IPC-D-279 EN.pdf - 第92页

LEDs and SOT -23 components appear to be more vulner- able than to other components, which is likely the result of the termination geometry . The thinner and more ductile copper SOIC leads appear to be more resistant to …

situations; the stress normally results in detection of plating

porosity, but is also known to result in loss of hermeticity

in sealed packages as well as loss of legibility of compo-

nent marking. The pH at which this test is conducted is

signiﬁcant to the test results.

E-14.0 TEMPERATURE/HUMIDITY/BIAS

The combination of the stresses, temperature, humidity,

and electrical bias, in the presence of conductive contami-

nation, particularly halides, results in electrochemical cor-

rosion and metal migration. Metallic dendrites of the com-

mon electronic metals (silver, copper, tin, lead, gold) have

been found on assemblies during high temperature/high

humidity testing; these dendrites occur on the surface of

the printed board. Dendrites are found within the bulk of

the printed board where voids allowed entrapment of con-

ductive solutions and within delaminated areas of ICs

where ﬂux residues were found. Under this combination of

stresses and where the bulk of the encapsulating material is

the source of the contamination, ICs are found to demon-

strate a time to failure described by

=(RH)

exp

(

)

where RH is the relative humidity (%), E

is the activation

energy (eV) in the range of (0.77 to 0.81 eV) and n is in

the range of -2.5 to -3.0. The RH range around 100% is

anomalous. See Appendix C .

Dendrite growth of silver, copper, lead or tin also occurs

outside and inside plastic encapsulated integrated circuits

and networks. Molding voids; internal and external pack-

age cracks at the bonding ﬁngers; manufacturing process

damage at trim and form (possible when the component

supplier process quality is inadequate); as well as delami-

nation during solder reﬂow create an ideal physical/electro-

chemical environment for dendritic growth.

E-15.0 SAND AND DUST

The results of exposure to sand - dust can include increased

friction between mating surfaces in disk and tape drives;

increased abrasion of mating plated surfaces- where the

plated surfaces form an electrical contact, contact failure

may be intermittent or complete; contaminated, abrasive

lubricants (Desert Storm demonstrated many ineffective air

and oil ﬁltering systems); clogged metering oriﬁces of air

dashpots; and clogged air ﬁlters with reduced cooling effi-

ciency.

Dusts with even slightly conductive constituents can reduce

insulation resistance and contribute hydrolyzable contami-

nants with subsequent effects noted under the moisture/

humidity section above. Highly conductive dusts have been

wiped up from the tops of dust hoods in ‘‘clean’’ factories.

E-16.0 MECHANICAL SHOCK

Mechanical shock may excite resonances in systems and

printed circuit assemblies. During surface mount technol-

ogy assembly, mechanical shock can be introduced by pick

and place machine collets; broken multilayer capacitors

and broken solder joints have resulted. During through-

hole (TH) assembly, mechanical shock may be introduced

by the TH insertion machine or by the lead trimming pro-

cess. High energy processes such as depanelizing (routing)

have resulted in displaced and broken wirebonds in inte-

grated circuits built with an open cavity around the bond-

ing wires. Peak stresses may overload or plastically deform

structures which may then fail by cracking, moving or

changing shape. Jamming or impairment of mechanical

functions may occur. Momentary disruption of electrome-

chanical functions may occur on disk drive arms, tape drive

heads, and relays; momentary opens may occur in

switches, connectors, and between components and their

associated sockets. Transportation is a source of mechani-

cal shock in service. Inappropriate use of elastomers

intended to isolate product from external sources of

mechanical energy may result in surprising damage from

energy stored in the elastomer.

