Session 3 Assessment of Long-term Reliability in Lead-free ...

Session 3

Assessment of Long-term Reliability in Lead-free Assemblies

Sanka Ganesan, Ji Wu, and Michael Pecht
CALCE Electronic Products & Systems Center

University of Maryland
College Park, MD 20742, USA

www.calce.umd.edu

Ricky Lee and Jeffery Lo
Hong Kong University of Science and Technology

Yun Fu, Yonghong Li, and Ming Xu
Aero Combined Environment Laboratory
China Aero-Polytechnology Establishment, Beijing 100028, P.R.China

Abstract 2 What We Know?

Abundant data exist on the short-term reliability (i.e. By definition, lead-free soldering assembly involves
less than 5 years) of lead-free solder joints under single the use of only lead-free materials. This applies both to
loading conditions. Data on combined loading conditions printed circuit boards (PCB) soldering materials, namely
and long-term reliability is scarce. The lead-free electronics solder paste or wave solder for surface-mount or through-
will be deployed in many products that serve markets hole assembly respectively, and finishes used on
where long-term (greater than 5 years) is a critical component terminals and PCB mounting pads. Ganesan
requirement. In many applications, electronics will be and Pecht [1] provide a summary of lead-free solder alloy
subjected to long-term exposure to temperature extremes compositions. At present, tin-rich alloys with some
(high and low), humidity, atmospheric contaminants, and combinations of silver, copper, bismuth, and antimony are
combined thermo-mechanical loading (temperature cycling the leading lead-free solder candidate materials. Among
and vibrations). those, tin-silver-copper (SAC) eutectic alloy, which has a
melting point of approximately 217°C, appears to be one of
To assess long-term reliability, an experimental plan the most promising compositions, based on both current
includes assessing the interactions among the applications industry trends, and recommendations by CALCE EPSC,
conditions. The interacting influences include the growth the International Tin Research Institute (ITRI), National
of intermetallics at the solder joints due to high Electronics Manufacturing Initiative (NEMI), and Japan
temperature exposure, electro-chemical degradation of Electronics and Information Technology Industries
electronics assemblies due to the exposure to humidity and Association (JEITA).
atmospheric contaminants, formation of tin pest due to
extended exposure to low temperature and the effects of There are over 300 lead-free patents for the ternary
combined thermo-mechanical loading conditions on the SAC alloy. Patents depend on such factors as “solder alloy
electronics assemblies. The test vehicle designs incorporate composition,” “solder joint” or “intermetallic compound”.
commercial variations in PCB pad finishes, component Patent and intellectual property issues on lead-free alloys
lead finishes. Three PCBs designs were developed to have been discussed in studies conducted at CALCE [2-4].
assess reliability of surface mount assemblies and single
sided through-hole assemblies. The PCB materials include PCB pad finishes can affect the reliability of solder
FR4, polyimide for SMT assemblies and paper-phenolic joint since they may change the final composition of the
type (CEM-1) for through-hole assemblies. solder joints due to the dissolution of species from the
board finishes and the reaction between the solder alloy
1 Introduction and the board finishes. Previous studies showed that Sn-
3.8Ag-0.7Cu alloy should not be soldered onto a PCB pad
The electronics industry is migrating to lead-free with hot air solder leveling (HASL) finish. This
electronics, both to comply with government legislations combination resulted in a weak interface after high
and to increase market share through product temperature aging [14, 15]. Solectron study showed that
differentiation. Considering that lead-based electronics HASL finish creates a low temperature phase in reaction
have been in use for over 40 years, the adoption of lead- with Sn-3.5Ag solder in wave soldering which in turn
free technology represents a dramatic change. The resulted in fillet lifting [1]. PCB pad finish selection also
manufacturing of lead-free electronic products involves depends on the manufacturability: solderability and ability
assembling lead-free components to lead-free printed to create good solder joints after multiple reflows (may be
circuit boards using lead-free solder alloys. Key issues that required in complex systems). Based on these
are being addressed by academia and industry include lead- considerations, PCB pads with Sn or Ag metallization may
free solder alloy selection, characterization of lead-free offer a robust solution especially in no-clean flux assembly
solder alloy properties and behavior under various stress processes.
loading conditions, lead-free manufacturing, logistics and
intellectual property issues, and lead-free assembly On components, the source of lead (Pb) is usually the
reliability assessment. Sn-Pb alloy at the terminals. For the peripheral components,
Sn-Pb coating is used as lead finish, while for the array

