IPC-D-279 EN.pdf - 第19页

3.1.11 Electrical 3.1.11.1 ESD Susceptibility and Damage Prevention All electronic components containing thin conducting or insulating ﬁlms are susceptible to electrostatic discharge (ESD) damage. These components includ…

3.1.7 Environmental Stresses The operating life of a

surface mounted assembly is dependent upon a number of

factors which include intended usage, usage environments,

strength of the materials and components to withstand the

stresses imposed by the usage and the environments, mate-

rial (variables), etc. In a surface mounted assembly, the

most critical element from the life cycle viewpoint are the

solder joints and PTVs. Cyclic (or fatigue) displacements

experienced during various phases of the product’s life

cycle are responsible for consumption of useful life of

material elements.

One of the major contributors to the cyclic loading is ther-

mal cycling due to the internal power cycling and external

environmental changes. Another important contributor is

vibration during the operational use, transportation, han-

dling, etc. An estimate of the fatigue life of the solder

assembly that will be consumed by these fatigue cycles

during the product’s life cycle can be obtained by perform-

ing a cumulative damage analysis.

It should be noted that the cumulative damage analysis

requires the knowledge of fatigue characteristics of the

materials involved. Also, a thorough understanding of how

the product will be used, handled and maintained, by the

user and under what environments, is necessary. See

Appendix E.

3.1.8 Temperature/Thermal Temperature is one of the

most important parameters in the use environment that

must be considered in the SM PWA design process. Tem-

perature history is the most signiﬁcant parameter affecting

the reliability of SM solder joints. For some product appli-

cations, the use environment consumes the most significant

portions of the required fatigue life; this is typically the

case where the product development cycle includes produc-

ing a prototype prior to the ‘‘ﬁnal build.’’

Therefore, DfR depends to a large extent on the thermal

design for the assembly, as well as the external thermal

environment. Temperature is also important, since many

materials in electronic assemblies have properties which

change signiﬁcantly with temperature.

3.1.9 Cyclic Temperature Swings Cyclic thermal excur-

sions, ∆T, cause thermal expansion mismatches due to dif-

ferent parts of the assemblies having materials with differ-

ent CTEs and/or being at different temperatures. Thus, the

size of the cyclic temperature swing is proportional to the

resulting loading. The larger the ∆T, the larger the threat to

reliability.

During cyclic temperature excursions, these cycles can

have different proﬁles, e.g., sinusoidal, trapezoidal, saw-

tooth, square-wave, etc. These differences are important in

understanding time-dependent and rate-dependent pro-

cesses. The transient parts of the proﬁle give the ramping

rates of the temperature change which can cause transient

temperature gradients and over-stress conditions. The

steady-state, or near steady-state, parts of the proﬁle deter-

mine the duration of the temperature dwells which are

important for time-dependent processes like creep and

stress relaxation.

3.1.10 Thermal Shock In thermal shock, the extremely

rapid temperature changes (∼30°C/minute and above)

result in warping of the surface mount assembly. The

warpage is caused by large transient thermal gradients

induced by the rapid temperature change when the boards

are plunged into a new thermal environment. The warpages

result in tensile and shear stresses where the tensile load-

ing dominates over the steady state expansion mismatch.

Thus, even assemblies with matched coefficients of thermal

expansion will exhibit solder joint failures when subjected

to thermal shock. The thermal shock loading mechanism is

summarized in Figure 3-1.

Thermal shock conditions can arise from several sources.

Examples of these are:

1) Rapid changes in external environment, e.g., sun-to-

shade in space, missile launch, wing-mounted avion-

ics, automobile start-up from very cold.

2) Rapid changes in power dissipation.

3) Various manufacturing/repair processes, e.g., reﬂow,

vapor degrease, rework, etc.

The distinction between thermal shock and thermal cycling

is not always addressed in designing reliability experi-

ments. There is a fundamental difference between thermal

shock and thermal cycling. The primary differences arise

from the mechanism of loading. Thermal shock tends to

result in multiaxial states of stress dominated by tensile

overstresses and tensile fatigue. On the other hand, as pre-

viously discussed, thermal cycling results in shear loads

and failure occurs from an interaction of shear fatigue and

stress relaxation.

