IPC-D-279 EN.pdf - 第123页

plastic and gasket materials (up to 1% water by weight) results in swelling; cyclic changes in humidity can result in ‘ ‘creeping’ ’ of the plastic or gasket material. V ery low lev- els of water vapor (low Relative Humi…

Appendix L

Corrosion Basics and Checklist

L-1.0 CORROSION BASICS

The result of corrosion is material loss due to corrosion of

the metallic conductors, permanent or intermittent continu-

ity loss due to build up of non-conductive corrosion resi-

dues (particularly between contacts) and permanent or

intermittent shorts due to build up of conductive corrosion

residues and conductive metal dendrites. Corrosion accel-

erates the failure of components under cyclic fatigue con-

ditions. Corrosion can also disrupt painted or plated prod-

uct coatings. Surfaces roughened by corrosion are less

effective as sealing surfaces. Water increases the oxidation

rate of oxidants such as SO

,SO

and O

. Water greatly

increases the corrosion rate and metal migration growth

rate of halides such as chloride and ﬂuoride. Water

enhances galvanic corrosion in the presence of dissimilar

metallic ﬁnishes; this issue is critical for EMI gaskets, EMI

seals and brazed joints to electroplated structures in

ceramic packages. In the presence of nutrient materials,

water increases fungus growth; the fungi release organic

acids in their waste products.

The oxides of tin, nickel and copper are not good conduc-

tors. Low interfacial pressure contacts to these metals can

become resistive or intermittent.

Salt atmosphere/spray and ﬂowing corrosive gas atmo-

sphere are excellent sources of hydrolyzable, conductive

contamination + water + oxygen. Salt atmosphere/spray

stress is required in military systems but is not commonly

encountered in commercial situations; it normally results in

the detection of plating porosity, but is also known to result

in loss of hermeticity in sealed packages as well as loss of

legibility of component marking.

Office and factory dusts have been found to contain high

levels of chlorides; water pastes made with dust from the

tops of benches and fume hoods are highly conductive. The

atmosphere of paper mills contains acidic sulﬁde and sul-

fate compounds. Unﬁltered forced cooling air can contrib-

ute to premature failure of peripherals and system.

The common halogenated cleaning solvents decompose at

high temperatures or in the presence of catalytic metal sur-

faces. Extremely high halide levels have been found in

droplets of water ﬂoating on the solvent surface in the cold

sump of vapor degreasing systems; in these cases, the

water absorbing cartridge had failed or the solvent stabili-

zation additives were exhausted. The halides deposit onto

assemblies which are ‘‘cleaned’’ in the cold sump.

A typical source of hydrolyzable or ionizable contaminants

which result in corrosion in electronics is human ﬁnger-

prints, spittle, and food. Fingerprints also contribute oily or

greasy residues which keep the conformal coatings from

fully protecting the conductors and lands from electro-

chemical corrosion.

L-2.0 CORROSION OF THE PWA

The common insulation system in a PWA is the printed

board, its solder mask and any conformal coating. Adsorp-

tion of water or condensation of water vapor on the surface

of insulators with dissolution of hydrolyzable contaminants

results in the subsequent loss of Surface Insulation Resis-

tance (SIR); this effect is seen particularly on porous sur-

faces such as uncoated printed boards which have been

contaminated with hydrolyzable materials and have not

been scrupulously cleaned and can lead to electrochemical

corrosion effects such as metal migration and dendrites.

Metallic dendrites of the common electronic metals (silver,

copper, tin, lead, gold) have been found on the surface of

PWAs contaminated with chlorides and operated under

high humidity.

Dendrites have also been found within the bulk of the

printed board where voids allowed entrapment of conduc-

tive solutions and within delaminated areas of IC’s where

ﬂux residues were found. See ‘‘A Review of Corrosion

Failure Mechanisms during Accelerated Tests.’’ The pres-

ence of water, DC bias and ionizable contaminants at the

interface between the resin matrix and glass ﬁbers between

PTHs, vias and conductors lead to interfacial electrochemi-

cal corrosion and dendrites or Conductive Anodic Fila-

ments (CAF). See Appendix C. See also IPC-TR-476.

L-3.0 CORROSION IN COMPONENTS

Common halogenated solvents can and have diffused

through the rubber seal of aluminum electrolytic capaci-

tors; the result is the dissociation of the solvent inside the

component, the release of HCl, the corrosion of the alumi-

num foil, and failure of the capacitor. A solution is the use

of capacitors where the elastomeric seal is augmented by a

hermetic or epoxy seal.

Absorption in the bulk of insulators with dissolution of

hydrolyzable contaminants results in the subsequent loss of

bulk Moisture Insulation Resistance (MIR) particularly in

printed boards, dielectric ﬁlm capacitors and plastic encap-

sulated electronic components such as integrated circuits,

networks, and hybrids. Dendrites have also been found

within delaminated areas of IC’s where ﬂux residues were

found.