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components, tin-lead-silver (SnPbAg) alloy balls are in use loading conditions was found to be significantly less than
as component terminals. To realize the transition of under temperature cycling. However, combined loading
peripheral components to lead-free, alternative coatings, conditions may be more representative of actual
such as pure tin (Sn), tin-bismuth (Sn-Bi), tin-copper (Sn- application environments than single loading tests. There
Cu) are being developed and implemented. There are also are no data on the long-term reliability of lead-free
commercially available preplated lead frame components electronics under combined loading conditions such as
with nickel-palladium-gold (NiPdAu) finish. From the temperature cycling and vibration conditions.
point view of plating manufacturability, tin plating (matte
finish) appears to be the leading candidate for lead 2.2 Intermetallic compounds
finishing, followed by Sn-Cu, Sn-Bi, and Ni/Pd/Au. A few
Japanese companies are implementing Sn-Bi plating as The formation of intermetallic compounds (IMC) at
lead finish material. However, there are possible concerns solder-pad interface is essential for creating a reliable joint.
in the long-term reliability of these components in various However, excessive growth of IMC will result in a brittle
application environments. Fine-pitch peripheral lead-free interface which can lead to solder failures during the
components have reliability concerns such as tin operating life of the product. IMCs form due to two factors:
whiskering for pure tin plating, and creep corrosion for wetting reaction of solder alloy with the pad metal on the
nickel-palladium-gold preplated lead frames. For array board during the soldering process; solid state ageing
packaging, tin-silver-copper appears to be the leading during the storage and operating life of the product. During
metallurgy for component terminals. the wetting reaction, the tin present in the solder
chemically reacts with the pad metal to form the IMC. The
Solder reliability is a concern in the transition to lead- extent of the IMC formation depends upon the type of pad
free electronics. There has been long-term, successful use finish metallurgies like copper (OSP), nickel (ENIG),
of tin-lead solders in terms of electronics manufacturing immersion silver or immersion tin. The copper-tin IMCs
and solder processing. By contrast, lead-free soldering form in the case of pad finish that contains copper,
assembly prompts many unanswered questions. The immersion tin and immersion silver. In the case of
prominent reliability concerns with the use of lead-free immersion silver, silver-tin IMCs can also form. Tin-
solders are solder joint durability, intermetallic growth, copper-nickel IMCs form in the case of pads containing
electro–chemical migration, tin pest and tin whiskers. nickel.

2.1 Solder Joint Reliability In solid state ageing, the IMCs form due to the
diffusion of reacting species through the initially formed
Over 90% of the previous studies on lead-free solders IMCs. Intermetallics growth in lead-free solder joints due
were undertaken using SnAgCu alloy. Although a variety to high-temperature ageing has been reported. The studies
of electronic packages have been considered, most of them conducted at CALCE EPSC and elsewhere [1, 14-16]
were surface-mount area array devices such as Ball Grid showed that intermetallics compounds grow during ageing
Arrays (BGA), Chip-Scale (CSP), Flip-Chip (FP) Packages, at high temperature exposure due to solid state diffusion of
Quad Flat Packages (QFPs). Consequently, there is reacting species. The growth follows the classical square
insufficient number of studies on long-term lead-free root of time dependence. For example, after 1000 hours of
solder joint reliability for through-hole components exposure at 150ºC, the SAC solder on copper pad exhibits
assembled by lead-free wave soldering; especially single an intermetallics thickness of about 7 microns. CALCE
sided boards. EPSC study also showed that IMC growth does not follow
the square root of time dependence in the case of
The thermo-mechanical durability of lead-free solder immersion silver due to the dissolution of silver in the bulk
joints in cyclic temperature conditions has been studied for solder [14]. However, the combined effects of
temperature cycle extremes of 0 and +100°C, -40 and intermetallics growth plus vibration have not been
+125°C, or -55 and +125°C, with -40ºC to 125ºC investigated.
temperature cycle being the most widely employed. The
number of cycles to failure under the above testing 2.3 Tin whiskering
conditions was typically found to be several thousand
cycles. Tin whiskering is one of the key reliability issues
associated with lead-free electronics components. Whiskers
Another aspect is the stress conditions experienced by are elongated single crystals of pure tin that have been
the solder joints during its operating life. The solder joints reported to grow to more than 10 mm (250 mils) in length
in packages like QFPs and PBGAs are subjected to lower (though they are more typically 1 mm or less) and from 0.3
stress conditions due to compliance of the solder joints and to 10µm in diameter (typically 1-3 µm). Whiskers grow
low thermal mismatch stresses compared to packages like spontaneously without an applied electric field or moisture
leadless ceramic chip carriers (large thermal mismatch (unlike dendrites) and independent of atmospheric pressure
stresses) with non-compliant solder joints. Previous (they grow in vacuum as well). Whiskers may be straight,
CALCE studies concluded that non-compliant lead-free kinked, hooked, or forked and some are reported to be
solder joints as in leadless ceramic chip carriers under hollow. Their outer surfaces are usually striated. Whiskers
performed Sn-Pb joints [5-13]. On the other hand, for can grow in non-filament type which is sometimes called
plastic QFPs and PBGAs, the reverse is true (lead-free lumps or flowers. Whisker growth may begin soon after
solder joints outperform the Sn-Pb solder joints), which is plating or initiate after years. The unpredictable nature of
consistent with the thermo-mechanical durability results whisker incubation and subsequent growth is of particular
reported from several independent studies across the concern to systems requiring long term, reliable operation
industry and academia. The time to failure under vibration [1].

