IPC-D-279 EN.pdf - 第133页

surface to prevent moisture from collecting. All conformal coatings are permeable to moisture. The key issue is to prevent the moisture from collecting at an interface between adjacent conductors. Moisture will collect a…

remove mask from clearance areas, results in the ﬁnal cov-

erage. Dry ﬁlm is the high cost choice; however, it has

several advantages over liquid screenprinted masks. It can

provide not only reliable tent vias but can also provide a

relatively thick, uniform coating with superior registration

and resolution. These features are especially useful in the

case of vias placed under low clearance components or

where impedance-controlled circuitry is present on the

external layers. It does have the distinct disadvantage of

resulting in a solder mask that extends higher than the

lands it surrounds. This relative geometry translates

directly into increased defects for surface mount joints dur-

ing the wave and reﬂow soldering processes. Where tent-

ing of via holes is not required, dry ﬁlm solder mask can

be difficult to remove from holes smaller than 0.5 mm and

may affect solderability in these areas.

Some dry ﬁlm solder masks (in combination with particu-

lar solder ﬂuxes) appear to increase the incidence of solder

balls, reducing the effective conductor spacing. This may

increase the risk of corrosion and reduce the reliability.

Solder balls reduce the solder in the attachments and may

reduce the fatigue life.

Cracked and ﬂaking dry ﬁlm solder mask may be the result

of thermal abuse or overcure; solder mask tented over PTH

and vias may crack and entrap liquid ﬂux. The ﬂux may

leak out and contaminate testing ﬁxtures; corrode the bar-

rel of the PTH or PTV; and contribute to the growth of

metallic dendrites or conductive anodic ﬁlaments. (See

IPC-TR-476 as well as Design for Testability.)

In direct response to these high defect levels, solder mask

suppliers have developed alternative dry ﬁlm processes to

address the unique needs of Surface Mount Technology.

The ﬁrst is a thinner solder mask 60 µm which utilizes an

additional process after hot roll lamination to ensure com-

plete encapsulation of the circuitry with the reduced thick-

ness needed for high soldering yields. The size of via holes

which can be tented with this thinner mask is limited to

about 0.6 mm, however, this new method provides a supe-

rior combination of features necessary for surface mount

assemblies. The second method uses a liquid photopolymer

underneath a thinner dry ﬁlm layer to help bridge the cir-

cuits and ﬁll the via holes to support the solder mask tent.

This results in a total mask which is about 50 µm thick.

Since the via holes are completely ﬁlled with the photo-

polymer, it is critical that the bare board fabricator com-

pletely cure this type of solder mask. Without a complete

cure, outgassing during subsequent soldering processes will

lift the tents as volatiles in the vias escape during these

thermal excursions. Some users have noted that this ‘‘com-

bination’’ mask increases in thickness around the SM lands

(up to 125 µm), thereby negating the advantage of using a

thinner solder mask.

N-2.1.3 Liquid Photoimageable Liquid photoimageable

solder masks are the most recent group of solder masks to

be developed. They provide a lower cost alternative to dry

ﬁlms where the tenting (or ﬁlling of via holes) is not

required. These materials can be applied to the printed

board by a variety of methods which include: open screen-

ing, curtain and roller coating or by electrostatic spraying.

Following subsequent photoimaging and development,

photoimageable solder mask provides very high feature

resolution. Combined with the fact that this solder mask

application is only 15 to 30 µm thick, it provides excellent

resolution where small features are required (such as

between lands for ﬁne-pitch components).

Designs which minimize ﬂux entrapment are permitted by

solder mask systems which combine application of dry film

and liquid ﬁlm solder mask materials; these composite sys-

tems may result in solder mask on and local thick ridges

around the land patterns and test lands.

N-3.0 TEMPORARY MASKS AND STOPS

Temporary coatings, tapes and solder masks can be used to

prevent damage to connectors, variable components and

switches by solder, ﬂux, cleaning solvents, and conformal

coating. In SMT, the barrier may have to withstand very

high process temperatures as well as very high solvent

spray pressures. The solvent damage to the components

includes corrosion, metallic dendrites, and loss of lubrica-

tion. In some cases, loss of the solder mask has plugged the

cleaning system plumbing.

Some temporary removable solder masks containing NH

can corrode base metals or contribute to galvanic corrosion

at exposed interfaces of dissimilar metals; residues from

this class of solder mask may also inhibit the cure of sili-

cone conformal coatings which are catalyzed by platinum

compounds. Other, removable masks decrease in their

‘‘removability’’ after exposure to high processing tempera-

tures.

Some temporary solder masks soluble in organic solvents

react chemically with ﬂux vehicle components during the

soldering process; the reaction products may be detrimen-

tal to reliability. The residues from this class of solder

mask may trap ﬂux and ﬂux residues, leading to later cor-

rosion. It is critical that the temporary solder mask leave no

residue. If a residue remains which is not cleaned off, it

may interfere with contact mating, solderability, or confor-

mal coating.

