The post below from Bob got me to thinking once again about tin whiskers.
Since there is a known correlation between compressive stress and whisker
growth, might an underfill which shrinks during cure actually promote tin
whiskers?
I understand that underfill, like a conformal coating, should help
mitigate the concern but given that the pitch of flip chip is headed towards 100um
and whiskers can grow to more than 50x that distance, is anyone not
concerned about using high tin solder tin for flip chip interconnection?
Moreover, since at temperature, the underfill modulus will drop and a fairly
massive heat spreader may be loaded atop the die, what might be the result?
That aside, I listened a couple years ago to a very sobering paper by an
Intel researcher on thermally/electrically accelerated Kirkendahl-like
voiding at high currents and temperature. This seems to have been some
corroborated at the event Laura reports on.
We are not out of the woods yet, it seems...Please correct me if I am
misapprehending anything.
By the way there is some great bits of visual imagery in Laura's technical
writing (Great job if you have your TechNet "ears" on Laura :-)
Joe
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http://www.electroiq.com/index/display/smt-article-display/6648914340/s-arti
cles/s-smt/s-volume-23/s-issue-5/s-columns/s-smt-advisory/s-challenges-of_le
ad-free.html
This article is from
Surface Mount Technology |
Challenges of Lead-free Electronics
Lead exemptions are going away in environmental legislation. Lead
substitutes for die bumps in packages, and electromigration with lead-free
assemblies are challenges prompting vigorous research.
I recently attended the Annual Meeting of the Metals, Minerals and
Materials Society (TMS). For more than 10 years, the Electronic Packaging and
Materials Committee of TMS has sponsored sessions on lead-free research, and
for the past 5 years there has also been a Sunday workshop, "State of the Art
-- Lead-free Technology." The 2009 session was attended by almost 150
people.
The challenges we face are many. From the regulatory point of view, the
limitations defined under the European Union's (EU) Restrictions on Hazardous
Substances (RoHS) and the EU's End-of-Life Vehicle (ELV) Directive
currently are under review. Exemption 15 of RoHS allows leaded solder for flip
chip connections within the package. This is frequently a high-lead solder
(>85%) covered under Exemption 8, but can also be eutectic tin/lead solder.
Under RoHS, this exemption expires at the end of 2014, but under ELV, the
exemption expires in 2010 unless it is renewed. While there are several
system-based approaches to removal of lead, there is no universal, reliable
substitute for all applications.
Potential replacements for the leaded bumps are:
Gold (Au) stud bumps attached by thermal compression or thermosonic
bonding for small die;
Tin/silver (SnAg) on copper (Cu) pillars has successfully been implemented
by Intel for finer-pitch die;
Hitachi is pursuing a composite nano-solder of Cu and Sn powder;
Some die attach alloys under study are lead/tin/silver (PbSnAg),
cadmium/zinc (CdZn), AuSn, gold/germanium (AuGe), aluminum/silicon (AlSi), and ZnAl;
Anisotropic conductive adhesives are a less expensive option but
conductivity and lifetime might be compromised.
While each of these materials fills a niche application, the bottom line
is that there is no robust highly reliable solution available today to meet
the ELV deadline of January 1, 2011 for under-hood applications.
Electromigration of lead-free alloys was another hot topic at the TMS
annual meeting and conference.
This failure mode has gained importance as we move to high-density
interconnects with fine-pitch ball grid arrays (BGA) and chipscale packages (CSPs)
because the current density through the solder ball increases as ball size
decreases. For example, a 50-µm ball with 0.2-A current will have a
current density of 104 µA/cm2.
Electromigration involves the movement of atoms in a metallic conductor
due to the electron wind caused by high current density. When the BGA under
bump metallization (UBM) is the cathode, current crowding occurs at the
point where the trace enters the solder joint. Here, current density is highest
at the component side of the BGA, causing an electron flow that depletes
the cathode, creating pancake voids. The voids further reduce the contact
area at the UBM, increasing the current density and the temperature of the
solder ball, ultimately leading to failure. At the same time, the copper
consumption at the anode creates intermetallic compounds (IMC) that migrate to
the anode, thickening this area. Compressive stresses at the board
interconnect are increased due to this IMC migration. These forces create Sn
"hillocks" and even Sn "whiskers," according to some authors. Whisker growth is
somewhat slowed by the consumption of Sn by intermetallic formation with Cu.
The common test system used by several researchers to study this
phenomenon consists of a fine-pitch BGA cross-sectioned to expose the center of the
balls in a given row. The samples are exposed to current density of 104
A/cm2 for a period of time, then the current is removed, SEM analysis
performed, and changes in morphology documented. Electromigration through the solder
ball causes a temperature increase and, when device temperature is not
controlled, this accelerates joint degradation, ultimately leading to failure.
When electromigration is monitored under conditions where the device
temperature remains constant, the time to failure is extended.
SnAgCu (SAC) solders with a lower silver content, such as SAC 105, have an
increased propensity for electromigration, even though they are desirable
for handheld products due to improved drop test performance. Small amounts
of Zn in the solder balls or in the UBM can also help to stabilize the
solder joints, reducing electromigration. It has been show that when nickel
(Ni) is present in the UBM, the rate of electromigration deceases, while the
presence of Cu increases the migration and reduces the time to failure. It
was pointed out at the conference that RF devices of 77 GHz and higher are
of concern because there is a skin effect that enhances the electromigration
phenomenon. Designers need to consider the potential for this failure mode
as they create next-generation products.
One other areas of note at the conference was the use of various doping
agents to create SACX alloys for specific applications. Alloying agents
discussed include Ni, Zn, bismuth (Bi), manganese (Mn), Al, and iron (Fe).
The October issue of the Journal of Electronic Materials is devoted to
papers from the Annual Meeting of the Metals, Minerals and Materials Society.
Laura J. Turbini, Ph.D., is an SMT Advisory Board Member, an adjunct
faculty member at the Universities of Toronto and Waterloo, and Chemistry Lab
Manager and Principal Scientist at Research in Motion. She also serves on the
Board of Directors at the SMTA. Contact her at (519) 888-7465, ext. 77744;
[log in to unmask]
This article is from
Surface Mount Technology
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