Test data requirements for assessment of alternative Pb-free solder alloys
despite having acceptable performance shown in Table 1. These detailed tests and required for all surface mount alloy reli-
when compared to SnPb. If they were only procedures, as well as test board designs, ability tests. Reworked samples have not
compared to SnPb, there would be no will be reflected in HP specifications for been specified for these four tests, as the
direct assessment against the current solu- qualifying solder paste alloys, wave solder requirement for properly reworked joints
tion. By comparing the new alloy against and minipot rework alloys and BGA/CSP is that they meet the same requirements as
both high silver SAC and SnPb controls, ball alloys. for as-formed joints. Standardized board
the acceptance criteria can be set with It is important to note that the goal of designs for these tests are being finalized
respect to historical performance of a well these tests is the assessment of the solder, and will be included in the HP alloy quali-
known material in addition to the current
solutions.
In many cases, reasonable simplifica-
tions were made in selecting controls and
“A significant obstacle to useful,
variables in an effort to reduce the number
of tests and combinations. For example, a
wide variety surface finishes are available,
data-driven assessments of alternate
however, in some tests the number of fin-
ishes was narrowed based on the interme-
Pb-free alloys has been the inconsistent
tallic compounds (IMC) formed or based
on known failure modes. For example, in
testing that has been performed on the
mechanical shock testing, NiAu and OSP
were chosen as surface finishes because
materials.”
they form the two IMCs of interest and
because NiSn intermetallics have been as-
sociated with sporadic, low stress fractures not the laminates, surface finishes, or fication specifications that will be publicly
in joints made with high silver SAC alloys. equipment capability. Many typical tests released in 2008.
Another significant benefit of selecting that may be found in other assessments TC1 (0°C to 100°C) is the default
control alloys is that they ensure the integ- have been excluded or structured differ- condition for accelerated thermal cycling
rity of the tests by allowing comparison to ently than traditional process optimization (ATC), although TC2 (-25°C to 100°C) is
published literature. experiments in order to remain focused on an acceptable alternative. The disadvantage
the alloy. of the smaller ΔT in TC1 as compared
contamination reports Although the number of tests, combi- with other test conditions is outweighed by
A central feature of this protocol is that nations, controls and samples may seem the widespread use of the 0 to 100ºC cycle
most tests require complete contamination overwhelming at first, these tests were actu- in TC1, which makes data generation, col-
reports on the actual materials used and ally selected from a much longer list of po- lection and comparison easier.
not only on a representative sample. This tential tests. This set of tests was viewed as Multiple thermal cycle conditions
requirement was established because many the minimum set required to generate suf- have not been specified in this assessment.
of the new alloys have been designed to ficient information to assess the suitability Although multiple conditions are required
include very low-level additions of dopants, of a high Sn solder in the SnAgCu, SnAg, in developing acceleration models, it was
which serve as active contributors to the or SnCu families intended for use in high not considered strictly necessary to assess
functionality of the materials. volume manufacturing of electronics. It is performance in multiple test conditions
Since most alloys with dopants have unlikely that a single supplier will complete to make reasonable decisions on the suit-
been patented based on claims that the all portions of the testing alone, but rather ability of an alloy. Of course, further work
additions make the solder different enough partners with appropriate capabilities will would be needed to fully characterize and
from existing materials to warrant IP probably collaborate to generate the data model a new alloy.
protection, the logical conclusion is that for a material. Combinations of alloys and solder balls
these dopants must be accounted for in all have been selected to evaluate the compat-
tests, even when the additions would have Reliability ibility of new alloys with existing technolo-
been considered contaminants rather than Mechanical and thermal fatigue reliability gies. For example, new paste alloys are
a functional part of the composition in the tests are the primary tests required for any evaluated using BGAs with SAC105 balls
past. Only complete contamination reports new alloy. Many reliability tests can be in ATC and with BGAs with SAC405 balls
on actual tested material will allow the fair used to evaluate alloys, but four key tests in shock, vibration and bend. These cases
comparison of alloys and the identification were chosen as the most critical for surface are likely to occur in common applications
of sources of confounding results, should mount solder alloys: and problems with these combinations
functional levels of dopants be present as would indicate potential compatibility
contaminants.
• Accelerated thermal cycling
issues with currently available technologies.
(IPC-9701-A, TC1)
No acceptance criteria have been specified
Tests and reporting
• Mechanical shock
and specific applications designed without
The required tests are divided into three
(JESD22-B111, Condition B)
these alloy combinations might be able
major areas:
• Vibration (IEC 60068-2-64)
to use a material that showed poor results
• Reliability
• Four point bend (IPC-9702)
in these tests. However, it is desirable to
• Manufacturability
Daisy-chained components and the use identify compatibility issues wherever pos-
• Material properties
of continuous monitoring during testing sible, even when it is not a formal criterion
The complete set of tests and parameters is
are essential for these evaluations and are for acceptance.
www.globalsmt.net Global SMT & Packaging – November 2008 – 13
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