IPC-D-279 EN.pdf - 第129页

The solder joint quality is dependent on the volume of sol- der present. T ypically , a stencil or screen will be used to deposit the proper volume of solder paste. While the per- formance range of the various SMT proces…

of the solder. The conditions require that the termina-

tion ﬁnish and the lands be solderable, the solder

paste/ﬂux is sufficiently active with no moisture in the

paste during reﬂow, and the reﬂow conditions be well

speciﬁed and executed.

• The large number of small, narrow, closely spaced

leads on a given component does not allow the evalu-

ation or measurement of the component lead solder-

ability; often, the production reﬂow operation demon-

strates the marginal solderability of the termination

ﬁnish on one (1) lead of one component with 68 leads.

Similarly, the large number of very small pads on a

given printed board does not allow the evaluation of

the solderability of the printed board. Some PWAs fail

because of one (1) failed joint out of several thousand.

Confounded with component termination solderability

is the preferential wetting of the termination by the

ﬂux and molten solder when soldering temperature is

achieved by the termination before the land.

• Repeated HAL or HASL (hot air solder leveling) pro-

cessing of the printed board can result in exposed

copper-tin IMC which rapidly oxidizes and has

extremely poor solderability.

• The move to ‘‘low residue,’’ ‘‘leave on’’ or ‘‘no-clean’’

ﬂux technologies results in ﬂuxes which are milder

and less able to penetrate oxidized termination and

land ﬁnishes.

• The effectiveness of the solder paste is sensitive to

many factors such as mean solder particle size, particle

size distribution (particularly the ﬁnes content), oxide

content of the solder particles, moisture content of the

ﬂux vehicle (as manufactured or after any subsequent

exposure to moist air). The SM process, if not properly

controlled, can exacerbate the degradation of the paste

through such process deviations as excessive drying

(loss of tackiness) prior to placement of SM compo-

nents, re-use of paste from day to day, or opening of

the paste jar without proper warming of the contents to

room temperature. Insufficient preheating prior to

reﬂow can result in inadequate activation of the ﬂux

while excessive or prolonged preheating prior to

reﬂow can result in oxidation of the solder metal par-

ticles and exhaustion of the ﬂux. Many of these factors

contribute to solder balling.

Solder balls are reported to result in short circuits by

bridging component pads of an MLCC connected to

Vcc and Gnd, resulting in a charred printed board.

Solder balls can be the result of improper handling and

processing of the solder paste or the injection of sol-

der through a via or PTH. The one or more solder

joints on the PWA may lack the solder in the balls and

solder joint reliability may be compromised.

• Some joints may not achieve the required temperature

for correct solder reﬂow under infrared (IR) conditions

because of the differences in thermal mass of compo-

nents (such as high pin count connectors or sockets

and those PGAs with affixed heatsinks), thermal shad-

owing effects from overhanging portions of taller com-

ponents and the component leads termination ﬁnish;

similarly, to avoid the heat sinking effects of ground

and power planes, component lead connections to

power and ground planes should be made using ther-

mal relief (isolated) pads connected to the larger

planes by thin conductors. Conductors to ﬁne pitch

land patterns also have solder thieving and heat sink-

ing effects during the reﬂow process.

• Formation of excessive brittle copper-tin IMC at tem-

peratures encountered in SM processes, including

rework/repair degrades the ability of the solder joint to

withstand temperature cycles. Photographs of these

IMCs are in Solder Joint Reliability and in Metallurgy

of Solder Joints in Electronics; Scanning Electron

Microscope (SEM) photographs of solder joints are in

Soldering in SMT Technology.

• Solder joints are subjected to thermo-mechanical stress

during cool down after mass reﬂow soldering where

the entire printed board is heated; this effect is exacer-

bated where the design includes components which are

long, stiff, or possess a low coefficient of thermal

expansion (CTE). Examples of these components

include RFI fences, ground bars, power/ground/ﬁlter

distribution laminates, and board stiffeners. Variation

of the time at temperature during reﬂow will be

reﬂected in the incidence rate of failed joints due to the

stress arising from cool-down. The cool-down phase of

the process can cause warping of printed boards or can

ﬂex warped boards such that solder joints and compo-

nents are stressed and broken; stresses may be reduced

if stress relief bends and curves are designed into the

part. A long, heavily ﬁlled SM connector which is not

mechanically restrained and in which the contact

inserts are captured may fall into this category. Hot bar

reﬂow at AT&T of 600 pin connectors, laser soldering,

single point TAB bonding and scanned light energy

sources have been used to address the issue of heating

the entire printed board.

Note that a 4-sided ‘‘box’’ of rigid components such as

a complete RFI fence will be robust with respect to

mechanical ﬂexure but will result in a ‘‘z’’ axis flexure

in the center of the ‘‘box’’ during SM reﬂow and sub-

sequent thermo-mechanical solder joint stress upon

cool down.

The hard, brittle IMCs embedded in the much softer solder

matrix require changes to the metallurgical sawing/

polishing/etching techniques normally used for homoge-

neous materials.

July 1996 IPC-D-279

117

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

118

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

July 1996 IPC-D-279

119