TECHNET Archives

February 2000

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Dan Fazioli <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 7 Feb 2000 15:19:57 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (77 lines)
HI BRETON,

First, your quest to find information on this subject probably exists, and no
doubt is the result of numerous stress analyses and DOE's performed by countless
individuals for their specific application, but quite frankly, I have not seen
this type of data openly published in any industry technical literature during
my career.  If it has been published I would appreciate knowing how to obtain it
as well.  Nevertheless, as you probably already know, the strain values that you
are looking for with respect to the limits SMT components can withstand during
temperature exposures is in fact, the result of the components material and the
components mounting media's materials mechanical properties, their physical
geometry's and their TCE's in conjunction with the temperature extremes which
these elements are exposed to with respect to the soldering/temperature profile
being used.  Also, it has been my experience that when components with these
type flaws go undetected (some components do get through environmental screening
processes) during the manufacturing and testing phases of a product the
historical evidence has shown that the high strain/stress results can have very
serious implications on the reliability of these type components when allowed to
be delivered and exposed to field natural and induced environmental conditions.
 Secondly, I assume that you are mostly concerned with the strain and stress
values associated with SMT passive "leadless" type components, because
personally I have not seen a cracking problem with "leaded"  type active or
passive components such as QFP's, Hybrids, discrete and etc.  Unfortunately
because my experience up to this point-in-time with BGA's has been very limited,
I can't offer any constructive comments with respect to this issue for these
types of component styles.   Perhaps some other Technetters can respond on-line
regarding their experience with types of component styles.  Perhaps some other
TechNetters can respond on-line regarding their experience with this type of
component style?   In essence, when a component has both ends fixed, such as SMT
passive components (chip capacitors, resistors, etc.) soldered to a PWB and/or
some mounting media, the elongation taking place during the temperature changes
will cause the component to bow due to the solidification of the soldering
operation.  Moreover, depending upon the relative magnitudes of the TCE's of the
component and its mounting media, the force generated will either be compressive
or tensile in nature.  The stress is normally determined using HOOKE's LAW, and
my personal experience has shown that when the stress calculated exceeds the
allowable tensile and/or compressive strength properties of the component's body
material, catastrophic fracturing/cracking has the potential to occur when the
physical size of the component's body is greater than 250-mils square, and
exposed to the normal soldering operations.  However, my experience has also
shown that using HOOKE's LAW to calculate the stress usually results in stress
values that are often higher than those observed empirically.  This is true
primarily because there are several factors which can contribute to the
reduction in the stress that are either often overlooked and/or not accounted
for correctly in performing the analysis, and consequently, this is the reason
why good engineering practices dictate the performance of verification testing
to prove or disprove the components reliability issue.  In summary, since this
topic is one of great depth, I suggest that you may want to consider contacting
an expert in this field like Mr. Werner Engelmaier (904-437-8747).  Or better
still, regarding this subject, perhaps Mr. Engelmaier would be receptive to
sharing his comments on-line with us fellow TechNetters?    Nevertheless,
although I don't know how much help, if any, I can be, please don't hesitate to
contact me off-line.

Best regards,

Dan Fazioli
Engineering Technical Mgr.
Smiths Industries Aerospace, IMS
Clearwater, Florida 33762
e-mail: [log in to unmask]
ph: 727-532-6333

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or
847-509-9700 ext.5365
##############################################################

ATOM RSS1 RSS2