N-4.0 CONFORMAL COATING

The primary purpose of conformal coating is to provide

environmental protection for the electronic assembly. Con-

formal coatings are polymeric materials which may be as

thick as 250 µm thick.

Conformal coatings provide environmental protection by

keeping contaminates from the circuits and adhering to the

July 1996 IPC-D-279

121

surface to prevent moisture from collecting. All conformal

coatings are permeable to moisture. The key issue is to

prevent the moisture from collecting at an interface

between adjacent conductors. Moisture will collect at an

interface if there is a loss of adhesion due to: a) thermal

stresses or b) the presence of contaminates which will trap

moisture. As contaminates trap moisture they vesicate the

coating providing a gap for corrosion to form. Moisture,

coupled with contamination to increase the conductivity,

creates an electrolytic cell between conductors which

results in corrosion. Conformal coating also prevents shorts

of adjacent conductors by loose metal fragments.

N-4.1 Selection of Coating Selection of conformal coat-

ing depends on the use environment, the design of the

electronic enclosure, and the manufacturing facilities avail-

able. In the worst environment, exposure to saltwater and

temperature cycling, the assembly may require potting or

encapsulation instead of conformal coating to be reliable.

The use environment needs to be understood in terms of

the temperature range, exposure to corrosives, chemicals

and solvents, and permissible outgassing. The design of the

enclosure also has an effect. An enclosure which is cooled

by fan drawing air from the outside is different than an

enclosure which is sealed or purged with dry nitrogen.

Some conformal coatings have been formulated to be cured

rapidly with ultra-violet light; in some cases a secondary

heat cure may be required.

N-4.2 Thermal Stress Design Considerations Thermal

cycling may cause a number of problems with conformal

coating. If the coating ﬁlls the gap beneath a component

which does not have sufficient stress relief in the leads, the

stresses, generated by the temperature excursions and a

mismatch in the coefficients of thermal expansion, may

fatigue the solder joints and result in solder joint failure.

Hard coatings like epoxy may apply excessive stresses to

glass bodied components and can crack them. Some coat-

ings like polyurethanes and silicones may be soft at room

temperature, but if they are cooled below the glass transi-

tion temperature, their elastic modulus may increase sev-

eral orders of magnitude. This may generate excessive

stresses like epoxies coatings. These stresses may be mini-

mized by selecting a coating with a T

lower than the low-

est exposure temperature and applying the coating in the

proper thickness. (Each coating has a recommended thick-

ness.)

Obtain the information on CTE, Modulus and T

from the

conformal coating supplier. Examine the design with this

information in mind.

Before application or re-application of any conformal coat-

ing, the surface of the PWA and the components must be

free of materials (water soluble, ionic contaminants, greasy,

oily or particulate), which might interfere with wetting, or

trap moisture; otherwise, mealing or vesication will occur

with subsequent corrosion and dendrite formation between

adjacent conductors.

N-4.3 Chemical Stress Design Considerations Some

coatings are not stable in hot, humid conditions and may

revert to a gel. Select reversion resistant polyurethanes.

Parylene coating will be attacked by oxygen and crack if

exposed for extended periods at temperatures above 125°C.

Silicones are attacked by some solvents; in addition, traces

of silicone may interfere with subsequent bonding and

painting operations. Acrylic conformal coatings are

removed by most cleaning solvents including alcohol. Do

not select acrylic coating where solvent resistance is

required.

N-4.4 Space Environment Design Considerations

Some conformal coatings outgas signiﬁcantly, making

them unsuitable for spacecraft. Fluorescent chemicals

added to the conformal coating outgas and may cause prob-

lems where optical clarity is paramount in systems with

lenses, mirrors and viewing ports.

N-4.5 Manufacturing Considerations There are several

key steps in applying conformal coating, the most impor-

tant being the cleanliness. An ionograph, although useful

for process control, is not sufficiently sensitive to detect a

level of ionic contamination which will cause vesication

(blistering). Non-ionic contaminates, such as silicone, will

interfere with adhesion of epoxy and polyurethane confor-

mal coatings. The cleaning process should include cleaning

with both polar and non-polar solvents. Other process con-

trols should include proper mixing, application and curing

of the coating.

The conformal coating may contain solvents which affect

the adhesion or integrity of component and printed board

markings, labels and legends.

Conformal coating of PWAs fabricated on ﬂuorinated plas-

tics will require pre-treatment to improve adherence to the

substrate. The coating will increase the effective dielectric

constant between surface conductors (reduce high fre-

quency performance).

N-4.6 Other Design Considerations Conformal coating

on test pads results in diminished test accessibility; testabil-

ity buss methodologies and structures may be required to

permit effective and efficient fault coverage. (See IPC-SM-

782.)

Reduced heat extraction from the PWA (and increased

junction temperatures) may result if conformal coating cov-

ers heat sinks such as card edge clamps and cold plates.

Where solder coated conductor surfaces are overcoated

with rigid CC, the solder melts or reﬂows during subse-

quent processing (particularly hot air solder leveling or

IPC-D-279 July 1996

122

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.

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