IPC-CM-770D-1996 - 第157页

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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IPC-CM-770

29.3.4 Toxic Solvents

Coatings may contain toxic sol-

vents and should be used with caution and in well venti-

lated areas. Supplies should include a “Warning” on the

label of containers of material which may have a toxic

effect on using personnel.

29.3.5 Clearances

Care must be exercised when coating

assemblies with flat base components with leads projecting

straight through the board (relays, crystal cans). These

components must be mounted sufficiently high off the sur-

face of the board to prevent the coating from completely

filling the area between the component base and board sur-

face.

29.4 Coating Techniques

29.4.1 Cleaning

All surfaces to be coated must be thor-

oughly cleaned prior to coating to ensure adequate adhe-

sion, minimize corrosion, and optimize electrical

properties.

29.4.2 Baking

Immediately prior to coating, the parts

should be baked to remove all traces of solvents. Optimally

this bake-out should be performed at a minimum of 65°C

for two hours and under a vacuum of

450

mm of mercury.

Difficulties in coating adhesion have been observed when

the bake-out is performed at below 65°C for

2

hours. When

higher bake temperatures are used, board material and

component manufacturing data should be considered.

29.4.3 Application

With the exception of paraxylylene,

which is applied by vacuum deposition, all of these coat-

ings can be applied by dipping, brushing, or spraying.

Spraying or vacuum deposition are the preferred system

where thin coatings are required. With reasonable operator

skill an even, thin coating can be obtained, without runs or

drips. With spraying, masking is easy, and can be carried

out by taping or sleeving the required components or areas.

Masking for paraxylylene requires more care. If quantities

make it economical, tooling can be made to protect those

parts of the assembly that are not required to be coated.

29.4.3.1 Brushing

Brushing is a simple way of coating

numbers of boards. It is difficult to obtain thin, uniform

coatings with this method, and coverage of densely packed

boards is not easy without getting blobs and runs. Care

must be taken to see that no bare spots are left. Masking is

not usually required with this method of coating, if the

operator exercises care in applying the coating.

29.4.3.2 Dipping

Dipping gives good coverage, and

ensures that all cracks and crevices are adequately coated.

Masking with the dipping method is more difficult than that

required for the brush method and is less critical than for

vacuum deposition methods. Care must be exercised during

masking or it is not likely to be fully effective.

If the appropriate viscosity of the coating material is cho-

sen, dipping can produce good results, but very thin films

are not obtainable, and some runs are almost inevitable.

Build-up of coatings in crevices and around components is

permitted, but every effort should be made to keep this to

a minimum. Suitable means of masking should be provided

to prevent intrusion of coating resin into adjustable compo-

nents, connections or other areas as specified.

29.4.3.3 Spraying

Spraying provides a quick method for

coating. Spraying requires good masking and control of

spray conditions to obtain consistent results.

29.4.4 Masking

Tapes, latex rubber, strippable polyeth-

ylenes or vinyl masks may be used to mask required areas.

If masking can be performed prior to cleaning (if masks are

compatible with cleaning operations) it is advisable, as

handling of the cleaned part should be minimized. Masking

material must be compatible with solvents in the coating

system and must not inhibit or interfere with the cure.

29.4.5 Handling

Care must be exercised in handling

cleaned assemblies to prevent recontamination. Lint free

gloves are recommended for this purpose.

29.5 Quality Assurance

29.5.1 Solder Joint Cracking

Rigid conformal coatings

under a flat component may contribute to solder cracking

of the joints of that component by virtue of the expansion

of the coating. For this reason flat components should be

mounted off the board, and avoid filling the gap with con-

formal coating.

29.5.2 Mealing

Problems associated with conformal

coatings are invariably caused by inadequate cleaning prior

to coating. One example of this is “mealing,” which shows

as small white spots under the coating. The white spots are

areas where the coating was bonded and; subsequently,

delaminated from the substrate or failed to bond to the

substrate.

Mealing is a result of a containment or contaminants on the

board surface causing loss of adhesion when exposed to

high humidity conditions. The mechanism for adhesion has

been theorized as desorption of the containment at the

interface by moisture in humidity testing or a result of

osmotic pressure build-up as the contaminants goes into

solution in moisture which is transmitted and absorbed by

the coating and the board itself. It is also a result of the use

of a coating which has moisture vapor transmission rate

(MVTR) about equal to or less than the printed board

material allowing interface pressure build-up. This may

explain why coating evaluations have shown that changing

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IPC-CM-770

Januaty

1996

the coating material to one of higher MVTR, eliminated or

minimized the physical problem of mealing or white spots.

