IPC-CM-770D-1996 - 第155页
January 1996 IPC-CM-770 The cleanliness tests of J-STD-001 should be used to vali- date the cleaning process chosen and should be performed at a frequency that ensures compliance. 29.0 CONFORMAL COATING-RELATED CONSIDERA…
IPC-CM-770
Januaty
1996
Care should be taken in clinching leads to ensure that the
stress-relief bends are not reduced by straightening.
Parts which generate heat, if mounted directly on the
board, can cause locally severe thermal coefficient mis-
matches and result in cracking.
28.0 CLEANING-RELATED CONSIDERATIONS
In the case where printed board assemblies are to be con-
formally coated, the assembly should be free of flux resi-
dues and other contaminants prior to the application of
conformal coating.
The cleaning agent(s) used for the removal of grease, oil,
wax, dirt, flux and other debris, should be selected for its
ability to remove flux residue, ionic, ionizable, nonpolar
and particulate contaminates. The cleaning agent should
not degrade the material and parts being cleaned. (See IPC-
SC-60 and IPC-AC-62.)
28.1 General Considerations
28.1.1 Electromigration
Metallic growth or conductive
contaminants connecting any conductors causes problems
ranging from micro-amp current leakage to electrical dead
shorts. It is difficult to detect and in many cases impossible
to see this contaminant. Metallic growth usually forms
after assembly and/or coating, since growth is fostered with
either time, moisture, applied voltage, or all three.
A more elaborate cleaning procedure may be desired for
extremely high reliability assemblies exposed to severe
environmental conditions in order to reduce the risk of
electromigration. A thorough cleaning is important, there-
fore, for this purpose.
28.1.2 Surface Mounting
Surface mount component
assemblies create unique characteristics, which should be
taken into consideration for cleaning. Vapor phase solder-
ing, with its low temperatures and high speeds, greatly
reduces polymerization and charing of flux rosins, and
immediate transfer into a vapor degreaser may be
sufficient.
If
flux cannot be removed from beneath surface mounted
components, it may pose a potential threat to reliability.
Some solder paste flux systems have been found to leave
residues that are hard to clean. It is important that the
cleaning process used be capable of removing all the flux
residues from the solder paste used in the assembly
process.
28.2 Pre-Cleaning
Printed boards and components leads
to be soldered may need to be cleaned prior to assembly
and/or soldering to improve solderability.
If
required, sur-
faces to be soldered should be cleaned as follows:
Grease, oil, and other foreign matter should be removed
from conductors and terminals by using suitable cleaning
solution. Cleaners should not remove markings or dam-
age the part in any way.
Oxides and varnishes should be removed by methods
which do not damage leads or parts, and which do not
cause contamination or hinder solder wetting.
Sand blasting should not be used.
Dust or other loose matter should be removed.
28.3 Post-Soldering Cleaning
(See IPC-SC-60 and IPC-
AC-62) When required, flux residue should be removed as
soon as possible, but not later than one hour after soldering
by applying cleaning agents. Some fluxes may require
more immediate action to facilitate adequate removal. Flux
used in the process of soldering is divided into three basic
types. The type characterization is related to factors based
on the corrosive or conductive properties of the flux or flux
residue. The three basic types are as follows:
Low or no flux/flux residue activity.
Moderate flux/flux residue activity.
High flux/flux residue activity.
Mechanical means such as agitation, spraying, brushing,
etc., or vapor degreasing and other methods of application
may be used in conjunction with the cleaning medium.
Ultrasonic cleaning may damage certain parts. Therefore,
tests should be conducted to determine the applicability of
the process.
The post soldering cleaning procedure should be as defined
in
J-STD-001,
depending upon end product requirements
and fluxes used as follows:
A rough cleaning step for the removal of most flux resi-
dues (ionic and non-ionic).
A rough cleaning step for the removal of most flux resi-
dues (ionic and non-ionic) followed by a fine cleaning
step for the removal of the remaining flux residues (ionic
and nonionic).
A rough cleaning step for the removal of most flux resi-
dues (ionic and non-ionic) followed by a fine cleaning
step for the removal of remaining flux residues (ionic and
non-ionic), then followed by a final cleaning step that
includes a solvent or solution removal operation for the
removal of final traces of contamination.
Because of generally smaller spacing between leads,
smaller clearances between the substrate and the compo-
nent body and large area beneath the devices, chip carriers
present a more difficult cleaning situation than through-
hole mounted devices. Clearance under the package should
be adequate to facilitate effective cleaning operation.
