+----------------------+-----------------------------------------------+
| ![](media/           | National Electronics Manufacturing            |
| image1.png){width="2 | Initiative, Inc.                              |
| .3333333333333335in" |                                               |
| height="1.95625in"}  | 2214 Rock Hill Road • Suite 110               |
|                      |                                               |
|                      | Herndon, VA 20170-4214                        |
|                      |                                               |
|                      | Tel: 703-834-0330 Fax: 703-834-2735           |
+----------------------+-----------------------------------------------+

Tuesday, November 14, 2023

To: University Center

Dear Prof. :

NEMI has undertaken a project on lead free solders with the goal of, by
April, 2001, providing sufficient information for NEMI member companies
to change over their assembly lines to lead-free solders if they so
desire. To carry out this project, we have formed several working groups
to concentrate on specific parts of the program. One group is
responsible for examining and characterizing suitable alloys.

The NEMI Alloy Group realizes that we do not have sufficient time or
resources to completely characterize the selected Sn/Ag/Cu system,
particularly with respect to mechanical properties. We are very
interested to know whether you or your colleagues are involved in such
studies, and what information is already available. We would appreciate
knowing your plans and encourage you to consider the industry priorities
shown below when planning your next project(s). Although NEMI is not a
source of research funding, we may serve with our support and help to
the appropriate funding agencies by demonstrating relevance of this
research with the electronics industry needs.

The attached documents provide background information on the solder
formulations of interest and describe the areas where we believe
research work is needed. We would appreciate hearing from you whether
you are currently involved in, or plan on initiating, research
activities in the area of lead-free solders.

Thank you for your consideration of this request.

Ronald W. Gedney

Vice President of Operations

703-834-2084

<rgedney@nemi.org>

There are two sets of needed data as described below. First, additional
data on the mechanical/ materials behavior of the lead-free solders
listed below are needed. Second, a database on a wider range of
lead-free alloy properties needs to be established. In the former case,
we are asking that you use this information as input when developing
your research plans. In the latter case we are asking that you send any
relevant data, including preprints, reprints, and even raw data, to the
NEMI Alloy Selection Group for inclusion in the database. Please send
this information to Tom Siewert, NIST, Mailcode 853.07, 325 Broadway,
Boulder, CO 80303. If you have any questions, please contact Tom Siewert
at(303) 497-3523/<siewert@boulder.nist.gov> or me at (703)
834-2084/<rgedney@nemi.org>

## Mechanical/materials characterization of lead-free solders

The main alloy under consideration for surface mount applications by the
NEMI lead-free task force is Sn3.9Ag0.6Cu. For wave soldering
applications, the main alloys are Sn0.7Cu as a first choice and Sn3.5Ag
as a second choice. Within the NEMI program thermal cycling tests on
assemblies will be conducted comparing the Sn3.9Ag0.6Cu alloy with the
Sn37Pb control. In order to supplement these test vehicle-based tests
and to provide a basis for models of thermomechanical fatigue for
lead-free alloys, a comprehensive mechanical/ materials property
database is needed. Such data for lead-free alloys are limited and need
to be collected using common measurement methods by the microelectronics
industry, national laboratories, and academic institutions.

Obtaining an understanding of how these alloys behave at least
comparable to our understanding of eutectic SnPb is required for the
following reasons:

1.  It is impractical (and probably impossible) to rerun \"standard\"
    reliability qualifications, such as accelerated thermal cycling
    (ATC), for all packages and assemblies currently in use.

2.  Reliability estimation / extrapolation techniques using mechanical
    testing data and thermal cycling test data to predict the
    package/assembly field performance do not exist for the new
    materials. These must be developed.

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i.  Finite Element Model (FEM) based projections require new valid
    constitutive equations (stress/strain rate /temperature relations)
    and new fatigue damage criteria based on thermomechanical loading
    history for the lead-free solders.

ii. Engineering acceleration factor models, such as the frequency
    modified Coffin-Manson criteria, are nonexistent or at least
    untested for the new materials in the limit of actual solder joints,
    such as BGA solder joints.

iii. Rules of thumb or engineering intuition relating such things as ATC
     test results or solder joint defects to field performance, have not
     been established.

At this point it is not clear what criteria should be used for ranking
and selecting of the new materials. Bulk solder mechanical properties
need to be characterized to provide a basis for FEM modeling with this
modeling used for the development of fatigue damage criteria based on
ATC test results. Several temperature ranges and cycling frequencies are
required to fully validate the fatigue criteria. ATC tests should
include multiple temperature ramp rates and extremes for developing the
damage criteria. Throughout the industry, different temperature cycling
regimes are used depending on end use and company tradition. In addition
to the --55°C to 125°C cycling regime,

-40°C to 125°C and 0°C to 100°C are gaining widespread use within the
industry. (--40°C to 125°C will be used in the NEMI lead-free thermal
cycling trials.)

