Peter,
Originally, antimony was used as a dopant to prevent tin whiskering (gray
tin formation). It's been found, however, that just about all (solder)
metals when alloyed w/tin will prevent this.
David Suraski
-----Original Message-----
From: Brooks, Peter <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Thursday, July 08, 1999 1:37 PM
Subject: Re: [LF] Background information: reliability testing done by the
NCMS project
>Carol:
>
>The use of pure tin lead finish on solid state products for military
>applications is currently prohibited due to tin whisker problems. In fact
as I
>understand it a recent satellite failure was traced to the use of pure tin
on a
>relay.
>
>Apparently the use of lead within tin inhibits the growth of tin whiskers.
Do
>the other materials within the new alloys (e.g Copper) also inhibits tin
>whiskers?
>
>Also how many of these materials can be plated?
>
>Pete Brooks
>
>
> -----Original Message-----
> From: Carol Handwerker [SMTP:[log in to unmask]]
> Sent: Thursday, July 08, 1999 12:17 PM
> To: [log in to unmask]
> Subject: [LF] Background information: reliability testing
done by
>the NCMS project
>
> NCMS tested seven lead-free solders and eutectic Pb-Sn -
>
> Solder - Liquidus Temperature
> Sn-37Pb - 183°C
> Sn-3.5Ag - 221°C
> Sn-58Bi - 139°C
> Sn-3Ag-2Bi - 220°C
> Sn-2.6Ag-0.8Cu-0.5Sb - 211°C (CASTIN)
> Sn-3.4Ag-4.8Bi - 210°C
> Sn-2.8Ag-20In - 187°C
> Sn-3.5Ag-0.5Cu-1Zn - 221°C
>
> Among the reliability tests were thermal cycling of surface mount
> reliability test vehicles (RTV-SM) tested for:
>
> 6673 cycles at 0°C/100°C
> 5000 cycles at -55°C/125°C
>
> with
> 84 J-leaded PLCC
> 132 gull-wing leaded BQFP
> 1206 discrete chip resistors - alumina body
> 1206 discrete chip capacitors - barium titanate body
> 44 castellated I/O LCCC (for getting early failure information for
>modeling
> only)
> on multilayer FR-4 epoxy boards.
>
> The components on the RTV-SM represent a variety of commonly used
>surface
> mount components. Component choice was based primarily on the
expected
> stress/strain response during thermal cycling as well as the lead
shape,
> body size, and availability in a daisy chain configuration.
Components
>were
> obtained either with a 100% Sn finish on the component leads, or
with a
> Sn/Pb finish that was stripped and refinished by a commercially
>available
> Sn-dipping process.
>
> Components were connected daisy chained. Performance was monitored
> electrically and by cross-sections. Double-sided assemblies were
used
>for
> electrical testing to maximize sample size, while single-sided
>assemblies
> were used for cross sectioning to facilitate sample preparation.
> Multilayer FR-4 epoxy glass boards were chosen for the RTV-SM to
match
> product requirements. Components were distributed across the board
so
>that
> each component type occupied both edge and central positions.
Components
> placed near the edge of the boards often fail earlier than those
located
> more centrally on the board. A commercially available immersion Sn
>finish
> was used because it exhibited the best solderability in
manufacturing
>trials.
>
> Copies of the NCMS report with the test results and the CD-ROM with
all
> project information can be obtained at
>
> http://www.ncms.org/3portfolio/1ProjectPortfolio/pubs.htm
>
> At 03:49 PM 7/7/99 EDT, you wrote:
> >In a message dated 7/7/99 9:35:30, [log in to unmask]
writes:
> >>Hi, Werner,
> >>What tests are sufficient to ensure "reasonable" reliability [of
>solder
> >joints]?
> >>Carol
> >
> >Hi Carol ,
> >This sure is a loaded question, and can not be answered by a brief
>statement
> >without being flip-so, you asked for it!
> >Personally, and from experience, I rather assure solder joint
>reliability
> >using an adequate modeling approach, taking into account both the
> >physics-of-failure and the statistical failure distribution. The
reason
>is
> >that it is no more error-prone than testing and much less
expensive and
>
> >time-consuming. The time and cost factors of testing has led to
test
> >short-cut, with sometimes catastrophic consequences.
