IPC-CM-770D-1996 - 第145页

IPC-CM-770

Januaty

1996

with this geometry. Cleaning operations are aided by this

geometry since it provides clearance between the bottom

of

the component and the substrate. Excess solder fillets on

this geometry can stiffen the lead and reduce any compli-

ancy advantage.

27.1.2.3 LeadlTermination Finishes

Finishes on the

leads of leaded component packages and on the metallized

terminations of leadless packages, whether for through-

hole or surface mounting, preserve and assure the solder-

ability of the interconnection surfaces without decreasing

the assembly yield or the attachment reliability. The large

number of solder joints, which frequently cannot be visu-

ally inspected not only because of their sheer numbers but

also because they are physically inaccessible, makes it

imperative that the solderability of the component I/Os be

positively assured prior to assembly. The lead finish that

accomplishes this is fused or reflowed tin or solder. The

solder coating can be obtained either by solder dipping or

tidlead plating with a subsequent reflow process. It might

be necessary to remove excess solder, which could inter-

fere with the assembly or socketing process, with a hot air

knife or similar method.

The above conditions expressly exclude gold plating, since

gold forms brittle intermetallics with the tin in the solder

causing solder joint reliability problems, and silver plating

since silver forms a brittle silver intermetallics. Caution

should be used when leads are coated with tin or tidead

plating without reflowing prior to assembly because the

plating does not always assure solderability. Organics

which are sometimes Co-deposited during the plating pro-

cess can also cause poor solderability.

In addition, some user specifications require fusing of tin

plate to eliminate the possibility of tin whisker growth.

The foregoing considerations are more critical for surface

mounting than for through-hole mounting. Through-hole

mounting provides considerable margin because of the

mechanical anchoring of the leads and the larger solder

volumes.

For devices which are exclusively destined for non-

soldered interconnections (sockets, connectors), the surface

finish coatings depend on the design of the mating contact,

the number of insertion/withdrawal cycles and the service

environment.

It should be noted that the reliability of a tin/lead applica-

tion can be increased by lubricating the contact surfaces

with proper contact lubricants. Lubricants prevent fret and

corrosion that may occur on tidead due to changing envi-

ronmental conditions. A suggested source for additional

information of recommended finishes is IPC-GH-850

Handbook of Interconnection Contact Finishes.

27.1.3 Component Placement

The orientation of com-

ponents on the assembly can have adverse effects on the

final solder joint quality. See the placement descriptions for

each of the component types in Sections

1

through

8.

27.1.4 Assembly Sequence.

See the assembly sequence

information in Sections

19

through 25.

27.2 Solderability

A successful solder joint requires cer-

tain conditions. Primarily, the surface to be joined must be

solderable; this is known as solderability.

Good solderability can improve production rates, increase

reliability, lower costs and improve joint appearance. Pro-

duction rates increase significantly if board solder joints do

not have to be reworked or touched up. Furthermore

manual touch up can damage printed boards. Therefore,

production increases as a result of reduced printed circuits

board scrappage due to touchup operations. Reliability

increases and costs drop for this same reason. With regard

to cost, obtaining better solderability definitely increases

cost; however, in most cases this added cost is less than the

cost of bad joints and will therefore actually save money.

The solderability of both component leads and printed

boards must be high to obtain an easily and well made

joint. Degradation of either part will impede the formation

of good solder joints. The solderability of printed wiring

boards should meet the requirements of J-STD-O03 and the

components should meet the requirements of J-STD-002.

The emphasis in the control of solderability has been

placed on component leads rather than on printed boards,

because of the following characteristics of components:

Longer storage times

More rigorous processing during manufacture

Greater variety of base materials

Lower cost compared to cost of reworking the PWB

Considerable work on solderability control has been

applied to printed wiring boards. This includes both testing

for solderability and improving the surfaces to be soldered.

Specific information on solderability can be found in:

J-STD-001, “Requirements for Soldered Electrical and

Electronic Assemblies” J-STD-002, “Solderability Tests

for Component Leads, Terminations, Lugs, Terminals and

Wires” J-STD-003, “Soderability Tests for Printed

Boards”

27.2.1 TinninglSolder Dipping

Component leads not

meeting the designated solderability requirements may pos-

sibly be reworked by tinningholder dipping, prior to sol-

dering. All portions of the wires or leads which come in

contact with the area to be soldered should be tinned. The

solder should penetrate to the inner strands of stranded

conductors. To permit inspection for nicks or cuts at the

point of insulation termination, solder and wicking should

not conceal the individual outer wire strands.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

All gold plated leads and wires that are hand soldered or

surface mounted should be pretinned or solder dipped to

remove the gold prior to component soldering. Care should

be exercised to not adversely affect the component ele-

ments during this treatment.