Recommended testing procedures are those described in the NCMS Lead-Free
Solder Project CDROM. The CDROM may be obtained through NCMS at
<http://www.ncms.org/3portfolio/1ProjectPortfolio/pubs.htm#LeadFreeSolderCD>

Additional details of the testing/procedures required are provided
below.

### Microstructure and microstructural aging

The test samples must have representative microstructures, i.e.
equivalent with the microstructure produced in actual solder joints.
This cannot be over emphasized. The plastic deformation behavior and the
fatigue mechanisms depend critically on the microstructure.

Microstructural aging processes coupled with deformation should be
studied for selected alloys. An evolving microstructure will control
inelastic/plastic deformation behavior of the materials. Aged joints may
be more indicative of actual field conditions.

### Mechanical characterization 

Mechanical characterization of strain rate, as a function of stress,
temperature and microstructure, of the materials should be undertaken
utilizing stress relaxation testing techniques along with some
conventional stress/strain testing under isothermal, constant strain
rate controlled, conditions. The strain rate range of interest is from
approximately 10^-3^ to 10^-7^ /s. This strain rate range should be
measured at temperatures from approximately -- 55°C to 150°C. Some
cyclic stress vs strain responses should be measured to fully understand
the monotonic and cyclic plastic hardening behavior as well as the
elastic behavior. The proposed stress relaxation testing is quite
general and equivalent to conventional stress vs strain testing over a
spectrum of constant strain rates. However, the stress relaxation
testing can be more economical and better suited to the low strain rate
end of the desired strain rate range. If the microstructure is evolving,
there are issues of how to capture such a microstructure evolution in a
database, especially since the evolution (for Pb-Sn anyway) is not
uniform. There are different evolution paths as a function of position
and time, as the joints unzip and local stress state changes.

The effect of pad finish (OSP, NiAu, Palladium, Silver) should be
incorporated into the material/ mechanical characterizations of the
specimens. Wherever possible, specimens should be prepared and tests
conducted, which are either standardized or related to standards, issued
by relevant international and national standardization authorities. For
example recent work on shear strength testing of solder joints has been
conducted to compare and correlate a particular test method with a
standardized shear test method used on other materials in industry
(ref.1).

### Finite Element Modelling

FEM \"ready\" constitutive models must be developed from the mechanical
testing data described above for the new materials and embedded in a
commercial FEM code. These models can be calibrated using the test
results discussed above. For instance, the conventional stress vs strain
curves can be generated from the load relaxation data, using the FEM
models. In addition it is important that the FEM models be verified
against the behavior of single solder ball joints or arrays of such
solder joints in isothermal load vs displacement testing to assess any
microstructural questions and to assess model accuracy .

### Fatigue criteria 

Fatigue criteria should be developed based on the ATC test data and FEM
modeling for various types of solder joints and packages. The FEM
analysis requires that valid constitutive (stress, strain, and strain
rate) equations and damage criteria be developed. The ATC testing should
include multiple strain ranges, temperatures, and cyclic frequencies
with explicitly determined ramp versus dwell time. Possibly, a general
fatigue model can be developed and used in conjunction with FEM
analysis.

If the testing outlined for the alloys were successfully undertaken,
then the resulting data would serve as a benchmark for reliability
modeling throughout the industry.

# Lead-Free Solder Database 

A list of the relevant materials properties needed for the alloy
database is provided below. Most of the mechanical tests would be
undertaken using the most common strain rates. There is also a list of
properties required to be determined for lead-free solderable coatings
such as PCB and component finishes. The priorities for specific
information is noted with \"1\" being of highest priority and \"3\" of
lowest priority. \*For eutectic SnPb, there is a need for comparison
data for the same measurement techniques used for the lead-free alloys.
The priorities for eutectic SnPb are the same as for the lead-free
alloys.

