IPC-D-279 EN.pdf - 第132页

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 a…

the PWA. Stenciling of solder paste is improved with

a ‘‘pads-only’’ approach.

• Application of solder mask to ﬂexible and ﬂex-rigid

PWAs may be difficult.

• Resolution of the ‘‘pads-only’’ approach is the same as

the resolution of copper etching.

The solder mask material system must be compatible with

the soldering process and its materials (temperature, dura-

tion of exposure, ﬂux, cleaning solvents); exposure to heat

and chemicals changes the reactivity and morphology of

the surface and can lead to adsorbed ﬂux residues and to

degraded SIR. Some laminates such as polyimide (and

some laminate reinforcement materials such as aramides)

absorb sufficient atmospheric moisture that the printed

board should be thoroughly dried prior to reﬂow; otherwise

delamination between the solder mask and the printed

board (or between the reinforcement and the resin) may

occur.

The solder mask material system must be compatible with

the other assembly processes such as marking, bonding and

component rework/repair (re-soldering) processes (tem-

perature, duration of exposure, ﬂux, cleaning solvents).

High printed board temperatures after exposure to moist

atmospheres may result in delamination at the solder mask-

printed board interface. High printed board temperatures

for long durations can result in thermal degradation of the

solder mask as well as measling of the base laminate. (See

IPC-R-700.)

Solder mask may be dissolved or degraded by the solvent

or material system in the conformal coating. Problems may

be avoided by informing each intended supplier of the sol-

der mask-conformal coating materials expected to be used

and of the service environment for which protection is

required before the material systems of the product have

been decided.

Open or untented PTHs and PTVs (no solder mask on

either side of the printed board) can allow liquid ﬂux to be

trapped with potential for corrosion, reduced SIR, contami-

nated test ﬁxtures and causing electrochemical corrosion.

(See IPC-D-275.) If solder mask is intended to plug or tent

these holes, it must do it consistently. Another method to

prevent ﬂux from being trapped in these vias is to plug

them with solder (which wave soldering does automati-

cally).

Solder mask overlap onto the land pattern (whether by

design or by loss of process control) resulting in solder

joint area reduction and reduced solder joint reliability.

(See IPC-D-275, IPC-SM-782, and IPC-SM-785.)

Solder mask overlap onto or residue on test lands (whether

by design or by loss of process control) reduces test reli-

ability. (See IPC-D-275 and IPC-SM-782.)

Solder masks are not recommended for application over

solder coated conductor surfaces. The solder melts or

reﬂows during subsequent processing (particularly hot air

solder leveling or wave soldering); delamination at the sol-

der mask/metal interface may allow contaminants to

become entrapped. If solder mask is required over solder,

cross-hatching is recommended for those conductors which

exceed speciﬁc dimensions.

The robustness of solder masks’ ability to minimize copper

corrosion in the presence of such chemicals as the polygly-

cols appears to depend upon material system, DC voltage

level and user process. Polyglycols are found in ﬂuxes used

with aqueous cleaning processes and in fusing ﬂuids used

with hot air solder leveling. Some solder masks may

exhibit cracks after exposure to low temperatures.

Design Requirements and Considerations Proper solder

mask design is essential to the construction of a reliable

surface mount assembly. The ideal solder mask design cov-

ers all the circuits and leaves all lands and surface mount

pads completely free to facilitate the formation of solder

joints. Solder mask design must evolve from interaction

between the board designers, printed board fabricators and

assembly manufacturers. Good solder mask design results

from the deﬁnition of clear ﬁnal product requirements

based upon the desired level of reliability and the impact of

board fabrication and assembly capabilities. Once these

issues are addressed, a concise set of design rules can be

formulated and tested.

N-2.1 Types of Solder Masks There is a broad range of

available solder masks, most of which can be categorized

into three types: liquid screenprinted, dry ﬁlm and liquid

photoimageable. Each type uses a different method of

application and produces a unique combination of advan-

tages and disadvantages.

N-2.1.1 Liquid Screenprinted Solder Mask Liquid sol-

der mask is applied by screen printing through a mesh con-

taining a blocking emulsion in areas where the material is

not required on the ﬁnished board. This is the original type

of solder mask and provides the lowest cost alternative.

Thickness of this type of mask is a direct result of the

screen emulsion thickness and can typically reach up to 50

µm. While tenting of holes with this solder mask is not

possible, vias can be reliably plugged for holes of up to 1.1

mm with a properly designed screen printing process. Due

to registration limitations and bleedout, liquid screen-

printed solder mask is usually not acceptable for high den-

sity surface mount applications.

N-2.1.2 Dry Film Dry ﬁlm solder mask starts as a pliable

sheet of photosensitive material of a speciﬁed thickness

(commonly ranging from 75 to 100 µm). By using a com-

bination of heat and vacuum, the lamination process

ensures encapsulation of board circuitry. Subsequent pro-

cessing through exposure to UV light and development to

IPC-D-279 July 1996

120

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

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