L-4.0 OTHER EFFECTS OF WATER AND WATER VAPOR

Absorption of water in the bulk of the insulating ﬁlm of

capacitors results in increased dissipation factor and

increased leakage current. Absorption of water by speciﬁc

July 1996 IPC-D-279

111

plastic and gasket materials (up to 1% water by weight)

results in swelling; cyclic changes in humidity can result in

‘‘creeping’’ of the plastic or gasket material. Very low lev-

els of water vapor (low Relative Humidity or RH) allow

ElectroStatic Discharge (ESD) voltages to build up.

Chemisorption of water into polymer systems such as

molding compounds results in a lower T

, lower strength

and increased total thermal expansion.

L-5.0 FRETTING ‘‘CORROSION’’

This category of corrosion is a mishmash of failure of con-

tacts from two causes:

• In contrast to the good performance of gold-gold (gold

to gold) contacts at low contact pressures and low cir-

cuit energy levels or of tin-tin contacts at high contact

pressures and high circuit energy levels, gold-tin con-

tacts fail even at high pressures and high energy levels

due to the formation of gold/tin intermetallics with low

conductivity at the contact points due to small relative

motions between the contacts (micromotion). Sepa-

rable connectors and sockets for components such as

ICs have contributed to system failure when the gold/

tin intermetallics are formed. The micromotions can be

caused by power cycling of the component as well as

shock, vibration or temperature cycling of the system.

• In the presence of hydrocarbon vapors and relative

contact movement as in relays, platinum group metal

contacts catalyze the development of an insulating

polymeric ﬁlm; the source of the vapors can be the

housing of a relay. In the presence of silicone vapors,

electrical arcing and relative contact movement, metal

contacts develop an insulating silica ﬁlm.

L-6.0 CORROSION DESIGN CHECKLIST

• Identify process, service, rework and repair environ-

ments and chemicals, particularly:

• Rapid transitions between high temperature/high

humidity and low temperature with immediate or

continuing operation.

• Long term low or battery power operation at high

relative humidity.

• Mitigate effects of long term low power operation by

requiring that low power ICs and other components

with DC bias be free of delamination and cracking

between encapsulant and the surface of the IC or sub-

strate; risk sites are covered with void free laminate,

solder mask or conformal coating (CC). The CC

should demonstrate no delamination, mealing or vesi-

cation after exposure to high humidity/DC bias (e.g.

by absence of delamination with Scanning Acoustic

Microscopy).

• Assure that substrate conductor and component lead

spacings are suitable for the environmental conditions

of the product (humidity, contaminating dust, thermal

cycling, bias, corrosive gases) where no additional

conformal coating is required. Minimum spacings

from conductors to (conductor, mounting hole, compo-

nent terminals) and from barrels to inner plane con-

ductors and other barrels are used only where needed.

Also a DfM guideline.

• Identify and design against potential damage to insu-

lating or moisture barrier solder mask or conformal

coating by stresses such as high temperature, UV, and

ozone exposure; for partially cured coatings, very low

temperature can cause cracking.

• In environments with anticipated vibration or thermo-

mechanical movements due to temperature cycling,

separable connectors and contacts are immobilized.

Particularly applies to the active catalyst platinum

family, including gold, to prevent the formation of

insulating polymers from condensible carbonaceous

vapors. See also ‘‘fretting corrosion’’ below.

• Identify susceptible metals used in the components and

general design; for those metals, quantify performance

under anticipated corrosion stress.

• Design assemblies and choose components which are

easy to keep clean or are easy to clean. Minimum use

of EIAJ ‘‘O’’ clearance components, particularly

where using water soluble ﬂux. Minimum use of dis-

crete component spacers which introduce multiple

crevices (sugar or adhesive spot under components is

OK). Also a DfM guideline.

• The assembly process requires maintenance of clean

condition of components and assemblies (e.g. by the

use of gloves during handling); clean ambient condi-

tions during laminate handling, lamination, storage;

cleaning of printed board prior to solder mask applica-

tion. There is no hydrophilic solvent (e.g. polyglycol)

in ﬂuxes or soldering oil. Solvent or water mechani-

cally removed prior to air dry (e.g. air knives); non-

polar solvent used to remove organics which trap or

mask water soluble contaminants; polar solvents or hot

DI water used to remove ionizable contaminants; ultra-

sonic cleaning or high solvent velocity to get between

crevices. Where applicable, periodic SIR testing is

conducted to verify continued efficacy of the cleaning

method.

• Avoid all identiﬁed potential moisture traps associated

with components, including open cavity packages

sealed with polymers and low clearance packages over

bare conductors. Corrodible portions inside the cavity

are sealed with hydrophobic materials such as silicone.