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2.4 Tin pest between the lead-free solder joints. This issue may be
exacerbated in the situation of fine pitch components
Another issue associated with the use of lead-free tin- assembled with lead-free materials and process. These
rich alloys is the formation of “tin pest”. The allotropic issues need investigation to ensure long-term reliability
transformation of white (β) tin, which has a body-centered requirements.
tetragonal structure, to brittle gray (α) tin, which is a
diamond cubic crystal, is known as tin pest. This 3 What We Need to Know?
transformation occurs at 13.2°C. Above this temperature,
white tin is the stable form. Since the spontaneous All the changes to the bill of materials for electronics
formation of α-tin is rare in conventional Sn-Pb solder assembly in the context of lead-free migration are being
alloy, and tin pest has generally been ignored. This may not driven by high volume consumer, computer, and portable
hold in the case of some lead-free solders since the communication industry where the reliability requirements
microstructure of lead-free solders typically contains pure are not very stringent and the product life cycle is less than
tin grains with tin-copper and tin-silver intermetallic 5 years. However, the impact of these material changes on
compounds dispersed along the grain boundaries. The long-term or over 20 years of reliability is not understood.
allotropic transformation from white tin to gray tin is
accompanied by a 26% increase in volume. The volumetric Lead-free electronics will be deployed in many
strain in a solder joint can potentially have a detrimental products that serve markets where long-term (greater than
effect on its lifetime. More over, the gray tin is also brittle 5 years) is a critical requirement. In many applications,
and weak which in turn can result in joint failures, electronics will be subjected to long-term exposure to
especially in vibration loading conditions. temperature extremes (high and low), humidity,
atmospheric contaminants, and combined thermo-
Kariya studied pest formation of pure tin specimen mechanical loading (temperature cycling + vibrations).
(cast ingots) by aging it at a constant temperature of -18°C Thus, long-term reliability studies should comprehend the
for up to two years [18]. 40% of the specimen surface was interactions among these applications conditions and the
transformed into grey tin (tin pest) after ageing for 1.5 long test times. Thus the objectives of this long-term
years, and this percentage increased to approximately 70% reliability assessment of lead-free soldered assemblies are
after exposure for 1.8 years. The test results indicate that an to assess and compare the lead-free technology against the
incubation period is necessary for tin pest formation. No tin lead-based for long-term (10 to 30 years) reliability
pest was observed on the Sn-37Pb specimen. Kariya’s requirements, and provide recommendations to reduce
studies also provided the time that different alloy additions reliability risks to achieve long-term reliability goal of 10
retard tin pest transformation [19, 20]. He found that the to 30 years of operating life for lead-free products.
small additions of soluble alloying elements like bismuth
and antimony can retard the formation of tin pest. The key results expected from this long-term
reliability studies include: (1) extent of the growth of
During the life cycle of electronics, tin-rich lead-free intermetallics in the solder joints as a function of
solder joints can be subjected to temperature cycling commercially available PCB pad finishes and component
(typically between –55°C to 125°C) and/or prolonged finishes, (2) assessment of any yet-unknown risks, such as
exposure to temperatures below the transformation tin pest of high tin solders joints after long-term exposure
temperature, thereby subjecting the joints to allotropic to low temperature, (3) impact of PCB degradation due to
transformation. If the ambient temperature of the solder high temperature lead-free soldering in causing corrosion
joints is lower than the transformation temperature, the failures and or degradation in insulation resistance between
nucleation of the tin pest will be promoted. The nucleation solder interconnects, (4) vibration fatigue life and failure
of tin pest will be enhanced at lower ambient temperatures modes of lead-free solder joints with thicker intermetallics,
because the undercooling (difference between the service and possibly with tin pest, (5) Failure mechanisms, mode
temperature and the transformation temperature) increases. in solder joint failures in the combined temperature cycling
The impact of this phenomenon in solder joints has not + vibration tests, (6) Long-term life of lead-free assembly
been studied. Thus there is a need to understand and in comparison with the lead-based.
quantify the impact of tin pest on the reliability of lead-free
solder joints subjected to extended periods of low 4 Experimental Approach
temperature exposure in field applications.
The effort focuses on utilizing current knowledge,
2.5 Electro-chemical migration tools and resources to achieve the long-term reliability goal
for electronics products. The approach will be as follows:
The combined effects of moisture, temperature and
electrical bias on electro-chemical migration between lead- • Conduct experiments in accelerated environments
free solder joints on PCB assemblies have not been to assess the long-term reliability of commercially
considered. The electro-chemical migration is a concern available lead-free technologies.
because the boards are subjected to higher lead-free reflow
temperature, which may potentially cause higher • Perform failure analysis and interpret results.
degradation and higher migration of ions from the interior Experiments are designed with the objective of
of the boards to the surface compared to the boards determining the long-term (over 20 years) reliability of
subjected to the standard eutectic tin-lead reflow process. lead-free electronics assembly manufactured using
This effect may cause corrosion of PCB metallization. production qualified process. The study involves subjecting
Moreover long-term exposure to electrical bias and the test samples to short-term vibrations after long-term
humidity may cause migration of species (e.g., Ag) storage at high temperature (125ºC) and low temperature (-
55ºC), long-term exposure to high temperature (135ºC),

