IPC-D-279 EN.pdf - 第135页

N-6.0 JUNCTION COATINGS, ‘‘GLOB-TOPS’’ A specialized category of coatings includes the semicon- ductor junction, chip or die coatings such as the screen printed epoxies or polyimides, spun on polyimides, and chemically v…

wave soldering) and delamination at the conformal coating/

metal interface may allow contaminants to become

entrapped and blistering of the conformal coating. This

may contribute to electrochemical corrosion or migration.

Conformal coating removal must be performed in a manner

which:

• provides a mechanically and electrically sound, dry,

clean surface to which conformal coating material will

be applied;

• minimally degrades the underlying solder mask or its

adherence to the printed board. (Some CC removal

solvents also attack some solder mask materials;

excessive temperature during removal of epoxy or

polyparaxylylene CC may degrade the underlying sol-

der mask, or where the solder mask or laminate have

absorbed substantial water from the air, may cause

delamination of the solder mask from the laminate or

delamination within the laminate);

• minimally degrades the surrounding conformal coating

or its adherence to the printed board. (CC removal sol-

vents do not remain on the surface but can permeate

the CC ﬁlm and may affect the CC- solder mask inter-

face; CC removal solvent residue if not effectively

removed may lead to latent corrosion failures; thermal

removal of epoxy coatings with soldering iron, hot air

or laser, can char or otherwise degrade the surrounding

area. Mechanical abrading techniques can be used to

remove CCs and solder mask and to remove thermally

damaged coatings if ESD damage is not a concern);

• does not degrade or contaminate components which

are to remain in place. (CC removal solvents may con-

tain corrosive, polar or ionic materials and can attack

the polymers of which components are made; solvent

stress cracking may develop in the long term. See IPC-

R-700.)

Conformal coating rework or repair must be performed in

a manner which restores the functionality of the coating

with respect to adherence to the printed board and compo-

nents and with respect to freedom from delamination,

voids, and inclusions. (See IPC-R-700.)

N-5.0 COMMON CRITICAL PROPERTIES OF SOLDER

MASK AND CONFORMAL COATINGS

Conformal Coatings:

• Provide mechanical protection for the surface mount

(SM) PWA during assembly, test and service;

• Provide a smooth, chemically stable surface to reduce

SM PWA susceptibility to effects of condensing atmo-

sphere and electrochemical corrosion/migration during

test and service;

• Permeable to water vapor

CCs are dependent upon these factors for effective perfor-

mance under humid conditions:

• a clean underlying substrate surface free of water

soluble materials prior to application (to prevent

mealing, vesication, growth of metallic dendrites).

Non-polar solvents do not effectively remove these

soils;

• a clean underlying substrate surface free of solvent,

greasy, oily, particulate and other materials which

interfere with adhesion of the ﬁlm (to prevent

delamination and growth of metallic dendrites). Polar

solvents or water without detergents do not effec-

tively remove these soils. Additional, ultrasonic

energy may be required to remove and suspend par-

ticulates as well as contaminants in cracks and crev-

ices (see IPC-CH-65);

• a dry substrate free of excess moisture and solvents

(whose vaporization during exposure to the heat of

assembly processes may result in delamination of the

coating from the substrate);

• complete encapsulation of conductors - ﬁlm forma-

tion with no voids bridging conductors and no voids

along conductor edges;

• mutual compatibility of solder mask and conformal

coating where both are used;

• a proper cure for a strong, coherent ﬁlm free of

cracks, voids and inclusions. (See IPC-S-816, IPC-

PE-740.)

Some cleaning solvents such as chlorinated hydrocarbons

and alcohols attack or swell some solder mask formulations

and some conformal coatings.

Leaking capacitor electrolytes such as sulfuric acid, dim-

ethyl formamide, and gamma butyrolactone, particularly at

high use temperatures may degrade some coatings; these

electrolytes are also used to remove conformal coatings.

Liquid ﬂux entrapment in PTHs and PTVs may be mini-

mized by the selective deposition in and ﬁlling of the bar-

rels with liquid solder mask material or liquid conformal

coating material. Filling of the barrels with liquid solder

also accomplishes the same purpose.

Solder joint reliability under temperature cycling or power

cycling conditions may be reduced if the solder mask

touches the bottom of the component or conformal coating

ﬁlling the printed board - component gap. (See IPC-SM-

785.)

Degree of outgassing must be quantiﬁed and appropriate,

particularly for space applications, where outgassing is a

vital concern around optical surfaces such as lenses, mir-

rors and detector faceplates. Outgassing is also important

where unsealed relays, switches or separable connectors

are used; micromovement or electrical arcing can form

semi-insulating deposits of carbon or silica materials on the

contacts.

July 1996 IPC-D-279

123

N-6.0 JUNCTION COATINGS, ‘‘GLOB-TOPS’’

A specialized category of coatings includes the semicon-

ductor junction, chip or die coatings such as the screen

printed epoxies or polyimides, spun on polyimides, and

chemically vapor deposited poly-paraxylylene which are

deposited on the chip in wafer form and patterned; ‘‘glob-

top’’ epoxies and silicones which are deposited on the

interconnected chip; chemically vapor deposited poly-

paraxylylene which are deposited on the chip and wire-

bonds, unpatterned; and special purpose pyrolized sili-

cones. The application of junction coating material over

contamination creates a corrosion timebomb.