Conductors are sealed with solder mask or conformal

coating prior to assembly or component clearance

increased to effect complete cleaning.

• Avoid all identiﬁed, exposed galvanic couples such as

IPC-D-279 July 1996

112

terminations of copper/nickel/gold which are sheared

after plating (e.g. exposed base metals on the edges of

contacts). These conditions can also lead to tarnish

creep, the extension of corrosion products of copper

over the gold. See a sample galvanic compatibility

table below.

• Identify mechanical stress levels in susceptible metal

parts (particularly formed terminations for possible

contribution to stress corrosion or plating discontinui-

ties); alternatively, metal parts are formed in the

annealed state and postplated.

• Identify susceptible ceramic package brazed and plated

terminations which may have brazed joints of metals

constituting galvanic couples, may have exposed

plated interfaces, and may have been or need to be

trimmed and/or formed after plating.

• Use components which do not release corrosive mate-

rials. Wet slug tantalum with sulfuric acid electrolyte is

not recommended for new designs. Avoid aluminum

electrolytics with dimethyl formamide electrolyte

which degrades solder mask and conformal coatings.

Orient vent plugs of unavoidable electrolytic capaci-

tors to minimize damage consequent to component

failure—face vent plugs away from substrate.

• Identify and if possible avoid galvanic corrosion

couples; these practices may mitigate consequences:

• Plating is to be complete with no exposed interfaces

in plating systems such as copper overplated with

solder, tin, or gold. Alternatively, interpose nickel

plating between the copper and the overplate.

• Interpose an intermediate compatible material. For

instance, for a steel screw fastened to aluminum,

interpose a washer plated with cadmium (an environ-

mental no-no) or use ‘‘active’’ stainless steel for the

screw where the S/S passivation tends to isolate the

screw.

• Selectively metallic plate as required for reliable

electrical contact between pressure contacts; e.g. gold

to gold or tin to tin but not gold to tin (to avoid

‘‘fretting’’ corrosion).

• Coat surfaces (with polymeric or conversion mate-

rial) for insulation and moisture exclusion.

• Anodically ﬁnish for insulation and moisture exclu-

sion but be very careful; the coating is thin and brittle

and entrapped residual anodic processing ﬂuids are

corrosive.

• Design so that cathodic metal area is much smaller

than the anodic metal area.

• Assure that fretting ‘‘corrosion’’ has been minimized

or eliminated:

• The need for contact lubricants has been investigated

and satisﬁed.

• Contact ﬁnish and thickness/porosity/smoothness is

appropriate to the use environment, including fre-

quency of reconnections and current/voltage condi-

tions.

• Contacts are gold to gold or tin to tin but not gold to

tin.

• Card mounting stresses and ﬂex circuit ﬂexures

(static or dynamic vibration) are controlled by

clamps, screws, hold-downs; the stresses are not

transmitted to the connector or to the contacts.

• Identify conditions at possible electrochemical corro-

sion risk sites:

• bare metal

• tight spacing (very small diameter vias, complex

mechanical ﬁtted parts, connectors, switches, vari-

able elements...)

• relative humidity 65% or condensed water ﬁlms and

droplets

• ionizable contamination

• conductors with DC potential (particularly between

leads of ﬁne pitch elements and where pin assign-

ment is optional, avoid large potential differences

between adjacent pins of connectors);

• Pressure contacts are sealed from condensing mois-

ture, high humidity, and corrosive gases; conformal

rubber seals under continuous high pressure appears to

be effective. Be cautious of compression set effect of

rubbers at low temperatures.

• Avoid exposed silver plating, silver pastes, and silver

adhesives; overplate silver conductor material with

nickel or conformally coat or locate the component so

that water will not condense and run onto the silver.

Includes MLCC, DIP, rotary and slide switch, variable

resistor and buzzer packages.

• Tent all vias and PTHs on both ends if using active

water soluble ﬂux (paste or liquid); alternatively, open

vias and PTHs do not terminate under a component

with tight clearance. A third alternative is to ﬁll the

vias and PTHs with solder, epoxy or modiﬁed solder

mask/conformal coating material. These techniques

minimize barrel corrosion due to ﬂux entrapment and

avoid test ﬁxture corrosion and loss of SIR due to

drips of liquid ﬂux.

• Hydrolyzable materials completely removed from the

PWA prior to application of any solder mask; similar

cleaning prior to any conformal coating application.

This is critical where water soluble ﬂux systems are

used; otherwise, mealing and vesication can result

under high RH conditions.

• Avoid conductive anodic ﬁlament growth (CAF).

Evaluate printed board suppliers for delamination of

solder mask between conductors and laminate voids

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