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high humidity (85%RH) with electrical bias, long-term technology. Moreover, all the assemblies will be built
temperature cycling (~10000 cycles) with temperature using production qualified process in order to represent the
excursions between -40ºC to 125ºC, and the combined actual production conditions of the PCB assembly. On the
loading conditions of temperature cycling with random PCB pad finish issue, the objective is to create variations
vibrations. representing the actual manufacturing conditions. Thus, the
experimental builds include PCB pad finishes with
The effect of long exposure to high temperature is immersion Ag, immersion Sn, electroless NiP/Au (ENIG)
expected to enhance the growth of intermetallics in the and organic solderability preservative (OSP). The
solder joints. Because intermetallic compounds are brittle experimental builds also include the PCB assemblies with
in nature, they are expected to degrade the solder joint life the current lead-based materials and processes to serve as
under vibration stress conditions. Thus this interaction control.
between the intermetallics growth and vibration stress
environment will be studied in the proposed experiments. 4.2 Test vehicles
On the other hand, the low temperature exposure is
expected to enhance the tin pest formation (due to SMT and through-hole boards were designed for this
allotropic transformation in tin) in tin rich lead-free solder study. The SMT test boards are made of two materials:
joints. The effect of this phenomenon on the solder joint FR4 (the glass transition temperature is ~130ºC) and
life under vibration stress conditions is unknown at this polyimide (glass transition temperature is ~250ºC). These
point of time. The proposed experiments are expected to boards have a daisy chain pattern to enable resistance
shed some light on this phenomenon. Long-term exposure monitoring of the solder joints of each component. The size
of electronics to high temperature high humidity plus of the board is 8” by 7”. The board typically has 1 ounce
electrical bias will enhance the electrochemical processes, thick copper (~35 microns) pads/traces. The board is
which ultimately can lead to corrosion of metallization, mounted with the selected packages with the dummy
reduction in insulation resistance between the solder joints. silicon die. The packages have wire bonds connecting the
This mechanism can be aggravated in lead-free assembly adjacent leads (not to the die) in order to enable resistance
because of the higher PCB assembly temperature possibly monitoring of the solder joints when assembled on to the
causing a more degradation (material degradation, test board.
contamination migration) compared to lead-based
assembly.. In random vibrations test, the vibration stress level will
depend upon the position on the board. This aspect needs
4.1 Variations in bill of materials for electronics to be considered in the component placement for PCB
products layout. The symmetrical layout of the vibration test board
is shown in Figure 1. Figure 2 is the SMT board design for
Most OEMs build electronics products consisting of temperature cycling tests. These boards incorporate a “cut-
several types of commercially available components which out” feature around each component (except resistors) to
include SMT components in QFPs, BGAs, SOICs, leadless facilitate component removal during temperature cycling
carriers, passives, and through-hole technology tests to enable failure analysis on each failed component.
components. Thus the experiment includes the SMT and These PCBs also incorporate a structure to study the
through-hole components. The basis for the selection of the electrochemical migration in solder joints. This structure
components is to create variations in the compliance of the consists of pads with the center–to-center spacing of 0.5
solder joints (example: QFPs vs. leadless ceramic chip mm (20 mils). This spacing represents the current fine
carrier) and thermal mismatch stresses (example: plastic pitch pad spacing practiced in the industry. During PCB
BGAs vs. ceramic carrier) on the solder joints. assembly, the solder paste will be reflowed on these pads
to create solder islands. During HAST testing
Next consideration involves creating variations in the (130ºC/85%RH), the electrical bias will be applied
lead finish of the components. The lead finish variations between them to simulate the accelerated electrochemical
for QFPs include electroplated matte Sn, Sn-Cu and Sn-Bi. migration effect between two adjacent solder joints.
As for BGAs, SnAgCu solder balls are used.
Figure 3 shows the layout for the single sided through-
With respect to PCB assembly, again industry has hole technology test board. This board is made of CEM-1
converged to the SnAgCu (SAC) paste for reflow soldering material and has a size of 8” X 5.5”. This board will be
of SMT components and SAC or Sn0.7Cu for wave used for both vibration and temperature cycling tests. The
soldering of through-hole components. Thus these rationale for the inclusion of this design is to create a
materials will be fixed in the experimental builds based on representation for electronics assemblies deployed in the
the current process conditions. Thus, test boards will systems such as washers and dryers.
consist of two types: SMT and single sided through-hole

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Figure 1: SMT test board for vibration stress test
Electro-chemical
migration test
structure

Cut-out feature

Figure 2: SMT test board for temperature cycling tests with electrochemical degradation test structure for long-term
storage tests. Cut-out features are provided to enable easy component removal during the temperature cycling test.

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48 ld PDIP
48 ld PDIP
48 ld PDIP
48 ld PDIP
48 ld PDIP
48 ld PDIP

48 ld PDIP
48 ld PDIP
48 ld PDIP
48 ld PDIP
48 ld PDIP
48 ld PDIP

Figure 3: Single sided, through-hole board for vibration and temperature cycling tests

4.3 Testing conditions

The fundamental premise in the proposed assessment of long-term reliability of lead-free electronics is to investigate
and quantify the interactions between the physical phenomena like intermetallics growth and tin pest and the expected field
stress conditions (vibrations, temperature cycling) on the solder joint degradation. The experimental matrix is shown in
Table 1.

