IPC-7095C-2013.pdf - 第132页

Comparing SAC lead-free solder joint with SnPb solder joint, there are distinct intrinsic dif ferences in their respec- tive metallurgy that determines their dislocation systems, microstructures and metallur gical phases…

8 RELIABILITY

Reliability is the ability of a product to deliver designated

functions under given conditions, for a speciﬁed period of

time within an acceptable level of conﬁdence. The reliabil-

ity of electronic assemblies requires a deﬁnitive design

effort to be carried out during the developmental phase of

the product to meet requirements. Reliability is deﬁned in

IPC-SM-785.

Short term reliability is threatened by early life failures

generally attributed to insufficient production quality.

These ‘‘infant mortalities’’ can be reduced by appropriate

screening techniques prior to shipping, but not eliminated.

Longer term failures are the result of premature wear-out

damage caused by inadequate design of the assembly. The

design guidelines in IPC-D-279 are a good reference.

Reliability qualiﬁcation requirements are given in IPC-

9701 together with well-deﬁned test methodologies. For

lead-free solder attachments, IPC-9701A contains guide-

lines for accelerated solder joint reliability testing. In the

absence of an agreed-to acceleration model for the various

lead-free solders, reliability requirements based on acceler-

ated thermal cycling is difficult to establish. Most of the

existing models that have been developed are very product-

speciﬁc.

8.1 Reliability Drivers for BGAs When the soft solder

(tin/lead or lead-free) is used as BGA solder joints to con-

nect BGA packages onto the PCB substrate, solder joints

do ‘‘wear-out’’ during their service life. This degradation

process is unavoidable. The objective of solder joint reli-

ability is to make sure that solder joints do not degrade to

the level that they lose their required functionality, be it

electrical, thermal, or mechanical, under a designated ser-

vice life and anticipated service environments.

Solder joint failure, absent other extraneous causes, is often

attributed to thermo-mechanical reliability which occurs

due to fatigue and creep interaction. The solder fatigue/

creep damage is caused by the temperature excursion and

ﬂuctuation as the result of in-circuit functions, power

on-and-off, and/or external environmental temperature

exposure. These temperature changes inevitably generate

cyclic thermal stress, thus cyclic strain on solder joints, in

a thermal expansion coefficient mismatched system during

the service life of electronic packages and assemblies,

causing thermal fatigue (as opposed to mechanical fatigue).

Due to the recent advent of handheld consumer products,

mechanical reliability of the solder joints has become an

added requirement. Shock, transient and cyclic bending of

the board as well as vibration, are the major stress genera-

tion factors that can impair the solder joint functionality.

All these factors are described below.

8.1.1 Cyclic strain Heat is generated from electronic

devices and circuits during the power-on of an electronic

system, following the well-known Joule’s law. The heat

will then be gradually dissipated to external environment

while power is off through the basic heat transfer mecha-

nisms of conduction, convection, and radiation, making the

system reach ambient temperature again. Many such tem-

perature cycles during the service of an electronic system

are expected and inevitable.

Barring other extraneous causes, the solder joint intrinsic

degradation process engages two scientiﬁc phenomena—

fatigue and creep. Other extraneous causes include the

improperly formed solder joints due to wetting problem,

inadequate process issue, surface ﬁnish-induced problem at

or near the interface, such as Au-embrittlement and other

intermetallic issues, etc.

8.1.2 Fatigue Fatigue is a progressive and localized

structural damage (atomic and larger scale levels) which

occurs when the solder joint is subject to cyclic stresses

(loads and unloads). When the stress exceeds a certain

threshold, microscopic cracks will begin to form. This

localized damage mechanism separates fatigue from creep

behavior.

As the cyclic stress continues, a crack will propagate and

eventually reach a critical size, and the solder joint will

fracture. Generally the fatigue process undergoes three

stages: crack initiation, crack propagation and fracture. For

solder joints functioning as the electrical, thermal and

mechanical interconnections in a circuitry, a mechanical

fracture is generally not a practical criterion of failure. The

electrical performance, as measured by resistance increase

due to cracks, is used as a failure criterion. The electrical

failure often proceeds mechanical fracture. When in the

presence of corrosive elements, a corrosion-enhanced

fatigue could also occur.

Fatigue strength, correlating to fatigue life, is deﬁned as the

stress value at which a failure occurs after a given number

of cycles. The fatigue strength depends, not only on the

speciﬁc solder alloy material, the extreme high tempera-

ture, and the extreme low temperature, but also other fac-

tors: surface defects, notches, holes, residual stress, voids,

gas porosity, and inclusions. All of these ‘‘imperfections or

defects’’ could act as the stress concentration sites to initi-

ate the localized damage under the fatigue environment.

This is the reason why defects such as voids or surface

cracks should not be categorically dismissed even when

some test results do not reveal the performance difference

between the presence and absence of such defects.