27.2.1.1 Cleaning Prior to Tinning

Component leads

can be cleaned with a mechanical cleaning tool that does

not impart damage to the component or the component

lead. Other methods of cleaning can be used, provided that

these methods do not damage the component. Knives,

emery cloth, sandpaper, steel wool and other abrasive

should not be used.

27.2.2 Substrate Preparation

Printed wiring boards

should be tested for solderability before assembly.

If

sol-

derability is not acceptable, then pretreatment prior to

assembly may be required to enhance solderability and

subsequent quality of the soldered assembly. Solder mask-

ing may or may not be present dependent on specifications.

Typical pretreatments are degreasing (surface contamina-

tion, e.g., dirt, oils), brightening (chemical activation of

solder and/or copper), and baking (moisture). Baking of the

substrate may be necessary in order to prevent delamina-

tion of the substrate.

27.3 Materials

All soldering processes are only capable

of achieving optimum process yields if the materials used

in those processes are not substandard. Regardless of the

particular soldering process used (dip, wave, reflow), all

solder processes follow the same basic steps: flux, preheat,

and soldering. Some newer technologies use other materi-

als such as adhesives either in conjunction with or to

replace soldering. Adhesive attachment of components is

particularly attractive with temperature sensitive devices,

and for securing surface mount devices. The various types

of alternative solder materials will be discussed here.

27.3.1 Flux

Flux must have properties such that it

(1)

chemically removes the surface oxide or tarnish and

(2)

keeps the surface clean until the solder has melted and

flowed over the fluxed surface.

Soldering fluxes have been divided into three general cat-

egories. The traditional flux specifications classify fluxes

on the basis of their chemical make-up or flux base (rosin

base fluxes, for example, are classified as R, RMA or

RA).

J-STD-004 utilizes a unified approach to flux classification

based on fundamental, intrinsic corrosive and conductive

properties of flux and flux residues, rather than specifying

the flux base.

Flux are specified according to one of the following three

types per J-STD-004:

L

=

Low or no flux/flux residue activity

M= Moderate flux/flux residue activity

H= High flux/flux residue activity

Inorganic fluxes are not permitted for electronics soldering.

The flux and the cleaning process (or lack thereof) are

directly interdependent.

27.3.1.1

Many common fluxes use natural rosin as a

base. This natural product, derived from the gum of pine

trees, is a mix of abietic acid and numerous dehydrogena-

tion products.

Rosin is a glassy, non-crystalline mixture of organic acids

which are inert up to their softening point and only assume

an acidic nature when molten. After melting and resolidifi-

cation the hard glassy properties render the residue once

again inert. Residues from other added chemical activator

compounds usually become encapsulated in the rosin resi-

due, which renders them non- corrosive. The effectiveness

of this safeguard, however, depends on the quantity and

nature of the activator used.

The acidity of pure rosin alone is usually insufficient to

clean surfaces to be soldered,

so

rosin fluxes are usually

enhanced by a variety of chemicals called activators. Com-

mon activators are inorganic halides, organic halides, car-

boxylic acids, amines and halogenated amines. The degree

of activation achieved by the various chemicals depends

upon the compound used and the quantity. Since the over-

all activity developed is often a synergistic product of more

than one activator, activation is usually quantified not by

formulation alone but rather by some secondary property

such as the ability of the residue extract to dissolve a cop-

per from the “Copper Mirror Test”-or by its ionic con-

ductivity. The level of activator affects the rate of wetting.

27.3.1.2 Organic Acid (Water Washable) Fluxes

These

fluxes are significantly more active and aggressive than

rosin fluxes in removing oxides from the surfaces to be

soldered. They use strong organic acids and salts to achieve

these properties. As such they are more forgiving of poor

solderability characteristics of the surfaces to be soldered.

The increased activity yields flux residues which are more

corrosive than the residues of rosin fluxes. Because of their

characteristics it is necessary to completely remove the

residue with a post soldering cleaning operation to prevent

early failure of the soldered assembly.

27.3.1.3 No Clean Fluxes

(Low

ResiduelNo Residue).

This family of fluxes includes both rosin or modified resin

fluxes and non- rosin fluxes. The activators used are gener-

ally weak organic dicarboxcylic acids.

