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Page 66

March, 2024

Nihon Superior “Cracks the Code”
for Reliable Low-Temperature
Solder Formulation
By Keith Sweatman, Senior Technical Advisor, Nihon SuperiorT

he construction of IC packages, with their multiple
layers of different materials with different coefficients ofthermal expansion (CTE) means
that, as the temperature towhich they are exposed increases
beyond the temperature at which
the layers were bonded, thepackage will warp.
That problem has been recognized since the early 2000s
when, for example, the JapaneseElectronics and Information
Technology Association (JEITA)
proposed, on the basis of carefulmeasurements, the limit thatneeded to be placed on theamount of warpage that could be
tolerated without compromising
the quality of the solder joints.
However, the advance ofintegrated circuit capability
could not be constrained. During
the past decade, as the size andDiagram showing large IC package warpage.
complexity of IC packages in- quence of warpage exceedingcreased, package warpage moved that specification. Examples
beyond the limits of the proposed include “head-in-pillow,” which
JEITA specification. The conse- results from splitting of the fillet
quence was a steady increase in of molten solder between the padthe incidence of the solder joint and the package, and “non-wetdefects that are a direct conse- ted opens,” in which there iscomplete separation of themolten solder from the pad or thecomponents as the gap between
the component and the substratewidens further.
While a switch from polymer
to glass substrates offers thepromise of an ultimate solution
to the problem of warpage of IC
packages, technical and cost factors seem likely to mean that, forsome time yet, electronics manufacturers are going to have toassemble packages that suffer
significant warpage at the reflow
temperatures required by thecurrent generation of lead-freesolders.
Low-Temperature Solder
A solution to the warpage
problem, which could potentiallycome with several additional costand environmental benefits,
Continued on next page

March, 2024

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Page 67

Nihon Superior “Cracks the Code”
Continued from previous pagewould be to switch to a solder
alloy that can form reliable joints
at a peak reflow temperature
lower than the 482 to 500°F (250to 260°C) required by the Sn3.0Ag-0.5Cu (SAC305) alloy thathas become the standard leadfree alloy for reflow soldering.
On the basis of the characteristics of widely used IC packages the requirement for theavoidance of warpage-related
defects is that it should be possible to form a solder joint thatmeets the usual quality criteriawith a peak reflow temperatureless than 392°F (200°C).
Because of its relatively low
melting point and its uniqueability to react with most of thetermination finishes used in electronics, tin (Sn) is an essential
ingredient. Nature offers only
two elements that can reduce themelting point of the resulting
alloy to a temperature thatmakes reflow at less than 392°F(200°C) possible without introducing other complications: bismuth (Bi) and indium (In). The
high cost of indium means thatits use is likely to be limited tospecialized high value applications.
Bismuth, which is typicallycheaper than tin, provides a significant saving in material cost,
and the long history of the use of
bismuth salts for the treatment
of indigestion means that its
non-toxicity is well proven.
There is a tin-bismutheutectic, Sn-57Bi with a melting
point of 282.2°F (139°C) and wetting and flow characteristics thatsuggest that, with its sharp melting point, it might be the ideal
solution. However, because of
the particular characteristics ofthe near-pure bismuth phasethat forms in Sn-Bi alloys, a consideration that is as importantas its effect on melting point is
its mechanical properties.
Optimizing Bismuth Content
Studies of the failure mechanism in Sn-Bi alloys indicatethat at strain rates at the higher
end of the range to which solder
joints can be exposed in service
the Bi phase cracks in a brittle
manner with the crack propagating into the tin phase in which it
is embedded triggering brittle
failure of the whole solder joint.
The Nihon Superior view
was, therefore, that careful consideration had to be given to thevolume fraction of the bismuth
phase in the solder joint.
Recognizing that reality, in theirformulation of a solder thatwould meet the less than 392°F(200°C) peak reflow temperature
criterion Nihon Superior reduced
the Bi content to the lowest levelthat would still leave the alloy
with melting characteristics thatwould make it possible to meetthat criterion. That Bi level wasidentified as 37 wt% and thatprovided the basis for the alloy
that, with further optimization,

was introduced to the electronics
manufacturing industry underthe brand name TempSave B37.”
Nihon Superior acknowl-