Thermal shock is usually performed in dual chamber

arrangements or with liquid nitrogen injection whereas

thermal cycling is performed in single chamber cycling

equipment. Dual chamber arrangements will produce tem-

perature transition rates in excess of 50°C/minute.

Single chambers generally do not produce transition rates

even close to 30°C/minute which is the rate necessary to

induce thermal shock. The results of these two types of

testing are generally incompatible. Finally, thermal shock

testing for purposes of evaluating surface mount solder

joint reliability is only appropriate if thermal shock is

indeed a ﬁeld condition encountered by the product.

In some speciﬁcations, the deﬁnitions of thermal cycling

and thermal shock are not fully differentiated; the rates of

change are more closely associated with what we are call-

ing thermal shock.

July 1996 IPC-D-279

3.1.11 Electrical

3.1.11.1 ESD Susceptibility and Damage Prevention

All electronic components containing thin conducting or

insulating ﬁlms are susceptible to electrostatic discharge

(ESD) damage. These components include those fabricated

in high speed technologies (MOS, bipolar, GaAs), thin film

technologies (resistors, integrated circuits, magnetic heads,

MOS capacitors), and in future, wafer scale integration and

multichip modules.

3.1.12 EMC/EMI The electromagnetic spectrum is usu-

ally divided into categories ranging from the long-

wavelength radiation from power lines through radio, infra-

red, visible, ultraviolet, and x-rays, to gamma rays at the

short-wave end. All electromagnetic waves consist of an

electric ﬁeld and a magnetic ﬁeld. The relative magnitude

of these ﬁelds depend on the emitter (EM source), wave

propagation medium, and the proximity of the emitter to

the subject assembly.

Many electronic circuits are susceptible to electromagnetic

radiation and must be shielded to ensure proper operation.

One of the most important effects of the electromagnetic

radiation in the environment is electromagnetic interfer-

ence (EMI). EMI is the electro-magnetic disturbances that

impair the desired signal. In practice, EMI is often divided

into two categories: conducted EMI and radiated EMI.

Conducted EMI is an interfering signal resulting from an

undesirable voltage or current coupled into a signal or

other pertinent conductor. Radiated EMI is an interfering

signal resulting from an electric and/or magnetic ﬁeld

amplitude and frequency spectra intentionally or uninten-

tionally radiated by an electrical source. Examples of radi-

ated emission sources are radio and TV transmitters, light-

ning, digital system noise from electronic control systems,

etc. In military applications, an important effect is the inter-

action of electromagnetic radiation with electroexplosive

devices used as detonators. Improper EMI could acciden-

tally initiate the explosion.

EMC is the ability of electronic systems to operate in the

intended electromagnetic environment at designed levels of

performance and efficiency. The most direct approach to

protection is, in most cases, to avoid the limited region in

which high radiation levels are found. When exposure can-

not be avoided, shielding is the important protective mea-

sure. The material selected for shielding can be an impor-

tant factor. Ideal materials include steel, copper and nickel

coating. In the design process, apertures for cooling venti-

lation and cable connections on the shielding box should be

properly designed so that the EMI will have no inﬂuence

inside the shielded space.

3.1.13 Mechanical Shock and Vibration Shock and

vibration are common accelerators of failure in electronic

packaging. The most frequent vibration-induced failures in

surface mount are:

1. Flexing of leads and interconnects.

2. Dislodging or damaging of parts and structures.

Methods have been developed to counter the destructive

effects of shock and vibration. Generally, isolation of a

printed board against the effects of shock and vibration

requires that the natural frequency of the printed board be

substantially lower than the undesired frequency of vibra-

tion to avoid the resonance.

The basic system level isolators available are:

1. Natural or synthetic rubber, used to damp the vibra-

tion.

IPC-279-03

Figure 3−1 SMT Assembly Response to Thermal Shock

THERMAL

STEADY

STATE

THERMAL

STEADY

STATE

+ 125°CAT – 65°C

+ 125°C

– 65°C

IPC-D-279 July 1996

2. Metallic isolators including springs, metal meshes or

wire rope; the latter provide smooth friction damp-

ing.

There are also other isolators such as viscous dampers

which are velocity-sensitive. For high-frequency vibration,

viscous dampers tend to become ineffective.

