Test data requirements for assessment of alternative Pb-free solder alloys
The BGA packages selected for these not be robust enough for general use, and for assessing material suitability:
tests have relatively large pitches (0.5 mm characterizing the entire process window is
• Liquidus, solidus temperatures
and 1.0 mm). This choice was made in more important in this type of assessment
(ASTM E794-06)
order to focus on the performance of the than identifying the existence of single
• Electrical resistivity (ASTM B193-
bulk solder and not force failure modes point solutions.
02)
primarily associated with surface finishes or Understanding the process window
• Thermal conductivity
package types. and behavior over an extended temperature
• Young’s modulus, shear modulus
With respect to through-hole joints, range also helps to gauge the likelihood of
and Poisson’s ratio (ultrasonic stress
these four tests (ATC, shock, vibration and processes being run beyond component
wave propagation technique by
bend) are not as useful in completing reli- temperature limits. For example, consider
ASTM 1875-00)
ability assessments as they are for assessing a case in which an alloy yields better results
• Stress-strain curve, 0.2% yield
surface mount joints. Through hole joints as the temperature increases all the way to
strength, ultimate tensile strength
are known to be much more robust than very high temperatures. Even if the alloy is
and elongation (tensile testing)
surface mount joints in ATC and mechani- capable of soldering at lower temperatures,
• Density
cal tests and testing to failure would be at least some manufacturers will probably
• Vickers hardness (ASTM E92-
impractical. So in lieu of these tests, only a attempt to run the process at the highest
82(2003)e2)
pin push or pull test (with ATC, shock and temperatures to achieve the highest yield,
• Coefficient of thermal expansion
vibration preconditioning) is necessary for potentially damaging components. There-
(CTE) (ASTM E831-06)
reliability assessments of wave and minipot fore, characterization of manufacturing per-
rework alloys.
[29]
formance across the entire process window Determining some material properties
is needed for alloy assessment. Focusing on may be considered optional in the case of
Manufacturability characterizing the process window means evaluating SnAgCu, SnAg and SnCu alloys
The second major area for alloy evaluation that some tests in this protocol have more because of the expected similarity with
is manufacturability. The objective of these levels in the DOEs than might be used known materials. However, if this assess-
experiments is to characterize the complete for traditional process optimization. In ment approach is extended to materials
process window of a new material by deter- addition, the range of process temperatures beyond these alloy families, all tests should
mining the impact of time and temperature and variables is larger than usual in order be required.
process settings on important response to help determine the curvature of the
variables. This information is critical for response data. It should be noted that the conclusion
assessing the suitability of a material for use data from the three inner levels of the five This paper presents a core framework
in high volume electronics assembly. The level DOEs can still be used for process of tests that can be used to evaluate and
following tests have been included in the optimization. compare alternative Pb-free alloys based
manufacturing assessment for new alloys: With respect to the test boards, the on the reliability, manufacturability and
reflow test vehicle incorporates component material properties of the alloys. Detailed
• Wave solder process characteriza-
combinations to evaluate the compatibility test definitions and reporting requirements
tion (HP DOE)
of new alloys with SAC305 and SAC105. have been established in order to enable
• Reflow process characterization
The HP designed Culebra test board was meaningful data comparisons between
(HP DOE)
chosen as the test vehicle for the through alloy tests. This protocol is appropriate for
• Rework process characterization
hole experiments. A single thickness is used evaluating SnAgCu, SnAg and SnCu alloys
and copper dissolution assessment
for all tests (0.130 inches) because pin wet- for use in electronics.
(HP DOE)
ted length is the measured variable rather These tests will be reflected in HP
• Wetting balance (IPC J-STD-003,
than percent hole fill.29 By using pin wet- specifications that will be publicly released
F1)
ted length as the response variable, results in 2008 for the qualification of solder paste
Assessing the manufacturability of solder from thick boards can be used to predict alloys, wave solder and minipot rework
alloys in a reproducible way is challenging behavior for thinner boards. alloys and BGA/CSP ball alloys, although
because, unlike reliability tests, standard minor adjustments may be made as test
manufacturing test methods and test Material Properties boards and BOMs are finalized.
boards are not available. A wide variety of Many physical properties can be measured
equipment combinations and processes are for solder alloys; however, not all of them references
used, and reported data are rarely compa- are useful or relevant for assessing the suit-
1. US5833921, Lead-free, low-temperature
rable to other tests. In addition, most avail- ability or acceptability of new alloys. Only
solder, Paruchuri, Ford, 1999
able data are focused on process optimiza- the liquidus and solidus temperature mea-
2. US5871690, Low-temperature solder
tion and capability assessment, rather than surements are considered strictly required
compositions, Achari, Ford, 1999
identifying the complete process window. in the assessment protocol. Depending
3. US5487868, Tin based solder alloy con-
The goal of the manufacturing tests on the application, it may be necessary to
taining lead, antimony and tellurium,
in this protocol is to characterize the determine properties in addition to liqui-
Nishimura, Nihon, 1999
4. Schroeder, Hua, “Feasibility Study of
performance of new alloys over their dus and solidus temperatures in order to
57Bi-42Sn-1Ag Solder,” APEX, 2001
entire process window, not just to identify complete comprehensive engineering evalu-
5. US6180055, Lead-free solder alloy,
optimal points. Many alternate alloys can ations. For example, the stress-strain and
Nishimura, Nihon, 2001
be successful to some degree in highly elastic properties of the alloys are needed if
6. US6228322, Solder alloy composition,
optimized processes due to the inherent finite element analysis is to be performed.
Takeda, Sony, 2001
solderability of high Sn alloys. However, The following tests generate data on
7. US6229248, Solder alloy, Kusabiraki,
alloys with very small process windows may basic material properties that may be useful
Murata, 2001
18 – Global SMT & Packaging – November 2008
www.globalsmt.net
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