I am taking it that the testing you detailed is a one-off qualification
test and not normal Environmental Stress Screening (ESS), which is done for
each production run?
Environmental Qualification Test (EQT) is designed to prove through
accelerated aging, that your product will function reliably throughout its
intended lifespan in its normal operating environment. ESS is a mini
version of this to weed out any early life failures in conjunction with a
burn-in programme, but should not significantly shorten the life of the
survivors.
To comment on your questions:
a) Moisture absorbtion/transpiration is a natural part of boards and board
assemblies. To try to eliminate it in, or for, environmental testing will
provide a false result, unless that elimination is required in order to
match the intended operating environment of the finished product.
b) If you (or your fab house) has chosen the correct soldermask & hardener
for your application, mask adhesion must be unaffected by the the testing
or your product will have failed the test.
c) Oxidation is formed in the presense of oxygen (air) and moisture. The
degree of oxidation depends on exposure to these factors and also
temperature. Better chemists than I can explain this better, or you can
trawl the TechNet archives for more info. There was a sizeable thread on
oxidation (a.k.a. rust, tarnish, etc).
d) Did the burning to your vias occur during bare board testing or to the
finished assembly? Was the board under power at the time? If the board was
under power during temperature cycling, and it wasn't conformally coated,
then you've probably had a short circuit at the 'dew point', when any
moisture in the chamber air surrounding the boards has condensed onto the
board surface and given you a conductive path you didn't intend. Was the
via a power via?
e) If your product is not to be exposed to any harsh environment (extremes
of temperature, dust abrasion, salty air, vibration, shock, chemicals,
high/low pressures, and so on), then the degree of testing being carried
out is excessive. I wouldn't say unnecessary, because some form of EQT
and/or ESS is useful for product reliability. I'll re-state what I said at
the beginning of this response - the tests must be designed to prove that
your product will function reliably throughout its intended lifespan in its
normal operating environment.
Hope this helps a bit.
Peter Duncan
CircuitekAsia
@AOL.COM To: [log in to unmask]
Sent by: cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
TechNet Aero/ST Group)
<[log in to unmask] Subject: [TN] Enviromental Stress
ORG> Testing-----Benefits and Drawbacks
01/29/02
01:17 AM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
CircuitekAsia
To All Technetters,
Being from purchasing, I have been asked to rationalize the inherent cost
for demanding quality standards that my company now prescribes to.
Below, I have outlined our company's test criteria for environmental stress
testing:
Panel Allocation Test Description
Thermal Cycle -55C / 150C X1 15 Min x 2 x 100; Test PC boards without
components
Humidity 85/85 X2 7 Days; Test complete units
150C Bake X3 7 Days
Hot Plate/ Oil Temp Shock X4 10 Sec x 20 0C-100C Water Dip
Via Current X5 7 Days / 14 cycles @ 25C & 125C
Below, I have outlined several questions so that I can attain a richer
understanding of the intent, benefit and drawbacks of environmental stress
testing:
a. How would moisture absorption effect results of environmental stress
testing?
b. How would soldermask adhesion be effected by environmental stress
testing?
c. How is oxidation formed; is it a function of water
absorption/environmental exposure?
d. What could cause via current test (the via are burnt and opened) after
environmental stress testing?
e. If our product is not exposed to any harsh environmental conditions,
what is the benefit for this type of testing?
I would appreciate any and all comments and feedback all of you have to
offer.
Regards,
[This e-mail is confidential and may also be privileged. If you are not the
intended recipient, please delete it and notify us immediately; you should
not copy or use it for any purpose, nor disclose its contents to any other
person. Thank you.]
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