28.4 Quality Assurance
Proper storage and handling
will greatly reduce the probability of problems. To main-
tain cleanliness, assemblies should be stored in air tight
packages in a clean, moderate environment.
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COPYRIGHT Association Connecting Electronics Industries
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COPYRIGHT Association Connecting Electronics Industries
Licensed by Information Handling Services
January
1996
IPC-CM-770
The cleanliness tests of
J-STD-001
should be used to vali-
date the cleaning process chosen and should be performed
at a frequency that ensures compliance.
29.0 CONFORMAL COATING-RELATED
CONSIDERATIONS
A conformal coating is a thin layer of insulating material
which is applied to a printed board assembly. This material
follows closely the contours of the board and components.
It “conforms” to the shape of the assembly, and ideally
will produce a film of consistent thickness over the entire
assembled printed board. Assembled printed boards are fre-
quently given a conformal coating to assist them in func-
tioning under certain environmental conditions.
29.1 General Considerations
Correctly chosen, and
carefully applied, conformal coating will help to protect the
assembly from the following hazards:
Humidity
Dust and dirt
Airborne contaminants-.g., smoke, chemical vapors
Conducting particles-.g., metal chip, filing
Accidental short circuit by dropped tools, fasteners, etc.
Abrasion damage
Vibration and shock (to a certain extent)
Conformal coating is not a substitute for good design, or
the selection of adequate components and materials. It
does, however, assist the designer in producing equipment
which will live under hostile conditions. The conformal
coating should be compatible with any soldermask used on
the assembly. Conformal coatings protect the electrical
characteristics of the assembly by doing the following:
Preventing contamination of the dielectric surface by field
soil, which in humid environments can cause electrical
leakage.
Inhibiting the growth of fungus, thereby protecting the
electrical characteristics. Even non-nutrient surfaces can
support fungus growth when contaminated with field soils
such as oil vapor.
Suppressing electrical flashover between conductors at
high altitudes.
The secondary function of conformal coating is to help
support the components
so
that the entire mass of the com-
ponent is not carried by the solder joints.
Properties to be considered in quality test and/or
evaluations:
Appearance
Thickness
Fungus resistance
Adhesion
Shelf life
Pot life
Abrasion resistance
Solvent resistance
Flammability
Dielectric withstanding voltage
Moisture resistance
Thermal shock resistance
Thermal humidity aging
Fluorescence
Resonance
For additional information on conformal coating see IPC-
CC-830.
29.1.1 Selection Criteria
Conformal coating resins are
selected to fulfill the above requirements listed for adhe-
sives and protective coatings plus several other minor ones
such as transparency (to permit reading component values
after coating) and flexibility (to prevent damage to compo-
nents in temperature cycling). However, certain limitations
are inherent in conformal coatings:
Since they are permeable to water vapor and are not for-
mulated with corrosion inhibitors such as chromates, they
will not prevent corrosion caused by active electrolytic
salts on the part being coated or salts trapped under the
coating on the surface of the part.
Since they are permeable to water, their insulation resis-
tance decreases as the thickness of the film increases,
particularly in a fillet of resin around a component (such
as in integrated circuit).
Since coatings are organic and fill the voids between con-
ductors, they cause a marked change in interlead capaci-
tance.
Coatings have a high coefficient of thermal expansion,
so
they can exert a lifting force certain components, causing
solder joint on failure.
Coatings do not exhibit exceptional adhesion to metals,
particularly solder.
Paraxylylene coatings excepted, most conformal coating
resins are similar to organic finishes and will exhibit pin-
holing and thin spots on sharp points, edges of parts and
conductor edges.
The above limitations can be overcome through careful
design of the assemblies and care in the application
processes.
29.2 Materials
The conformal coating should be suffi-
ciently flexible to permit bending without cracking or craz-
ing at design temperatures.
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IPC-CM-770 Januaty 1996
Conformal coating should completely cover the surfaces
of
the assembly, including soldered joints, wires, and compo-
nents, other than those areas and components deliberately
masked. Typical thicknesses of the cured conformal coating
are specified in Table 29-1.
Table 29-1 Coating Thickness
Coating Recommended Thickness
Type
AR
....................................................
0.002 +0.001 inch
Type
UR .......................
0.05 +0.025mm (0.002 +0.001 inch)
Type
ER
.......................
0.05 +0.025mm (0.002 +0.001 inch)
Type FC
.......................