> >Using a 'Design for Reliability (DfR)'-approach, of course,
requires a
> >reasonably good model. For near-eutectic Sn/Pb solders and their
> derivatives,
> >we have the data on which a number of modeling approaches used in
the
> >industry are based. But for many of the newer soldering alloys,
>including
> all
> >of the lead-free solders, we do not have sufficient data to
determine a
>
> >fatigue reliability model. Thus, until such time, any modeling
requires
>
> >assuming that the solder in question behaves similar to Sn/Pb and
> multiplying
> >some safety (actually ignorance) factor (>=2xlife to acceptable
failure
>
> >probability).
> >For the new solders, side-by side cyclic testing with eutectic (or
>60/40)
> >Sn/Pb solder from -20<->+100C (125C if substrate glass transition
> temperature
> >is at least 150C) with 15 minute dwells at each temperature
extreme (24
>
> >cycles/day) with conventional chambers (or 5 minute dwells with
>chambers
> >modified to provide temperature uniformity in the whole test
volume;
>~100
> >cycles/day) with at least 32 equal continuity daisy-chains
monitored
>with an
> >Anatech Event Detector for both solders, is necessary. This will
give
>an
> >adequate data base to draw valid comparisons with near-eutectic
Sn/Pb
> >solders, and, to me more importantly, from which a fatigue
reliability
>model
> >can be derived.
> >Tests that are sufficient to ensure "reasonable" reliability will
be
>totally
> >dependent on the product design together with its application. But
to
>design
> >such a test, you have to have an appropriate fatigue reliability
model
>from
> >which a valid acceleration factor for the test vis-a-vis the use
>conditions
> >can be determined. Electronic application vary from easily met
>reliability
> >reqirements for consumer products (~1,000 cycles (3 yrs) @
delta-T's
>~10C &
> >10% acceltable failure probability) to difficult to meet
requirements
>for
> >low-earth-orbit satellites (~90,000 cycles (10 yrs) @delta-T's
~35C &
> 0.001%)
> >or automotive-under-hood equipment (~10,000 cycles (~3 yrs) @
delta-T's
> ~100C
> >& 0.1%).
> >
> >Werner Engelmaier
> >Engelmaier Associates, L.C.
> >Electronic Packaging, Interconnection and Reliability Consulting
> >7 Jasmine Run
> >Ormond Beach, FL 32174 USA
> >Phone: 904-437-8747, Fax: 904-437-8737
> >E-mail: [log in to unmask], Website: www.engelmaier.com
> >
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> >Assemblies.
> >Please visit IPC's Center for Lead-Free Electronics Assembly
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> >For technical support contact Gayatri Sardeshpande [log in to unmask]
or
> 847-790-5365.
> >################################################################
> >
> >
> **************************************
> Carol A. Handwerker
> Chief, Metallurgy Division
> NIST
> 100 Bureau Drive Stop 8550
> Gaithersburg MD 20899-8550
> Office:(301) 975-6158
> Fax:(301) 975-4553
> e-mail:[log in to unmask]
>
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> IPCWorks -October 25-28 featuring an International Summit on
Lead-Free
>Electronic
> Assemblies.
> Please visit IPC's Center for Lead-Free Electronics Assembly
> (http://www.ipc.org/html/leadfree.htm ) for additional information.
> For technical support contact Gayatri Sardeshpande [log in to unmask]
or
>847-790-5365.
> ################################################################
>
>################################################################
>Leadfree E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
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>with following text in the body:
>To subscribe: SUBSCRIBE Leadfree <your full name>
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>IPCWorks -October 25-28 featuring an International Summit on Lead-Free
Electronic
>Assemblies.
>Please visit IPC's Center for Lead-Free Electronics Assembly
>(http://www.ipc.org/html/leadfree.htm ) for additional information.
>For technical support contact Gayatri Sardeshpande [log in to unmask] or
847-790-5365.
>################################################################
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To subscribe/unsubscribe, send a message to [log in to unmask]
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To subscribe: SUBSCRIBE Leadfree <your full name>
To unsubscribe: SIGNOFF Leadfree
################################################################
IPCWorks -October 25-28 featuring an International Summit on Lead-Free Electronic
Assemblies.
Please visit IPC's Center for Lead-Free Electronics Assembly
(http://www.ipc.org/html/leadfree.htm ) for additional information.
For technical support contact Gayatri Sardeshpande [log in to unmask] or 847-790-5365.
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