+----------------------------------------+---------+-----+-----+-----+
| **Properties of Lead-Free Solders**    |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
|                                        |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Thermal, Electrical, and Wetting       | Sn3.9   | S   | S   | Sn  |
| Properties                             | Ag0.6Cu | n0. | n3. | 37P |
|                                        |         | 7Cu | 5Ag | b\* |
+----------------------------------------+---------+-----+-----+-----+
| Coefficient of Thermal Expansion (CTE) | 2       | 2   | 2   |     |
+----------------------------------------+---------+-----+-----+-----+
| Vol. Change on Freezing (%)            | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Specific Heat (JKg-1K-1)               | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Latent Heat (KiloJ Kg-1)               | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Thermal Diffusivity (mm2/s)            | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Thermal Conductivity (Wm-1K-1)         | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Electrical Conductivity (%IACS)        | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Electrical Resistivity (micro Ohm cm)  | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Surface Tension(mNm-1) at Temperature  | 3       | 3   | 3   |     |
| of solder/T(mp of solder)              |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Surface Tension at (mNm-1) ) at        | 3       | 3   | 3   |     |
| Temperature of Solder/T(mp of solder)  |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Wetting Time @ 0 Force (sec) as a      | 3       | 3   | 3   |     |
| function of Temperature of solder/T(mp |         |     |     |     |
| of solder)                             |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Wetting Time @ 2/3 Force (sec) as a    | 3       | 3   | 3   |     |
| function of Temperature of solder/T(mp |         |     |     |     |
| of solder)                             |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Max Wetting Force (micro N) as a       | 3       | 3   | 3   |     |
| function of Temperature of solder/T(mp |         |     |     |     |
| of solder)                             |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
|                                        |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Mechanical Properties                  | 1       | 1   | 1   |     |
+----------------------------------------+---------+-----+-----+-----+
| Ultimate Tensile Strength(MPa) at room | 1       | 1   | 1   |     |
| T                                      |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Shear Strength (MPa)                   | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates, e.g.      |         |     |     |     |
| 10^-3^ to 10^-7^ /s)                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Ring in Plug Shear (kg)                | 3       | 3   | 3   |     |
|                                        |         |     |     |     |
| (at particular strain rates, e.g.      |         |     |     |     |
| 10^-3^ to 10^-7^ /s)                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Elastic (Young's) Modulus (GPa) at     | 3       | 3   | 3   |     |
| 25°C                                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Elastic (Young's) Modulus (GPa) at     | 3       | 3   | 3   |     |
| 50°C                                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Elastic (Young's) Modulus (GPa) at     | 3       | 3   | 3   |     |
| 100°C                                  |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Total Elongation (%)                   | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates, e.g.      |         |     |     |     |
| 10^-3^ to 10^-7^ /s)                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Uniform Elongation (%) at room T       | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates, e.g.      |         |     |     |     |
| 10^-3^ to 10^-7^ /s)                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Yield Strength (MPa) at room T         | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates,           |         |     |     |     |
| e.g.10^-3^ to 10^-7^ /s)               |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Work Hardening Coefficient             | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates,           |         |     |     |     |
| e.g.10^-3^ to 10^-7^ /s)               |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Creep Resistance (at particular strain | 1       | 1   | 1   |     |
| rates)                                 |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Min.Creep Strain Rate \@Stress of      | 1       | 1   | 1   |     |
| 20MPa (1/s) at room T                  |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Min.Creep Strain Rate \@Stress of      | 1       | 1   | 1   |     |
| 20MPa (1/s) 125°C                      |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Hardness (HV)                          | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Thermomechanical Fatigue Resistance    | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates, e.g.      |         |     |     |     |
| 10^-3^ to 10^-7^ /s)                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Isothermal Fatigue Data                | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates e.g.       |         |     |     |     |
| 10^-3^ to 10^-7^ /s)                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Thermal Fatigue Hysteresis behavior    | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates e.g.       |         |     |     |     |
| 10^-3^ to 10^-7^ /s)                   |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Constitutive Behavior                  | 1       | 1   | 1   |     |
|                                        |         |     |     |     |
| (at particular strain rates)           |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Stress Rupture                         | 2       | 2   | 2   |     |
|                                        |         |     |     |     |
| (at particular strain rates)           |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+
| Acoustic measurements                  | 3       | 3   | 3   |     |
+----------------------------------------+---------+-----+-----+-----+
| Fracture Toughness(MPa- M1/2) Room T   | 2       | 2   | 2   |     |
|                                        |         |     |     |     |
| (at particular strain rates)           |         |     |     |     |
+----------------------------------------+---------+-----+-----+-----+

There is very little information on the properties of lead-free
component and board surface finishes, their performance during assembly,
and their effect on the thermomechanical fatigue properties of the
assemblies. Useful data on lead-free surface finishes will include:
solderability before and after aging (aging conditions: room
temperature, dry aging, steam aging, temperature and humidity aging),
whisker formation testing for tin coatings after aging at 50°C for at
least 2 months and as a funciton of component lead/coupon deformation,
microstructure (grain size, preferred crystal orientation), hardness,
ductility or elongation, organic content in the deposit, and wear and
contact resistance.