Table 1: Experimental matrix

Pre-treatment Accelerated stressing Solder and PCB pad finish

Sn-Pb SAC SAC SAC SAC

HASL Immersion Immersion ENIG OSP
Ag Sn
9 9
High temp. storage (125ºC/100 Vibration test 9 9 99
hours) 9
High temp. storage (125ºC/350 Vibration test 9 9 9 99
hours) 9
Low temp. storage (-55ºC/500 Vibration test 9 9 9 99
hours)
Low temp. storage (-55ºC/1000 Vibration test 9 9 9 99
hours) 9 9 99
None (Control) Vibration test
HAST (130ºC / 85%RH / 9 9 99
Not applicable 672 hours) + Bias (for
corrosion test structure) 9 9 99
None Temp. cycling (-40ºC to
125ºC)

None Temp. cycling + vibration 9 9 9 99

The growth of intermetallics at the solder joints can be activation energy for IMC growth of 1.05 eV) [16], it is
enhanced by subjecting the electronics assemblies to high expected that the test conditions of (125ºC/350 hours) will
temperature storage. The time and temperature conditions simulate the growth of intermetallics that will be observed
for this test are determined based on the acceleration factor in the field after exposure to 55ºC in 28 years. In the
that relates the extent of growth in the test to the field proposed experiments, the growth kinetics of intermetallics
conditions. Based on a previous analysis (based on the

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and their structural morphology will be studied for all the of electronics assemblies in temperature cycling and the
combinations of lead finish-SAC-PCB pad finish. combined temperature cycling with random vibrations.

The lead-free alloy that will be studied in this proposal 4.4 Analysis methods
is tin rich Sn3Ag0.5Cu alloy. The microstructure of this
alloy in the solder joint consists of tin grain matrix with As assembled PCB samples will be examined under
interdispersed Sn-Ag, Sn-Cu intermetallic compounds. Tin optical microscopy to assess and compare the quality of the
pest may occur at low temperatures. To accelerate this solder joints. Specifically, analysis will look for difference
phenomenon of tin pest and investigate its impact on long in the wetting characteristics, appearance of the joints (dull
term reliability, the PCB assemblies will be subjected to vs. shiny), fillet shape, any evidence of bridging, solder
low temperature (-55ºC) storage of 1000 hours. At this balling, and any evidence of fillet lifting in through-hole
point, we do not know the acceleration factor for this test. components.

Vibration test involves subjecting the test assemblies Cross-sectioning and microscopy (Optical, SEM/EDX)
to resonant frequency of vibration in the vibration chamber. will be performed on each type of component/pad finish
The appropriate power spectral density level for long-term combinations after exposure (up to 1000 hrs.) to high
vibration tests will be determined through simulation and temperature storage test to determine the extent of
step stress experiments. Sensors will be mounted and intermetallic compounds growth, their chemical
monitored on the PCB assemblies to measure vibration constituents (EDX), morphology (SEM, TEM) and
stress. The resistance of each daisy chain will be monitored stochiometry (XPS). Similar analysis will be performed
continuously using the in-house built test equipment during for samples after exposure to low temperature to
vibration tests. This equipment will monitor the resistance investigate for the evidence of tin pest in the tin rich solder
of each chain in parallel with detection window of ~70 ns. joints. Because tin pest is associated with crystal structure
The resistance threshold (a programmable quantity in the change and which is more likely to initiate from the surface
system) will be set to 100 ohms. A resistance spike of the solder joints, ion scattering spectroscopy (ISS)
exceeding the threshold will be counted as a failure event techniques will be used to determine the tin pest
phenomenon. Microscopic analysis will also be performed
when the increased resistance persists for at least 1µs. on the solder joints after the
There is a counter which stores these failure events with temperature/humidity/electrical bias tests to assess the
the time stamp. This parallel and continuous measurement electrochemical degradation. Optical/SEM analysis of the
method is expected to ensure that no failure event solder joints after vibration, temperature cycling and
(including intermittents) is missed within each vibration combined vibration + temperature cycling tests will be
period (can range from 2.5 ms to 12.5 ms). A daisy chain performed to determine the failure mode and morphology.
in vibration test will be considered as failure when there
are consecutive 15 failure events. Time to failure for the The plan also includes characterizing the morphology
daisy chain will correspond to the time at which the failure of the solder joint microstructures, intermetallics, tin pest
event occurred. in solder joints and tin whiskers in lead and PCB plating
finish metallurgies under long-term aging (high
The temperature cycling tests will be conducted in air temperature exposure, low temperature exposure,
convection chambers. The temperature of each board will temperature cycling) using several characterization
be independently monitored using thermocouples installed methods including TEM (transmission electron
at the center of the boards. The temperature profile that microscopy), XPS (X-ray photoelectron spectroscopy: to
will be used in this study is: temperature excursion determine the stochiometry of intermetallics compounds)
between -40ºC to 125ºC with 15 minutes dwell time at both and ISS (ion scattering spectroscopy: to determine the
high and low temperature extremes and a ramp time of 15 surface crystallographic structures, film thickness, and
minutes (~10ºC per minute ramp rate) between extremes. possibly surface stresses).
The resistance of the daisy chains in temperature cycling
will be monitored using commercially available Agilent 5 Printed Circuit Board Assembly
data loggers. The data logger switching (polling)
frequency will be set to 82 chains per minute. All the tests The printed circuit board assemblies for this study
will continue till failure. Failure is defined as one or more were assembled in Jabil’s San Jose facility. Jabil Circuits
resistance excursions greater than 300 ohms occurring in Inc. is one of the leading edge electronics manufacturing
each cycle for 10 consecutive temperature cycles. To service (EMS) providers in the world. Jabil offers
define cycles-to-failure, we go back to the first of the ten manufacturing services to world leading electronics
final thermal cycles that registered the consecutive failure companies that include consumer, telecommunication,
readings. computing, instrumentation, medical, and automotive
industries. Jabil has design, manufacturing and repair
HAST test (135ºC/85%RH with bias for 672 hrs) will centers in every major region of the world. Jabil's
also be conducted to investigate any electrochemical manufacturing operations comply with ISO 9000 - Quality
migration and corrosion phenomena. This study will use Management System Standards, ISO 14001 –
the corrosion test structure that is incorporated on the Environmental Standard, and ISO 18001 - Health and
temperature cycling test board. This structure will enable Safety Standard.
application of electrical bias and to quantify the
electrochemical degradation phenomenon. The Jabil’s San Jose facility supports high and low-
mix PCB and backplane assembly for volumes that range
The experimental builds include the Sn-Pb assemblies from just a few units for prototypes, to modest quantities
as controls to assess and compare the long-term reliability for pre-production, to hundreds of thousands in volume