Two common defects caused by package warpage.

edges that by reducing the Bi
content to 37 wt% they haveincreased the melting behavior
from the sharp 282.2°F (139°C)
of the eutectic to a melting rangeof 282.2 to 345.2°F (139 to174°C).
There was a view in theindustry that such a melting
range would increase the likelihood of a phenomenon known as
hot tearing,” the result of which
is a cavity or crack.
However, the behavior ofmetals in what is sometimes
known as “the pasty range” isContinued on page 72

www.us - tech.com

Page 72

March, 2024

The State of the Art in Flying Probe Test

Continued from previous page

and the CAD data from thedesign to automatically generate
a complete test program.
In order to minimize thesetup times for prototype, preseries, and production tests thisprocess is streamlined to be fastand easy, and the optimized

management of the software
allows the parallelization of different types of tests, saving precious operational time.
It can also include a smart
analysis functionality that, using
a set of algorithms based on theprinciples of artificial intelligence, further optimizes the test

The material matters in material handling

flow during execution, while
keeping coverage objectives
unchanged.
Full compatibility with thesmart factory is now an essential
requirement, and the software
platform of an advanced flying
probe system is able to provide
smart integration with all
aspects of the customer’s production processes (data collection,traceability, interaction withMES, repair operations).
The platform can also
include onboard machine industrial management solutions forremote monitoring of current
and voltage consumption, checking the status of the power network and temperature level, forthe status of the light indicators,
parameters useful to indicate thecorrect functioning of the test,
and to provide information related to predictive maintenance inreal time.
Seica’s Solution
Seica’s PILOT VX is a flexible and configurable test system,with a set of technologicallyadvanced tools and features,

capable of providing the test solutions required by the enormous
diversity that characterizes today’s electronics.
An extraordinarily powerful
resource, the system is usable inevery phase of the product lifecycle.
Its ability to provide immediate and extremely precise
access to all test points, combined with a vast suite of hardware and software measurement
tools, allows the testing and validation of prototypes and preseries quickly, without requiring
operators to have in-depth andspecific training on how to generate and execute test programs.
The PILOT VX has established itself as a new benchmark
in terms of speed and performance, a fully automated test solution that responds to the fundamental concerns of electronicboard manufacturers who want
to optimize their investments.
Contact: Seica, Inc., 110
Avco Road, Haverhill, MA 01835% 603-890-6002
E-mail: davidsigillo@seicausa.com
Web: www.seica-na.com r

Nihon Superior Attains
Reliable Low-Temp
Solder Formulation
Continued from page 67
complex and solidification over atemperature range is not theonly prerequisite for the occurrence of hot-tearing. As predictedby traditional solidification models, hot tearing has not been anissue for the TempSave B37alloy.
With the optimal bismuthlevel determined, the next stepin the development of TempSave

B37 was to fine-tune themechanical properties of the tin
and bismuth phases with furtheradditions. The objective was toadjust the mechanical propertiesof those two phases until, when
the solder was subjected tostress, they worked together toaccommodate the resulting
strain in a way that ensured
maximum ductility.
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Ball shear testing identified
the levels of Sb, Cu and Ni that
optimized the toughness of thejoint between the Sn-37Bi solderball and the copper substrate asmeasured by the energy
absorbed during the test andthat is the alloy that Nihon
Superior have introduced to themarket as TempSave B37.
Extensive testing by an independent laboratory and commercial evaluations have confirmedthat the TempSave B37 has lived
up to the expectation created by
the development process andoffers the prospect of high-yield
low-temperature reflow of large
IC packages.
Contact: Nihon Superior
USA, LLC, 1395 Hawk Island
Drive, Osage Beach, MO 65065% 573-280-2357
E-mail: k.howell@nihonsuperior.co.jp
Web: www.nihonsuperior.com rMarch, 2024

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