In the atomic level, the solder joint fatigue mechanism

starts with dislocation movement and then forms a slip

band that nucleate short cracks. Microstructurally, grain

coarsening is often a result as observed under the SEM

examination. Grain size is important to fatigue—the

smaller, the better. Yet when other defects, such as a sur-

face defect exist, a defect rules.

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Comparing SAC lead-free solder joint with SnPb solder

joint, there are distinct intrinsic differences in their respec-

tive metallurgy that determines their dislocation systems,

microstructures and metallurgical phases, which in turn

dominate the fatigue behavior and degradation mechanism

in response to various service conditions, and thus the

fatigue life.

It should be noted that stresses acting upon solder joints in

real world applications are usually random in nature rather

than ‘‘regularly’’ cyclic as the commonly used parameters

in an ATC (accelerated temperature cycling) test. One chal-

lenge is to extrapolate from cyclic test data to the behavior

under actual random stresses. Based on the underlying met-

allurgy, this challenge escalates in SAC lead-free system in

comparison with tin/lead eutectic.

In practical terms, a fatigue phenomenon involves chance,

randomness, and probability. This is why one simple ATC

test is hardly able to determine a conclusion. And it stands

to reason that the test data should be in congruence with

the principles of science and needs to be checked-and-

balanced with the underlying scientiﬁc principles; in this

case, the fundamental metallurgy and fracture mechanics.

8.1.3 Creep Additionally, soft solders, even under ambi-

ent temperatures (298 ± 5°K), reach a homologous tem-

perature well beyond 0.5. Therefore, creep behavior is also

expected to occur, which complicates the overall degrada-

tion behavior and failure mechanism, as the creep and

fatigue processes operate interactively. In contrast to

fatigue, solder creep is deﬁned as a time-dependent defor-

mation, which is irreversible and global.

When a stress is applied and sustained, solder responds

with an instantaneous strain in the elastic and/or plastic

region. As time progresses, solder may continue to deform

in a time-dependent manner and eventually fail. Theoreti-

cally, this time-dependent deformation can occur at a tem-

perature above the absolute zero, albeit very slowly. In the

low temperature region, the creep strain is very limited and

deformation normally does not lead to eventual rupture.

The strain accumulates in a logarithmic rate. In the high

temperature region, the creep curve departs from the loga-

rithmic relationship.

A typical creep curve (deformation vs. time) comprises

three stages: primary, secondary and tertiary. In the primary

stage, the transient strain rate decreases rapidly from a very

large initial value due to structural alterations when a sol-

der deforms. The secondary creep prevails at the tempera-

ture above the half of the alloy’s melting temperature and

correlates well with the self-diffusion process. The disloca-

tion climb or glide is generally considered to be the rate-

determining step. The creep rate in this stage reaches a

steady state, as a result of a balance between two compet-

ing metallurgical processes—strain hardening and recovery

(a softening effect). The more the solder is plastically

deformed, the more difficult it becomes to continue the

deformation. The work hardening operates through the

generation and the interaction between dislocation defects

acting as obstacles to further the deformation process. At

higher temperatures, work hardening may be partially or

fully counteracted by recovery. Under this condition, a

time-dependent, thermally-activated, strain-energy-

releasing softening process occurs. The deformation

reaches a dynamic steady state in which the rate of work

hardening is equal to the rate of recovery.

When the softening effect and rising stress overcome the

strain hardening, the tertiary stage sets in, where the creep

rate accelerates until the rupture occurs. This stage is nor-

mally associated with structural changes such as the onset

of recrystallization, coarsening and formation of internal

cracks and/or voids that are precursors to fracture.

Separately, each of these two phenomena is associated and

controlled by a different set of parameters in microstruc-

tural as well as atomic-level material properties.

8.1.4 Creep and Fatigue Interaction Solder joints in the

real world performance of electronics are exposed to the

conditions that induce both fatigue and creep processes and

they operate interactively. Simply put, this environment can

be considered as creep under cyclic thermal loading or

fatigue under high temperature. This interactive nature

between creep and fatigue complicates the solder joint deg-

radation behavior and the underlying failure mechanism.

Thus, the solder joint intrinsic degradation leading to an

eventual failure can hardly be a creep failure or a fatigue

failure, rather is the result of interactive fatigue and creep.

And the mechanisms of creep and fatigue are expected to

operate in a competitive, alternative or mutually promoting

manner, depending on the service conditions that combine

both the external climate and the in-circuit operation envi-

ronment. In the commonly adopted Accelerated Tempera-

ture Cycling test, the creep and fatigue processes also oper-

ate interactively. From an engineering perspective, one can

consider the degradation phenomenon as either creep-

aggravated fatigue or fatigue-accelerated creep.

Reliability is a relative term. The system’s operating con-

ditions must be speciﬁed and its functional period of time

be also speciﬁed. In engineering, reliability is the probabil-

ity that a system will perform a required function without

failure under a given set of conditions for an intended

period of time. Weibull distribution, a continuous probabil-

ity distribution has been widely used in reliability engineer-

ing and failure analyses. As the Weibull plot is one conve-

nient method of calculating the parameters of the Weibull

distribution, almost all publications related to lead-free reli-

ability include the Weibull plot, which plots empirical

cumulative distribution of data: time-to-failure.

