Solder joint embrittlement—it’s not just gold Solder joint
embrittlement—it’s not just gold
The various crystal platelets formed of IMCs of Au, Ag
or/and Pd with Sn will cause ‘embrittlement’ of the solder joints if they exist in sufficient concentrations either indi- vidually or in combination. In addition, Sn itself under- goes a ductile-to-brittle transition.
Cu6Sn5 Werner Engelmaier
Ag3Sn
Figure 1. Etched solder joint showing the crystalline platelet-structure of AuSn, AuSn3
Ingemar Hernefjord, Ericsson Microwave Systems, Sweden.
and AuSn4 I
have, of course, talked about the inter- metallic compounds (IMCs) necessary to form a good metallurgical bond1
as well as the fact that some IMCs are the cause of the ‘embrittlement’ of solder joints2
. Te embrittlement possibility was
recently expanded with the push for a new surface finish—ENEPIG (electroless nickel/electroless palladium/immersion gold)3-5
of ENIG (electroless nickel/immersion gold) without the disadvantages6
. ENEPIG provides the advantages .
cation7
I recently collaborated in the publi- of an investigation for which, as
a consultant, one does not oſten get the opportunity, because of not having access to the necessary equipment. Tis work fol- lowed a number of related publications that provided new insights in the structure of Sn-based solders8-9
. Te embrittlement of solder joints
with the platelet-like crystals formed by the intermetallic compounds (IMCs) of Sn with Au, Ag and most recently Pd is obvi- ous from the pictures in Figures 1 through
‘embrittlement’ of the solder joints if they exist
consisting of AuSn, AuSn3 or Pd3Sn, Pd2Sn and others10
in sufficient concentrations
will cause either
, Ag3
individually or in combination. Tese IMC-platelets are not weak themselves but weaken the solder matrix because of their structure. However, any inclusion of IMC crystal platelets causes stress concen- trations, reducing the necessary fracture energy, and in a sense will ‘embrittle’ the solder joints. Te ductile-to-brittle transition of
Sn-based solders is a different mecha- nism associated with the crystal structure of β-Sn, which is the main constituent of Sn-based solders. Figure 5 shows the impact energies required to fracture solder specimens at various temperatures;
the
reduction in the required impact energy below the ductile-to-brittle transition tem-
58 – Global SMT & Packaging – February 2011
4; these figures show the crystal-platelet structures of IMCs of Sn with Au, Ag, Ag and Pd, respectively. Tese IMC-crystal platelets, variously Sn,
, AuSn4 IMCs. Courtesy of
Figure 3. Etched section of solder joint showing the crystalline platelet-structure, as well as some needle-structure of Ag3
IMC. Courtesy of Reza Ghaffarian, JPL, USA.
Sn
Figure 2. Micro-photo showing the fragile crystalline platelet-structure of Ag3
Sn IMC
having caused fractures in the solder joint after 1750 cycles of –55⇔+150°C ATC. Source: H. Walter et al, AMIC GmbH/TU Berlin/Fraunhofer Institut, Germany.
perature (DBTT) is clearly evident. Te late Roger Wild of IBM seems to be
the first to have observed the practical reli- ability consequences of the ductile-to-brit- tle transition11
. In Figure 6, in a Manson-
Coffin plot of thermal cycling data from eutectic Sn-Pb solder joints that were sub- jected to one of three temperature cycles: TC1: -50⇔+25°C, TC2: -50⇔+100°C, and TC3: -50⇔+100°C, are plotted. Tis plot shows that at a given level of shear strain, the number of cycles-to-failure for TC3 are significantly less than those of
the other
two thermal cycles. It is believed that this is caused by the synergy of low-tempera- ture embrittlement with high-temperature creep-fatigue. Cracks nucleating during the high-temperature
creep-fatigue stage of
solder damage will more easily propagate in the solder during the low-temperature part of the TC3 thermal cycle, due to the notch sensitivity of the partially embrittled Sn-rich phase in the eutectic Sn-Pb solder. Te contribution of both low-tempera- ture embrittlement and high-temperature
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