The low solids rosidresin based fluxes leave small

amounts of residue after soldering and except where these

may interfere with bed of nails testing or other post solder-

ing operations or operating characteristics generally have

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770 Januaty 1996

no deleterious impact on reliability and do not need to be

removed from the assembly with a post solder defluxing

operation.

Other no clean fluxes, no residue, are formulated with no

rosin and utilize a mixture of one or more weak dicarbox-

cylic acids and wetting agents to provide the activation

needed to enhance soldering. These fluxes leave little or no

residue and do not need to be removed from the assembly

with a post solder defluxing operation.

An inert gas atmosphere, such as nitrogen, is often used to

enhance the soldering process when using these more

benign fluxes.

27.3.2 Solders

Solders are generally metal alloys with

melting points in the range -150°C to -400°C. Below this

temperature range, alloys are commonly called fusible

alloys: above this they are called brazes. Tin-lead alloys are

most common, although more complex compositions had

been developed for special applications.

For electrical soldering, alloys near the eutectic composi-

tion (63% tin 37% lead) have the required combination of

properties. Although compositions either side of the eutec-

tic have higher liquidus (completely melted) temperatures

desirable for higher ambient temperature applications,

remember that the initial melting point (at which solder

softens) is 183"C, the same for all tin-lead alloys with

compositions between 20% and

98%

tin.

The range of properties of tin-lead alloys can be varied by

adding other metals such as bismuth or indium to lower the

melting point, or antimony, silver, etc. to increase hardness

and fatigue resistance. Alloys containing less than

10%

tin

are used for applications involving temperatures below

-40°C.

27.3.2.1 Solder Alloy Selection

Selecting a solder alloy

for a particular assembly depends on the expected operat-

ing conditions and on the types of components used. For

example, the mechanical and fatigue properties of solder

may be more important for surface mounted components

than through hole components.

With respect to temperature, consider the initial melting

point of the alloy when seeking an alloy for high temper-

ature applications or for multiple step soldering applica-

tions. Table 27-1 is a list of the most common solder alloys

and their melting temperatures.

27.3.2.2 Solder Purity

As soldering processes become

progressively more automated, final product quality

depends increasingly on consistent material quality and in

particular on solder purity. Solder should be purchased to a

standard of purity, as specified in J-STD-006.

Even very pure solder will pick up impurities from the

substrates, component leads, jigs and even the solder bath

Table 27-1 Solder Alloys

Alloy Melting Temperature

("C)

63/37 tin/lead

.....................................................................

183

60/40 tin/lead

..............................................................

183-1 91

60/40 lead/indium

.......................................................

195-225

52/48 indiumkin

..................................................................

11 8

62/36/2 tin/lead/silver

.........................................................

179

96/4 tin/silver

......................................................................

221

95/5 tin antimony

........................................................

235-240

10/88/2 tin/lead/silver

..................................................

268-290

1/97.5/1.5 tin/lead silver

.....................................................

309

43/43/14 tin/lead bismuth

...........................................

144-1 63

and machine tracks themselves. This limits the time life of

a solder bath. Typical limits for various likely impurity ele-

ments and the defects these can cause are also shown in

Table 27-2. Tin content should be tested with the same

frequency as testing for copper/gold contamination. The

balance of the path is lead and/or the items listed in Table

27-2.

27.3.2.3 Solder Forms

Various forms of solder can be

used to join components and printed board structure lands.

Solder can be supplied in ingot, wire, platings, preforms,

and pastes.

The solder form used in a particular soldering operation is

dependent on the method of heat application, component

placement methodology, and solder alloy selected.

A. Solder Pastes

Solder pastes are rarely used for soldering through board

mounted components, but they are used extensively for

surface mount where the variety of application methods

lends variety and flexibility to component mount and place-

ment schemes. These pastes can be applied to printed board

structures using three basic methods: screen printing, sten-

ciling and dispensing.

The properties of the solder paste affect the application

results. Solder pastes consist of a fluhinder and solder

alloy particles. The rheology of the fluxhinder keeps the

heavy solder particles

(85

to

89%

by weight) evenly dis-

persed under normal temperature and pressure conditions.

The fluhinder contains pure rosin or synthetic resins, acti-

vators, solvents, lubricants and thickeners. The activator

removes surface oxides and tarnishes during the soldering

process. The solvents adjust the solder paste to proper vis-

cosity. Since the solvents alone do not provide adequate

body to support the metal powder, thickeners are added.

They also give the solder paste the proper texture and life-

time. Solder particle shape and size also significantly influ-

ence deposition characteristics. Well rounded particles flow

more readily through small openings and give better pat-

tem definition, while the irregularly shaped particles or

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