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production. The facility’s capabilities extend from single was used. The stencil was sourced from Beam On
sided through-hole to double sided, high density surface Technology (a preferred supplier of Jabil-San Jose
mount assemblies with chip scale packages, mixed assembly facility). A laser-cut stencil with 5 mils nominal
technology assemblies, lead-free assembly, flip chip, chip thickness was used for printing. The quality compliance
on board, and fine pitch high pin count press fit connectors. certificate from the stencil supplier is shown below.
The SMT process flow is shown in Figure 4. Figure 5
shows the process flow for through-hole assemblies. A 10-zone Vitronics-Soltec XPM2 convection reflow oven
was used for reflow. The reflow gas was nitrogen. The
The assembly involved two surface mount designs temperature was monitored for PBGAs and resistors.
(vibration and temperature cycling) and one single sided PBGAs were selected due to their sensitivity to popcorning.
through-hole design. The surface mount designs, fabricated The PBGA saw a peak temperature of 246ºC. The peak
using two PCB materials FR4 and polyimide were of 2- temperature of all the PBGAs was same. The resistors saw
metal layer construction. The components were mounted a peak temperature of 250ºC. The time duration for the
on top side of the surface mount boards only. The single PBGAs in the oven above the melting point of the solder
sided through-hole design boards were fabricated using (217ºC) was 78 seconds. The time duration between 150ºC
CEM-1 material and were assembled using Jabil’s selective to 217ºC was 73 seconds. The Pb-free reflow temperature
soldering technology. profile used for the assembly is shown in Figure 6.

The total number of assemblies included 306 surface For the control Pb-63Sn assemblies, the thermal profile
mount designs (vibration design=169, temperature cycling (Figure 7) was used. In this assembly the PBGAs saw the
design = 137) and 70 through-hole design. The build peak temperatures between 213ºC to 215ºC. In this process,
sequence started with PbSn assembly for the SMT resistors saw a higher peak temperature of 219ºC. The time
vibration and temperature cycle designs. This was followed duration above the melting point (183ºC) for this process
by Pb-free assembly of SMT vibration design boards and ranged from 67-71 seconds. The time duration between
temperature cycling design boards. Single sided through- 150ºC to 183ºC ranged from 88 to 100 seconds. By
hole boards were assembled last due to scheduling comparing two processes, there is a difference of 31ºC in
constraints. peak temperatures experienced by PBGAs. All the
assemblies were subjected to two reflow profiles in order
5.1 SMT assembly process to simulate actual top and bottom assembly process. Figure
8 shows the assembled board after reflow.
A no clean-type 3-Kester R905 paste (Sn3Ag0.5Cu)
was used for the Pb-free assemblies. For control Sn-Pb
assemblies, no-clean-type 3-Kester 256 paste (Sn37Pb)

Bill of Materials Incoming inspection PCB Shipping media
Quantity verification of PCBs serialization transfer (for CLCCs)

(sample basis)

Stencil print Component Component Reflow
paste placement placement
(resistors) (PBGAs, LQFPs,
CLCCs)

Automatic Defects No defectives Flying probe
X-ray and optical assessment (resistance
measurements)
inspection

Rework Defectives Shipping

Figure 4: Process flow for the SMT boards assembly

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Bill of Materials Incoming inspection Hand-placement Selective soldering
Quantity verification of PCBs of components technology
(Versaflow)
(sample basis) (48 PDIPs)

Automatic Defects Defectives Rework
optical inspection assessment

No defectives Flying probe Shipping
(resistance
measurements)

Figure 5: Process flow for single sided through-hole boards assembly

Figure 6: Lead-free reflow profile

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Figure 7: Sn-Pb reflow profile