0.05 +0.025mm (0.002 +0.001 inch)
Type
SR .....................
0.05 TO 2 mm (0.002 TO
0.008
inch)
Type
XY
................
0.015
to
0.05 mm (0.0006
to
0.002 inch)
The following coatings are typical of those that are accept-
able for use with printed board assemblies.
29.2.1 Oleo Resin Varnish
(Temperature range -55°C to
+125”C.) A general purpose coating for nondemanding
conditions. Easy to apply, can be removed by readily avail-
able solvents, such as xylene or toluene. Easily repairable,
good appearance, etc.
29.2.2 Acrylic Varnishes (Type AR)
(Temperature range
-60°C to +135”C.) A general purpose coating for use where
excellent electrical properties are required. Thin coatings
can be “soldered through.” Low solvent resistance, can be
removed with solvents such as methylethylketone (MEK),
toluene, etc. Easily repairable, good shiny appearance.
29.2.3 Epoxy Coatings (Type ER)
(Temperature range
-60°C to +200”C.) A general purpose coating for use where
the best electrical properties are required. Thin coatings can
be “soldered through,” otherwise coating must be
mechanically removed. Removal is difficult. Can be
patched, has a good appearance, is more difficult to apply.
Not recommended for application on assemblies with glass
components. Possesses high solvent resistance.
29.2.4 Polyurethane Varnish (Type
UR)
(Temperature
range -55°C to +125”C.) A good coating where resistance
to moisture and abrasion is required. Usually specified for
military applications. Thin coatings can be “soldered
through,” otherwise coating must be mechanically
removed. Can be patched, has a somewhat dull appearance,
is more difficult to apply. Moderate solvent resistance.
29.2.5 Silicone Varnish (Type
SR)
(Temperature range
-55°C to +260”C.) A good coating where good dielectric
and arc resistance properties are required. Also of value
where higher temperature operation is necessary. Can be
patched, good appearance. Easy to apply. Moderate solvent
resistance.
29.2.6 Silicone Rubber Coatings (Type
SR)
(Tempera-
ture range -55°C to +260”C.) A good high temperature
coating, with good abrasive properties. Primer required for
optimum adhesion. Flexible, transparent, moderately diffi-
cult to remove. May be mechanically removed or soaked in
an appropriate solvent to aid in removal. Good appearance,
not difficult to apply.
29.2.7 Paraxylylene (Type
XY)
(Temperature range
-65°C to +145”C.) Vacuum deposited polymers which pro-
vide excellent protection against humidity and abrasion.
Being deposited from a vapor they are true conformal coat-
ings, penetrating all crevices, and coating all surfaces with
a layer of constant thickness. Can be deposited in very thin
films. Cannot be replaced using conventional techniques.
Application requires specialized equipment. High solvent
resistance. Paraxylylene resins are covered by U.S. patents.
29.2.8 Polystyrene
(Temperature range -55°C to
+85”C.) Polystyrene coatings are slightly permeable to
moisture vapor, but possess the best dielectric properties of
the common conformal coatings. Suitable for use where
optimum dielectric properties are required.
29.3 Caution Notes
29.3.1 Chlorinated Solvents
Trichloroethylene and
some other chlorinated solvents should not be used on
glass-silicone resin laminated material, per Specification
MIL-P997. Delamination and surfacekomponent damage
may result with their use.
29.3.2 Glass Diodes
When applying epoxy, polyure-
thane varnish, or polystyrene coatings in thicknesses
greater than 0.003 inch to boards containing glass diodes,
the glass diodes should be fitted with soft vinyl sleevings
prior to application of the coating.
When thick layers of rigid conformal coatings are used
over brittle components, the differential expansion of coat-
ing, component, and board can cause cracking of the com-
ponent or solder joint. For this reason it is necessary to
keep the coating thin.
If
there is any doubt as to the possi-
bility of component damage, a buffer coating of pliant
material can be used on the brittle components. (This is not
usually the case for polyurethane, silicone, and thermoplas-
tic acrylic coatings.) A suitable material is heat shrinkable
sleeving, polyethylene terephthalate.
29.3.3 Shelf Life
Polyurethanes may have a minimum
shelf life of 3 months; epoxies, silicones, and polystyrene
may have a minimum shelf life of 6 months and acrylic
varnishes may have a minimum shelf life of 12 months
when stored in original unopened containers in accordance
with manufacturers’ recommendations. Date of manufac-
ture should be marked on the container.
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