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Wed, 01 May 96 17:34:02 PST
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     When MIL-STD-2000A was written, the issue of the SOT's didn't arise 
     because at that time, Military didn't allow use of non-hermetic ICs 
     and thus we didn't consider criteria for these parts.  I recall as I 
     chaired the Acceptance Criteria Committee for the DoD for 
     MIL-STD-2000A and am still having nasty flashbacks!!!!  What you see 
     in 2000A was a general requirement that solder not contact the part 
     body lead seal for reasons of thermal transfer (4.23.7.5  Lead heel 
     fillet.); this wasn't a defect as the idea was that the part's 
     functionality was 100% checked AND solder may wet up to near/abut lead 
     if the lead is solderable (e.g. gold).  The part about not touching 
     the bottom of the part was in 4.23.7.7  (J-leaded and V-leaded device) 
     and had to do with solder under the device acting as a stress loading 
     point during low temp exposure.  Remember, we were talking hermetic 
     (ie. ceramic) devices.
     
     When ANSI/J-STD-001A was being developed (A rev), it was noted,as you 
     point out, that solder contacting the sides of the part (on SOT/SOICs) 
     is fairly common and no failures were noted, thus the condition was 
     allowed in ANSI/J-STD-001A in table 9-2 note 1.
     
     Call if you have questions.
     
     Jim Maguire
     Senior Principal Engineer
     Boeing Defense & Space Group
     PO Box 3999 M/S 3W-97
     Seattle, WA  98124-2499
     Phone 206-657-9063
     FAX    206-657-8903
     Email:  [log in to unmask]
     

______________________________ Reply Separator _________________________________
Subject: ASS- X S Solder on SOT-23
Author:  [log in to unmask] at esdigate 
Date:    4/30/96 2:32 PM
     
     
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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Date: Tue, 30 Apr 1996 06:09:41 -0700
To: [log in to unmask]
From: [log in to unmask] (Kenny Bloomquist) 
Subject: ASS: X S Solder on SOT-23
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