Figure 8: Test board after the reflow

5.2 Inspection area of the PBGA. The PBGAs were rated as MSL 3 parts
and were out of the moisture proof bags for a maximum of
A HP 5Dx X-ray tomography system was used to 2 hours before assembly. The defects were hypothesized to
inspect the quality of solder joints in the assemblies. The be due to the combined effects of popcorn phenomenon
X-ray images of the solder joints of all the component occurring in PBGA packages during Pb-free reflow and
types are shown in Figures 9-13. Figure 9 shows the x-ray PBGA-PCB warpage during higher temperature Pb-free
image of good PBGA solder joints from the assembled reflow. With this hypothesis, it was decided to bake all the
boards PBGAs before lead-free assembly of the rest of the boards
in order to eliminate popcorning as one of the potential
The 5Dx system was also used to capture the images causes. After baking the PBGAs at 125ºC for 24 hours
of all the defective lead-free PBGA solder joints (see before reflow, the rest of the SMT lead-free assemblies did
Figure 144). Defects in solder joints of the SMT vibration not exhibit the defects based on the X-ray inspection
assemblies (FR4 with immersion tin, immersion silver, results.
ENIG, and OSP finishes, polyimide boards with ENIG
finish) in non-baked Amkor’s 256 Pb-free PBGAs after the It can be concluded that the single factor which had
lead-free reflow. The solder joint defects were categorized the most influence in preventing defects was the baking of
in to three major types: open and or insufficient solder, Pb-free PBGAs. Further analysis of the failed PBGAs
shorts (bridging) and the combination of shorts with confirmed that moisture induced interfacial delamination
insufficient solder. All the defects occurred under the die and popcorning was the root-cause of the observed solder

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joints defects after lead-free reflow. All the affected process.
assemblies were reworked with assembly house rework

Figure 9: X-ray image of 256 PBGA solder joints

Figure 10: X-ray image of 100 ld LQFP solder joints

Figure 11: X-ray image of 44 ld CLCC solder joints

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Figure 12: X-ray image of 2512 SMT resistor
Figure 13: X-ray image of 1210 SMT resistor

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Over-sized Insufficient
solder joint solder joint

Bridged
solder joint

Figure 14: X-ray image of the first article on lead-free FR4 vibration board

5.3 Single sided through-hole assemblies programmable and operating modules: a micro-drop fluxer
mounted on an XY-table, a fix-mounted pre-heating
The assembly steps for single sided through-hole module and a solder module positioned on an XYZ-table.
assembly involved hand- placement of 48 ld PDIPs and To reduce the formation of oxides in the solder bath and to
wave soldering. The wave soldering for this study was improve the quality of the solder joints, the solder bath is
accomplished with Jabil’s selective soldering technology inerted with nitrogen. The temperature profile used for the
where in individual solder joints are created with the high through-hole assemblies is shown in Figure 16. The
precision solder nozzle in ERSA Versaflow equipment (see Sn3Ag0.5Cu solder alloy (Kester 905) was used for Pb-
Figure 15). free assemblies and the standard eutectic Pb63Sn solder
alloy (Kester 256) was used for control SnPb assemblies. A
The selective soldering technology is the preferred no-clean, Lonco65 flux was used during soldering. Figures
method in high mix, low volume assembly facilities. The 17 and 18 show the top side and bottom side of an
ERSA VERSAFLOW consists of 3 independently assembled through-hole board respectively.

Figure 15: ERSA Versaflow equipment used for single sided through-hole assemblies

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Figure 16: Temperature profile for single sided through-hole lead-free assemblies
Figure 17: Top side of an assembled through-hole board