For contaminated boards it is not a solution, only a mask.

Mealing is caused by residues from chemical processing as

follows:

Use of copper etching solutions which are not free rins-

ing, especially those which result in insoluble precipitates

and copper complexes on dilution. Also those solutions

which do not readily etch copper foil treated surface.

Use of immersion tin plating solutions which result in

insoluble precipitates on dilution with H20.

Use of improper and inadequate post rinse operations and

cleaning and neutralizing baths.

Flux residue from reflow solder, flow solder, and solder

touchup, not removed in flux cleaning operation.

Poor quality rinse water used in processing.

Moisture or solvent not baked from board material.

Other chemical processing not covered above-solder

brighteners, protective film residues, etc.

Handling contaminations.

When curing is completed, the coating should have the fol-

lowing characteristics:

A smooth homogeneous appearance, free from bubbles,

pin holes, blistering, wrinkling, cracking and peeling.

Unless the coating is deliberately made opaque, it should

not mask the colors or identification marks on compo-

nents or the base laminate.

The coating should not discolor or damage any of the

components or other items coated.

29.5.3 Appearance

When completely cured the coating

should be free of bubbles, blisters, cracks, other defects. It

should not contain dust, hairs, or any other contaminants.

Unless the coating is deliberately made opaque, it should

not obscure the colors and markings of the components.

29.5.4 Rework

The specific coating to be removed may

have one or more of the characteristics defined in 29.2, and

consequently the removal method selected should consider

the composite characteristics. (See IPC-R-700.)

30.0 QUALITY ASSURANCE TESTING

The goal of a quality assurance strategy is to prevent non-

conformances. A high number of nonconformances means

low yields, wasted materials, labor, and capacity, high

probably of shipping nonconforming products (i.e., greater

opportunity for defect screening efforts to fail), high

inspection cost, high rework cost and higher than necessary

cost over the long run. A quality assurance strategy consists

of the following elements:

A. Nonconformance Prevention.

Nonconformances can be prevented by:

Assuring process, material, and design compatibility.

Designs should demand as little as possible from the

materials and process to achieve end item form, fit and

function requirements. Conversely, process and material

capabilities should match the demands of the end item

requirements.

Raw material control

Raw material suppliers must

understand customer requirements and use the defect

prevention systems to insure that nonconforming prod-

uct is not shipped. The customer has a responsibility to

understand the capabilities and performance of raw

material vendors.

Process control and definition

Processes must be con-

trolled and predictable. Statistical process control can be

used to monitor process performance (irregularities,

degradation). Elements of the process which require

operator or technician input must be documented

clearly. Periodic auditing is a tool to ensure that ele-

ments of the process that require ongoing attention do

not fail.

B.

Nonconformance Detection

This is a necessary nonva-

lue added cost that can be minimized by

(1)

preventing as

many defects as possible in the first place, (2) clear under-

standable definition of reject criteria,

(3)

judicious use of

statistical sampling plans, and

(4)

efficient and credible

inspection techniques.

C. Nonconformance Monitoring

Scrap and use as is parts

must be monitored and Pareto analyzed to understand

causes and trends.

D. Corrective Action

Corrective action priorities must be

based on statistical process control and nonconformance

monitoring data. Corrective action must be aimed at root

causes of problems (e.g., Is the shop process specification

correct?; Is the design as producible as possible?; Do the

raw materials conform to requirements?) Corrective actions

must be implemented through design, process, manufactur-

ing, andor vendors.

Quality assurance is an ongoing procedure which should

begin as materials and components are accepted on deliv-

ery. Consequently, certain quality assurance tests and

inspections relating to assembly processes should be com-

pleted before any components are placed or soldered.

These include solderability, surface cleanliness and coating

quality.

Following assembly, product should be inspected for

defects and other workmanship features and then verified

against the standard agreed upon between customer and

vendor. Functional and quality tests, both mechanical and

electrical, should be a part of this process. The “as pro-

duced” quality of a product will not be maintained unless

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