IPC-D-279 EN.pdf - 第122页

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-co…

GUIDELINE AREA OF QUALITY/RELIABILITY IMPROVEMENT

Eliminate Engineering Changes on Released Products

Fewer errors due to changeovers & multiple revisions/versions Lower assembly error rate

Make Assembly Easy and Foolproof (Poka-Yoke)

No ‘‘force ﬁtting’’ of parts

Parts cannot be assembled wrong

Obvious when parts are missing or wrong orientation

Assembly tooling designed into part (self-aligning/securing)

Less damage to parts, faster and better serviceability

Lower assembly error rate

Use Repeatable, Well-Understood Processes

Part quality easy to control

Assembly quality easy to control

Higher part yield

Higher assembly yield

Choose Parts that Can Survive (are Compatible with) Process Operations including Rework, Repair and Maintenance

Less damage to parts

Less part degradation or latent damage through prior

evaluation

Plastic Encapsulated Surface Mount Components -

IPC-SM-786

SM Connectors - IPC-C-408

Other SM Components - IPC’s ‘‘Solvent Compatibility’’

No silver termination ﬁnish

No nickel termination ﬁnish

No thick gold printed board or termination

Minimize number of TH components

Maximize printed board Tg (glass transition temperature)

Printed board thickness compatible with placement machine

Dry ﬁlm solder mask and solder paste compatibility

Components and solder mask result in adequate clearance

to printed board

Higher yield

Higher reliability

Avoid silver leaching, weak solder joint

Increase solderability, joint strength

Increase solder joint visual yield ﬁnish

Lower defects due to solder bridging wider TH and

SM process latitude

Decrease hand loading, manual soldering

Decrease patent and latent printed board damage at high

process temperatures

Minimize solder balls

Enhanced cleaning. Higher SIR PWA robust to dendrite

formation

Choose or Design Process for Compatibility with Susceptible Parts

Less part damage or degradation

Ceramic components thermal shock < 4°C/second

Sensitive components preheated so that ∆T < 100°C

Susceptible Plastic Encapsulated Surface Mount Components

handled per IPC-SM-786

Don’t impact ceramic parts with pick and place tooling

Reﬂow process adjusted for thermal unbalance due to

thermal masses of parts (PGA, Heat Sinks)

Dry susceptible substrates before reﬂow

Component terminations not used for in-circuit testing

Higher yield

Higher reliability

Layout Parts for Reliable Process Completion

Less damage to parts during handling and assembly

Orient parts for non-interfering single axis insertion

Sequence parts for insertion for easy disassembly

Orient similar parts similarly

Parts not hanging over solder quality (own or neighboring

parts). No shadowing

Parts can automatically be placed or inserted

Higher yield, higher reliability

Less part damage

Easier rework, repair and maintenance

Fewer orientation sensitive bridging or solder joint failure

modes

Consistent solder joint

Lower assembly error rate

IPC-D-279 July 1996

110

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

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