E-17.0 MECHANICAL VIBRATION

Cyclic peak stress and fatigue lead to loss of strength or

material failure such as cracking, brinelling, spalling, or

displacement. Mechanical modulation of contacts (sockets,

relays, connectors, insulation displacement connection

cables...) results in momentary opens and intermittent fail-

ures such as no trouble found (NTF). Repeated ﬂexing of

materials (cables, joints, 1 piece hinges...) results in work

hardening, fatiguing and cracking. Fretting, which is

mechanical wear in localized areas due to micromotion,

results in continued generation of intermetallic compounds

in contact systems where gold plating opposes tin plating,

thinned or ruptured plating where a soft metal opposes a

hard metal or oxide, pileup of oxide and polymeric con-

tamination (particularly in the case of the platinum family

of contact materials) and subsequent increased contact

resistance and intermittent or total contact failure. Wear or

cut-through of panel coatings and cable insulation can

occur where contact occurs between insulated parts and

sharp edges. Unbalanced cooling fans may be an inadvert-

ent source of vibration.

Ultrasonically (U/S) enhanced cleaning systems have been

found to cause fatigue failure of the wire bonds in open

cavity integrated circuits where the resonant frequency of

the wire spans is close to the frequency of the ultrasonic

generator; this is probably not a cause of concern with sol-

idly molded (non-cavity) components. The vibration (cavi-

tation) of the cleaning liquid also results in erosion of the

solder joints and fatigue of component external connections

with high U/S energy densities. Fatigue and cracking of the

joint or of the component termination appears likely only

where the component termination system is mechanically

resonant near the U/S generator frequency; solder joints to

IPC-D-279 July 1996

LEDs and SOT-23 components appear to be more vulner-

able than to other components, which is likely the result of

the termination geometry. The thinner and more ductile

copper SOIC leads appear to be more resistant to U/S

induced fatigue than other terminations.

High energy repetitive assembly processes such as routing

result in both in-plane and out-of-plane vibration modes in

PWAs. In one case, accelerations on the order of 40-130

g’s have been documented, with broken wire bonds,

welded relay contacts, and broken crystal oscillator con-

nections.

It is reported that gold plated relay contacts (such as those

in reed relays) can suffer fretting corrosion during high fre-

quency mechanical stress.

E-18.0 MECHANICAL OVERLOAD

Solder joints, initially good, are subjected to mechanical

stress each time the assembly is ﬂexed, shocked, or

vibrated, for instance, TH component insertion, depaneling

(by routing, shearing or scribe/break) or testing. Bending or

ﬂexing an SMT board after it is soldered puts severe stress

on solder joints and components, as will the ‘‘straighten-

ing’’ of a warped board during insertion of the printed

board into a cardguide. Board support/retention/clamping

and vibration dampening during these operations, and

printed board design aspects such as part orientation have

been found to be techniques to minimize failed joints and

terminations. One MLCC supplier, specifying a test condi-

tion of components mounted centrally on a coupon sup-

ported on 90 mm centers, requires that X7R and Z5U

capacitors survivea2mmdeﬂection and COG capacitors

survivea3mmdeﬂection.

A simple technique was developed to predict areas of high

stress due to ﬂexure inﬂuenced by rigid components. On a

layout of the PWA, construction lines are drawn from the

corners of the rigid components (such as connectors) and

the construction line intersection deﬁnes the area of high-

est stress. Another area of high stress is the ‘‘fold-line’’ of

an ‘‘L’’-shaped printed board. TH laminated buss bars, TH

radio frequency interference (RFI) fences and PGA pack-

ages are also in this ‘‘rigid component’’ category.

Failed solder joints and components adjacent to reworked

components have occurred due to mechanical ﬂexure and

pressure stress during the component removal process.

Failed solder joints occur where connectors are assembled

without mechanical restraints to minimize rotational or

other movement of the connector relative to the printed

board. These restraints could be springy (snap-in) detents

(for wave soldering), rivets or screws (for hand soldering

or possibly posts secured by heat-staking or ultrasonic

deformation). Note that this is an order ranking from a

Design For Assembly/Manufacturability (DfA/M) view-

point.