Junction coatings are intended to provide:

• Mechanical environment protection from scratches of

the chip surface and disturbance of interconnections.

• ‘‘Reliability without hermeticity’’ by excluding liquid

water and by limiting halide and alkali metal content

to very low levels under hot water extraction condi-

tions.

• Low stress upon cool down, where the coating is

cured.

• In some cases, protection for DRAMs from alpha par-

ticle upset.

• In the case of poly-paraxylylene, increased bond wire

strength.

• In some cases, ‘‘planarization’’ of the chip surface in

preparation for encapsulation by thermoset molding.

Junction coatings may also introduce additional reliability

concerns such as:

• Shear stress on the surface of the chip during cool

down.

• Compressive stress on conductors on the chip surface,

increasing possibility of hillocks and voids.

• Shear stress on ball bonds during temperature cycling

with the junction coating is about as thick as the bond

is high.

IPC-D-279 July 1996

124

Appendix O

Aerospace and High Altitude Concerns

O-1.0 INTRODUCTION

Use of Surface Mount Technology for aerospace and high

altitude applications has the same problems that are expe-

rienced with through-hole technology. The problems are as

follows:

• lack of air for convection cooling

• larger thermal excursions

• contamination

• radiation environment

• change in dielectric property of gases with pressure

• lessened gravitation or pseudo-gravitational effects.

The degree which these problems impact the design of sur-

face mount assemblies depends on the precise space envi-

ronment, the purpose of the mission, and the system

design. A detailed analysis may be needed for each PWA to

determine the extent of the problems.

O-2.0 THERMAL DESIGN

At sea level, natural or forced air convection greatly assists

in the cooling of electronic components; however in

un-pressurized compartments, there is little or no air for

convection and additional thermal considerations must be

made. Since radiation is negligible at the temperature of

interest for most SM PWA the only way to remove heat is

by conduction. Many components generate heat which

requires dissipation to prevent excessive junction tempera-

ture; heat removal may be enhanced with thermal materials

placed beneath these components. However, the thermal

material placement must not result in excessive thermo-

mechanical stress on the solder joint and subsequent solder

joint failure by fatigue when the assembly is thermally

cycled. Simulation, analysis and testing is needed to deter-

mine the thermal control measures needed for components

and SM PWA in space.

O-3.0 LARGE THERMAL EXCURSIONS

In addition to the normal thermal issues in packaging

design, the space environment adds a new consideration;

thermal excursions due to the effect of the sun/shade expo-

sure. While exterior surfaces of the spacecraft may be

exposed to 100,000 temperature cycles from −60 to

+120°C, through temperature control, the temperature of

internal regions of the satellite may be limited to a few

temperature cycles of 5°C. In a deep space mission, the

temperature may be as low as −270°C. These differences

are critical in the selection and use of materials and a sys-

tems level as well as a PWA level, thermal analysis is criti-

cal in designing electronic packaging.

The following are a few general guidelines in selecting

materials for wide temperature extremes:

• Exercise caution in using polymeric materials with

glass transition temperatures (T

) in the temperature

range of test or use. Material properties change rapidly

and drastically near T

• Ensure compliant joints and bond lines between mate-

rials with different coefficients of thermal expansion.

Thermally induced stresses may be very large and

cause solder fatigue failures, adhesive bond failures,

crack ceramic and glass materials or permanently

deform metal. Thermal stresses may also be generated

if there are large, although perhaps transient, thermal

gradients from one part to another or within an indi-

vidual part.

• Ensure that metals do not undergo a phase change or a

change of the heat treatment in the temperature range

of interest.

• Ensure that polymers do not crystallize in the tempera-

ture range of interest.

Space applications exposed to severe environments require

packages robust to temperatures in the −55°C to +125°C

range. Hermetic packages may be required to be robust to

life cycle environment which include high relative humid-

ity; under extreme levels of shock and vibration, hermetic

packages (with ﬂying internal leads) are less robust than

plastic encapsulated packages.

O-4.0 CONTAMINATION

Contamination in spacecraft comes in two forms, particu-

late and condensed outgassed vapor. Particulate contamina-

tion may create false stars around the spacecraft by reﬂect-

ing sunlight causing problems with scientiﬁc and

navigational equipment. For some spacecraft missions,

condensed outgassed vapor is a minor concern; for others,

even a monolayer of condensed outgassed vapor is exces-

sive. Condensed outgassed vapor can cloud optical surfaces

causing decreased reﬂectance of mirrors, degraded clarity

of lenses and reduced solar cell output with subsequent

degraded star tracking capability, possible false data from

spectrum analyzers, and other degraded performance in

optical equipment. The outgassing species of most concern

in a vacuum are compounds which have sufficiently low

molecular weight to volatilize from a warm surface and

condense on cold surfaces. Gases (oxygen, water, and

nitrogen) usually are not a concern because they do not

form permanent contaminating ﬁlms. The worst sources of

outgassing material are incompletely reacted monomers

and plasticizers found in polymers.

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