.
I, and some of my comrades, share the concerns experessed by
Mr. Hersey and Mr. Bloomquist. But we haven't the wherewithal to test
out our concerns.
Isn't there some data out there somewhere to support this reliability
concern?
Oops! Broke a nail! Better go.
Thanks folks !
----------
From: Ralph Hersey
To: IPC Technet; JMcGee
Subject: ASS- X S Solder on SOT-23
Date: Tuesday, April 30, 1996 11:33AM
Mail*Link(r) SMTP ASS: X S Solder on SOT-23
Kenny,
Your response of "no response" I find very interesting.
IMO, I would not allow the filling of the strain reliefs in the component
leads for any "serious" Class 3 product. The following are my
concerns/thoughts:
1) With solder against the component body, significant heat has been
transfered through the component lead(s) through the lead and into component
body seal. This could have degraded the adhesive bond between the lead and
plastic body molding compound.
2) Differences of thermal expansion coefficients between the component and
the printed board interconnection substrate during thermal/power
cycling/shock
will strain the (assumed weakened) bond between the component lead and
component body. Re-occuring cyclic reverse-stress could lead to component
failure.
3) Mechanical shock/vibration will likewise subject the component lead and
component body to the same type of possible failure mode as in
concern/thought
2).
However it may be acceptable, depending on the life-cycle environment (as
you
will read, this pun was not intended) for the poroduct. If the component
were
mounted on an assembly that was a medical implant, such as a heart
pacemaker,
it would probabibly be acceptable, because if the inside of the ol' human
body
is subjected to thermal shock/cycling or mechanical shock/vibration to
damage
the assembly/component, I think some other part of the 'ol body will
deteriorate functionality before the SOT-23's soldered connection.
Disclaimer - this is my opinion, and would most probably not endorsed by my
employer.
Ralph Hersey
e-mail: [log in to unmask]
--------------------------------------
Date: 4/30/96 7:17 AM
From: Kenny Bloomquist
On Friday, April 26th I wrote:
We are discussing excess solder on a SOT-23 for a critical DOD application
per MIL-STD-2000A. The condition is solder contacting the body of the
component between the PWB and the body, also solder contacts the lead seal.
Per 2000A this "shall not" be done but it is not in Table 1 as requiring
disposition. My real question is, what is the risk for this condition and
does anyone have any supporting data (good, bad or otherwise)?
I was suprised that no one sent a response. I know this is a touchy subject
but any information would be greatly appreciated.
Thank you in advance for all responses.
[log in to unmask]
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