IPC-D-279 EN.pdf - 第130页

Appendix N Adhesives, Solder Mask and Conformal/Other Coatings N-1.0 ADHESIVES Moisture and Adhesion The molding compounds used to encapsulate SM electronic components are reported to decrease slightly in the value of Tg…

The solder joint quality is dependent on the volume of sol-

der present. Typically, a stencil or screen will be used to

deposit the proper volume of solder paste. While the per-

formance range of the various SMT processes vary, studies

have shown that over 63% of defects identiﬁed after reflow

originate during the solder paste screening (or stenciling)

step. Correct solder paste volume is controlled by the sten-

cil or screen emulsion) thickness, aperture opening size,

solder mask height and stencil process parameters (such as

squeegee hardness and pressure).

While either stencils or screens may be used for surface

mount leaded parts of 1.2 mm pitch and greater, stencils

are preferable for ﬁne-pitch work. Although stencils cost

more and require longer lead times for construction, they

clog less frequently, and provide longer wear life and a

greater degree of control.

The challenge for the screen or stencil designer is to pro-

vide for the correct volume of solder paste to the corre-

sponding land area while preventing bridges caused by

excessively wide apertures. Actual aperture sizes and sten-

cil thicknesses must be determined from solder volume

calculations or experimentally to suit the paste type, board,

tinning thickness and inspection expectations.

If the surface mount assembly contains a mix of various

pitch packages on the same board, it will be necessary to

engineer a stencil which can deposit various amounts of

solder without compromising the integrity of any compo-

nent’s attachments. There are at least four alternatives in

this situation:

(1) step-down stencils which have a thinner foil thick-

ness in the ﬁne-pitch land areas

(2) stencils which have the land apertures reduced in

only the ﬁne-pitch areas

(3) modiﬁed land stencils which have aperture openings

on alternating ends of the ﬁne-pitch lands and

(4) stencils which use fancy shaped apertures (such as

tear-drops, triangles, etc.).

The option chosen must be based upon design, as well as

manufacturing considerations.

IPC-D-279 July 1996

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Appendix N

Adhesives, Solder Mask and Conformal/Other Coatings

N-1.0 ADHESIVES

Moisture and Adhesion

The molding compounds used to

encapsulate SM electronic components are reported to

decrease slightly in the value of Tg and to lose adhesion to

the other materials in the assembly with increased moisture

weight gain. Older anhydride cured epoxies suffered from

‘‘reversion’’ or the chemisorption of water and the subse-

quent conversion of epoxide ring to carboxylic acid.

N-1.1 Electrically Conductive Attachment Materials

Permanent interconnections include metal and carbon par-

ticles in thermosetting adhesive or thermoplastic adhesive

matrices. The metals include silver, nickel, silver-plated

nickel, copper, silver-plated copper, gold, silver-plated

glass spheres. The thermosetting adhesives include epoxy,

polyimide and bismaleimide resin systems. The thermo-

plastic adhesives include acrylics.

Electrically conductive attachment materials include epoxy,

polyimide and bismaleimide polymers containing metal

particles of silver, gold, nickel, copper, silver-plated nickel,

silver-plated copper, and silver-plated glass spheres. Some

of these systems are 100% solids and require only heat to

cure. Others contain some solvents to reduce the viscosity

and require a drying phase prior to cure. These materials

may also be used as thermal conductors, if electrical isola-

tion is not required.

A reliability concern with conductive adhesives is the loss

of conductivity at the interface between a cured rigid filled

epoxy and a reﬂowable metal termination ﬁnish such as

tin-lead solder when the assembly is exposed to tempera-

tures approaching the melting temperature, Tm, of the sol-

der. The movement of the solder away from the rigid epoxy

adhesion interface can lead to an increased electrical resis-

tance.

A moisture-related concern is the loss of electrical conduc-

tivity at the interface between a metal ﬁlled epoxy and a

metal termination ﬁnish other than of silver or gold; tin,

lead, and nickel oxides formed as a result of moisture per-

meating the epoxy are not highly conductive and lead to an

increase in interfacial electrical resistance with time.

N-1.2 Thermally Conductive Adhesives Thermally con-

ductive attachment materials include epoxy polymers con-

taining such ﬁllers as alumina, cubic boron nitride, and

zinc oxide. The function of this class of materials is to ﬁll

the void or space between the power dissipating component

and the heat dissipater. Thermally conductive materials

which are not attachment materials include lands, tapes and

stamped shapes of elastomeric materials either ﬁlled with

alumina, cubic boron nitride, or zinc oxide or laminated to

aluminum or copper ﬁlms; these forms require that

mechanical pressure be applied between the component

and the heat dissipater.

N-1.3 Mechanical Attachment Adhesives Mechanical

attachment adhesives include SM adhesives intended to

secure the component during wave solder, or to secure the

component while it is hanging upside down on the sub-

strate through a reﬂow operation, or to secure such compo-

nents as crystals in service. These materials include very

thick resin or rosin ﬂux, and epoxy or acrylic polymers

(cured by UV, heat, or anaerobically) and hot melt glues.

N-2.0 SOLDER MASK

Solder mask is a thin polymer coating that is applied to the

surface of printed boards during fabrication. Due to its

nature, solder mask is often used to protect areas of the

printed board from environmental effects caused by dust,

moisture and contamination. The capability of a solder

mask to insulate and protect the assembled board is essen-

tial to reliability. The pertinent performance and qualiﬁca-

tion requirements for solder mask are deﬁned in IPC-SM-

840 which covers the range of mechanical, chemical and

electrical properties which a solder mask must possess.

However, a ‘‘pads-only’’ approach also achieves these

goals and has many other advantages that should be con-

sidered.

Solder masks provide a variety of functions when applied

to selective areas of the printed board. In addition to pro-

viding a thermal and electrical insulation layer, solder mask

prevents the formation of bridging during soldering pro-

cesses and reduces weight gain due to solder. Whether or

not solder mask is necessary for a particular design may be

based on many factors.

Many multilayer military and space applications use a

‘‘pads-only’’ outer layer design. By submerging all conduc-

tors and power planes in the innerlayers, only the land

areas are exposed on the board surface. Connection to the

sublayers is then accomplished by small plated and ﬁlled

vias inside the land area. If a ‘‘pads-only’’ approach is not

feasible, then it is crucial that solder mask be applied for

surface mount designs to act as a dam to solder migration.

‘‘Pads-only’’ approach has several advantages worth not-

ing:

• The ‘‘pads-only’’ construction is compatible with sol-

dering processes, conformal coatings and common

cleaning solvents.

• The electrical and dielectric performance of the pads-

only construction is no different than the remainder of

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119

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

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