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8.1.5 Mechanical reliability Mechanical reliability refers

to the response of the BGA interconnect to a mechanical

stimulus. These stimuli include shock events, which may

occur during shipping, installation, or ﬁeld use, transient

bend, which may occur during manufacturing (in-circuit

test, functional test, installation, etc.) or in the ﬁeld, cyclic

bend, such as repetitive keystrokes near a BGA, or vibra-

tion due to fans, or motors near or in the system. All of

these stimuli, and others, can have an effect on the

mechanical integrity of the BGA interconnect, and all of

them should be considered when using BGAs, which are

particularly susceptible to ﬂexure based stressors.

8.1.5.1 Shock A shock event is characterized by signiﬁ-

cantly high-speed, which does not allow time for intercon-

nect materials to respond to the applied force. Shock can be

induced via drop or impact, and can occur at any time dur-

ing the life of the product, although shipping and end-user

environment shock events are most typical. Shock events

can result in a separation, either partial or full, of a BGA

interconnect, at any of the interfaces making up the full

interconnect attachment. Even a partial interfacial fracture

can eventually, over the life of a product, cause an electri-

cal failure in a BGA. More brittle materials and interfaces

are particularly susceptible to shock induced failures.

8.1.5.2 Transient Bend Transient bend, or ﬂexure, is

characterized by a low number (typically 1-10 ﬂexure

events) of relatively slow strain. These events can be

induced during manufacturing, especially in test ﬁxtures

such as ICT or functional testers that use pogo-pin style

probes, during connector insertion, or screw-down of the

PCA into a chassis. Transient bend can also be induced

during unit repair, shipping, and in the end-user environ-

ment. As with shock, these failures typically occur at an

interconnect interface and can result in either a complete

open, or a latent ﬁeld risk as partial fractures can eventu-

ally open in the ﬁeld.

8.1.5.3 Cyclic Bend Cyclic bend is typiﬁed by multiple

relatively low-level strain events. These events can run into

the thousands or above, and are usually caused by repeti-

tive actions such as keypad actuation, transportation, and

connector insertion or extraction (such as in notebook

docking stations). Over time, fatigue failures may result.

8.1.5.4 Vibration Vibration is characterized by a con-

tinuous low-amplitude displacement of relatively high fre-

quency (as compared to the other mechanical stresses

described above). Vibration can impinge on a BGA due to

proximity of the component to a vibrating subsystem, such

as a motor, fan, or disk drive. It is possible, if the intercon-

nect system has already been stressed to the point of inter-

facial fracture, that vibration may exacerbate the issue,

leading to an electrical open; however, this is not yet well

understood.

8.2 Damage Mechanisms and Failure of Solder Attach-

ments

The reliability of an electronic assembly depends

on the reliability of the sum of the individual elements of

the mechanical thermal and electrical interfaces (or attach-

ments) between these elements. One interface type, the sur-

face mount solder attachment, is unique since the solder

joint not only provides the electrical interconnection, but is

also the sole mechanical attachment of the electronic com-

ponents to the printed board. It often provides the critical

heat transfer function as well. A solder joint in isolation is

neither reliable nor unreliable; it becomes so only in the

context of application.

The characteristics of these three elements—component,

substrate, and solder joint—together with the use condi-

tions, the design life, and the acceptable failure probability

determine the reliability of the surface mount solder attach-

ment.

The general characteristics of most lead-free solders as

compared to tin/lead solders, include (1) signiﬁcantly

increased alloy stiffness, (2) signiﬁcantly slower creep

rates, (3) greater difficulties of proper spread, and (4) sig-

niﬁcantly higher soldering temperatures.

The consequence of the greater stiffness or lower ductility

and the slower creep rates are higher stresses on the whole

solder attachment structure during either temperature

changes, causing thermal expansion mismatches or PCB/

component warping, or bending. These higher stresses,

combined with less strong solder to base material connec-

tions due to inadequate wetting or interfacial structural

weaknesses, may cause brittle interfacial failure.

8.2.1 Comparison of Thermal Fatigue Crack Growth

Mechanism in SAC vs. Tin/Lead BGA Solder Joints

Eutectic tin/lead solder joints develop different damage

mechanisms than SAC BGA solder joints during Thermal

Fatigue Tests, such as Temperature Cycling. Eutectic tin/

lead solder microstructure is multi-grained, with each BGA

solder joints having many individual grains of lead-rich

(dark phase) and tin-rich (light colored phases) precipitated

out, as seen in Figure 8-1. In contrast, each SAC BGA sol-

der joint has very few grains of tin. These grains can be

seen under cross-polarizing light. Figure 8-2 shows two

examples of SAC BGA solder joints. The one on the left

has 6 grains, whereas the entire solder joint on the right is

comprised of a single grain.

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