E-19.0 EM SUSCEPTIBILITY, RADIATION,

INTERFERENCE

If electromagnetic (EM) energy is induced in the traces

associated with integrated circuits inputs or outputs, the

results may be manifested as intermittent failure or second-

ary EOS. CMOS latchup may be initiated. The higher

clock rates and faster transition times associated with the

higher speed ICs can result in more EM energy generated

and radiated from the product. The radiated energy may be

gathered by other circuits or products and become an inter-

fering signal. See also the literature on ESD, EMI, EM

Control.

E-20.0 LOW ATMOSPHERIC

PRESSURE/HIGH ALTITUDE/VACUUM

These stresses result in the expansion or explosion of gases

in voids, resulting in overstressed sealed containers, pack-

ages and cavities (the ‘‘blowing’’ of liquid electrolytic

capacitors vent plugs falls into this category); decreased air

density and less efficient cooling (assemblies and systems

may overheat, fuses with leaky seals will tend to open pre-

maturely); decreased air density and decreased dielectric

strength (initiation of corona, arcing and insulation break-

down with subsequent thermal and ozone damage as well

as sprayed conductive materials, voltage breakdown and

leakage in relays and connectors and other open [unsealed]

conductors). ‘‘Sealed’’ cavity components may lose air and

suffer the consequences of reduced gas dielectric strength.

E-21.0 IONIZING RADIATION

The most common source of ionizing radiation has been

the outer package material (ceramic or plastic molding

compound) of dynamic random access memories

(DRAMs) where alpha particles are emitted by the heavy

metals, such as uranium or thorium in the ceramic or oxide

constituents; the alpha particles are absorbed by the storage

elements of the DRAM and contribute electronic charge to

the storage elements. This additional charge, on the order

of one million hole-electron pairs per particle, can result in

soft errors. Relatively thick layers of pure organic materi-

als, such as polyimide, can ‘‘stop’’ the alpha particle from

penetrating the silicon; unfortunately, these ‘‘stopping’’ lay-

ers can contribute to delamination of the molding com-

pound from the surface of the protected die and subsequent

corrosion.

Transient leakage currents and possible EOS can result if

the energy of X-rays from a cathode ray tube is sufficient

to penetrate the outer system package and the component

package. See also CMOS Latchup.

Visible light is normally blocked by electronic component

packaging; however, there are instances where light pen-

etrates the packaging material, is absorbed in a semicon-

ductor junction and contributes hole-electron pairs; opto-

couplers and photodetectors in plastic packages are

July 1996 IPC-D-279

susceptible to light induced leakage, noise and

malfunction.

E-22.0 SOLVENTS

Exposure of components and assemblies to various sol-

vents (not normally considered a ‘‘stress’’) during assem-

bly and repair operations can result in the following

scenarios:

• loss of lubricant with subsequent accelerated wear,

particularly in variable resistors and capacitors and in

switches

• absorption and diffusion through polymeric seals with

subsequent corrosion, particularly in aluminium

capacitors when halogenated solvents, such as CFCs,

have been used

• dissolution of some wire insulation ‘‘varnish,’’ mark-

ing inks and label adhesives with subsequent contami-

nation of the cleaning system, loss of the insulation

function, or loss of identity

• dissolution or softening of plastics such as polycarbon-

ate and polystyrene, even when low activity solvents

such as CFCs have been used, solvation effects are

exacerbated when solvent blends containing methylene

chloride are used

• and absorption and deformation of silicone rubber

parts and seals in CFCs.

Because CFC based solvents are being replaced by other

solvents due to concern over ozone depleting effects, great

care must be exercised in the replacement process. For

instance, cleaning of SMT boards with a ‘‘plug in compat-

ible’’ solvent prior to paste application has been found to

greatly increase the frequency of ‘‘solder balls’’ apparently

due to chemical reactions between the thermal decomposi-

tion products of the solvent and the tin in the solder during

the soldering process. The composition, location and quan-

tity of the residual contamination will also change with

changes in the solvent.

The very high velocity solvent sprays used in the ﬂux

removal systems designed to achieve very clean PWAs par-

ticularly in SMT are intended to clean under and between

components which are tightly ﬁtted together. Marginal

‘‘O’’-ring seals may succumb.

IPC-D-279 July 1996