Figure 18: Bottom side of an assembled through-hole board

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6 Summary Time to Failure,” 34th International SAMPE
Technical Conference, Baltimore, MD, 2003
Abundant data exist on the short-term reliability (i.e.
less than 5 years) of lead-free solder joints under single [7] Zhang, Q., Dasgupta, A., and Haswell, P. “Creep
loading conditions. Data on combined loading conditions and High-Temperature Isothermal Fatigue of Pb-
and long-term reliability is non-existent. The lead-free Free Solders”, Proceedings of IPACK 03:
electronics will be deployed in many products that serve International Electronic Packaging Technical
markets where long-term (greater than 5 years) is a critical Conference and Exhibition, July 6-11, 2003, Maui,
requirement. In many applications, electronics will be Hawaii, USA, 2003
subjected to long-term exposure to temperature extremes
(high and low), humidity, atmospheric contaminants, and [8] Zhang, Q., Haswell, P. and Dasgupta, A.
combined thermo-mechanical loading (temperature cycling “Isothermal Mechanical Creep and Fatigue of Pb-
+ vibrations). Thus, this long-term reliability studies free Solders”, International Brazing &Soldering
comprehend the interactions among these applications Conference, San Diego, CA, February 16-19,
conditions. These interacting influences include 2003
unacceptable growth of intermetallics at the solder joints
due to high temperature exposure, electro-chemical [9] Zhang, Q., Haswell, P., and Dasgupta, A. “Cyclic
degradation of electronics assemblies due to the exposure Mechanical Durability of Sn-3.9Ag-0.6Cu and
to humidity and atmospheric contaminants, formation of tin Sn-3.5Ag Lead-Free Solder Alloys”, Proceedings
pest due to extended exposure to low temperature and the ASME IMECE 2002, New Orleans, LA, 2002
effects of combined thermo-mechanical loading conditions
on the electronics assemblies. [10] Zhang, Q., Haswell, P., Dasgupta, A., and
Osterman, M. “Isothermal Mechanical Fatigue of
7 Acknowledgements Pb-free Solders: Damage Propagation Rate &
Time to Failure”, 34th International SAMPE
CALCE Electronic Products and Systems Center has Technical Conference, Baltimore, MD, November
initiated the long-term lead-free reliability program and is 4-7, 2002
collaborating with Aero combined Environment
Laboratory of China Aero-Polytechnology Establishment, [11] Haswell, P. and Dasgupta, A., “Viscoplastic
Hong Kong University of Science and Technology, Poland Constitutive Properties of Lead-free Sn-3.9Ag-
Technologic University, and many companies engaged in 0.6Cu Alloy,” MRS Proceedings, San Francisco,
telecommunication, industrial, aerospace, and oil CA, 2001
exploration businesses.
[12] Haswell, P., “Durability Assessment and
8 References Microstructural Observations of Selected Solder
Alloys,” Ph.D. Dissertation, University of
[1] Ganesan, S. and Pecht, M., Lead-free Electronics, Maryland, College Park, MD, 2001
2004 Edition, Edited by, CALCE EPSC Press,
University of Maryland, College Park, Maryland [13] Haswell,P. and Dasgupta, A.
“Microthermomechanical Analysis of Lead-Free
[2] Casey, P., S. Ganesan and M. Pecht, “Challenges Sn3.9Ag0.6Cu Alloys, Part I: Viscoplastic
in Adopting Pb-free Interconnects for “Green” Constitutive Properties, and Part II: Cyclic
Electronics,” Proceedings of the IPC/JEDEC Durability Properties”, Paper N2.1, MRS
Second International Conference on Lead-free Proceedings, Vol. 682E, MRS Spring Symposium
Electronic Components and Assemblies, pp. 21- on Microelectronics and Microsystems Packaging,
32, Taipei, Taiwan, 2002. Editors: Boudreaux, Dauskardt, Last, and
McCluskey, Chicago, 2001
[3] P. Casey and M. Pecht, “Assessing Lead-free
Intellectual Property,” Circuit World, Vol. 30, No. [14] Zheng, Y., Hillman, C., and McCluskey, P.
2, pp. 46-51, 2004. “Effect of PWB Plating on the Microstructure and
Reliability of SnAgCu Solder Joints”, presented
[4] P. Casey and M. Pecht, “The Technical, Social on AESF SUR/FIN 2002 June 24-27, Chicago, IL,
and Legal Outlook for Lead-Free Solders,” IEEE 2002
International Symposium on Electronic Material
and Packaging, pp. 483-492, Kaohsiung, Taiwan, [15] Zheng, Y., Hillman, C., and McCluskey, P.
December, 2002 “Intermetallic Growth on PWBs Soldered with
Sn3.8Ag0.7Cu”, presented on Proceedings of the
[5] Zhang, Q., A. Dasgupta and P. Haswell, 2003, 52nd Electronic Components & Technology
“Viscoplastic Constitutive Properties and Energy- Conference, pp. 1226-1231, San Diego, 2002
Partitioning Model of Lead-free Sn3.9Ag0.6Cu
Solder Alloy”, ECTC 2003, New Orleans, [16] Lee, T.Y. et al. “Morphology, Kinetics, and
Louisiana, USA, 2003 Thermodynamics of Solid-State Aging of Eutectic
Sn-Pb and Pb-free Solders (Sn-3.5Ag, Sn-3.8Ag-
[6] Zhang, Q., A. Dasgupta, P. Haswell and M. 0.7Cu and Sn-0.7Cu) on Cu,” Journal of Materials
Osterman, 2003a, “Isothermal Mechanical Fatigue Research, Vol.17, No.2, February 2002, pp.291-
of Lead-free Solders: Damage Propagation and 301.

0-7803-8807-0/05/$20.00©2005 IEEE 154 [17] Hilty, R. D. “Lead Free Components in
Automotive Applications”, IPC/JEDEC Fourth
International Conference on Lead Free Electronic

2005 International Conference on
Asian Green Electronics

Session 3

Components And Assemblies, Frankfurt,
Germany, October 21-22, 2003

[18] Kariya, Y., Gagg, C., Plumbridge, W.J., “Tin pest
in lead-free solders”, Soldering & Surface Mount
Technology, vol.13, no.1, 2001. p. 39-40, 2001

[19] Kariya, Y., T. Morihata, E. Hazawa and M.
Otsuka, “Assessment of Low-Cycle Fatigue Life
of Sn-3.5mass%Ag-X (X=Bi or Cu) Alloy by
Strain Range Partitioning Approach,” Journal of
Electronic Materials, Vol. 30, No. 9, pp. 1184-
1189, 2001

[20] Kariya, Y., Williams, N., Gagg, C., and
Plumbridge, W., “Tin Pest in Sn-0.5 wt% Cu
Lead-Free Solder”, Journal of Materials, June
2001, pp.39-41, 2001

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Asian Green Electronics