There are two approaches that may be taken when shock

and vibration are present: either isolate the printed board or

design it to withstand the shock and vibration. Studies have

shown that the ideal goal is to design equipment to be

resistant to shock and vibration, rather than to isolate it

from these forces.

Because the surface mount components are generally

smaller compared to the through-hole counterparts, they

are more vibration resistant due to the lower inertia. As the

PWA vibrates, the components mounted on the board are

subject to stress from two different effects. First, the mass

of the components is subjected to an acceleration that pro-

duces a force. The body of the component is kept in equi-

librium with reactive forces developed in the leads. Sec-

ond, the printed board ﬂexes, which tends to bend the leads

back and forth at their joints with the board. Because of the

shorter lead lengths in SMT, the mechanical stress induced

by shock and vibration in leads is considerably smaller

compared to that in through-hole leads.

To detect/precipitate the ﬂaws, stress screening with ran-

dom vibration should be performed. Usually, stress screen-

ing should be designed so that it causes minimal damage to

properly designed printed boards. This requires careful

determination of the screening intensity.

Depending upon the applications, the frequency ranges are

very different. For instance, the range for submarine elec-

tronic equipment is about 4-34 Hz, but for avionics the

range is 15-2000 Hz.

The following considerations must be included in the

analysis design for shock and vibration:

1. The location of the surface mount components rela-

tive to the supporting structure (i.e., edge, corner, or

center of the supporting structure).

2. The orientation of the components with respect to the

anticipated direction of the shock or vibration forces.

3. The maximum deﬂection of the printed board.

Mechanical shock to, and ﬂexure of, a SM Printed Wiring

Assembly can occur during its life cycle. Each incident

could be characterized by the energy to be absorbed by the

assembly, the shock peak amplitude, duration and time rate

of change for correlation to sample shock testing:

• PWA struck by tooling during assembly (component

insertion or placement, riveting, testing, test ﬁxtures,

probing, or depanelling)

• PWA installed into a card carrier or motherboard

• PWA dropped on a hard bench or ﬂoor

• Assembly dropped on a hard bench or ﬂoor

• Boxed product bounced and jolted during transporta-

tion

• Boxed product dropped on a hard bench, ﬂoor, truck

bed

• Product (in use) dropped on a hard work surface or

ﬂoor

• Product (in use) struck by passing equipment

• Product stored in racks, PWA ﬁxtures, or rework sta-

tion grips.

• PWA subjected to combined environmental stress in

ﬁeld use.

The assessment of the potential quality/reliability degrada-

tion of a printed wiring assembly (PWA) resulting from

exposure to the use environment cannot always be deter-

mined by considering each environmental stress in isola-

tion. For instance, consider the non-linear combined effects

of severe thermal and vibration stress application. A given

magnitude of random vibration at elevated temperature

(near or above T

) may produce increased PWA ﬂexure

because of increased board softness or ﬂexibility. Simulta-

neously, solder joint pull strength is decreased at the

elevated temperature. The result may be overload, that is, a

PWA conﬁguration for either the elevated temperature or

the vibration applied at room ambient temperatures may

fail prematurely in service when exposed to the two factors

together. Board stiffness and damping as discussed in sec-

tion 4.0 are key factors to be considered. The adequacy of

a proposed design may require combined-environment

stress testing of prototypes prior to full-scale production.

See also the references by Steinberg and Engel regarding

vibration, shock and thermomechanical effects on PWA,

section 9.16.

3.1.14 Insulation Resistance

The emerging advanced technologies are characterized by

denser packaging resulting in ever ﬁner conductor line

widths and spacings. Without changes in the material and

the operating environment, which for economic and practi-

cal reasons are not likely, ﬁner lines and spacings result in

reduced insulation resistance and increased threat of CAF

(conductive anodic ﬁlament) formation. The DfR principles

listed in Appendix C need to be kept in mind in the design

and application of these emerging technologies.

The damage mechanisms work generally in two distinct

regions: at the surface and in the bulk of the electronic

assemblies, particularly the printed board. The measured

insulation resistance will depend upon the nature of the

laminate, solder mask and/or conformal coating under

investigation. It will also depend upon the degree of cure

of the polymers and for printed boards on the quality of the

drilling process for the plated-through holes (PTHs) and

vias (PTVs), and will be affected by soldering ﬂux/paste

July 1996 IPC-D-279