IPC-CM-770D-1996.pdf - 第146页

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

6-9

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

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

6-10

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

Table

27-2

Solder Bath Contamination Limits

Solder Joint Characteristic Guidelines

1/ The tin content of the solder bath shall be within 1: of the limits of 00-5-571 for the solder specified and tested at the same frequency as testing for copper/

2/ The total of copper, gold, cadmium, zinc and aluminum contaminants shall be lead and/or the items listed above

3/

An operating day constitutes any 8-hour period, or any portion thereof, during which the solder is liquified and used

gold contamination. The balance of the bath shall be lead and/or the items listed above.

&/

See paragraph 4.19.2.2

5/ Not applicable for Sn62 solder

-

limits to be 1.75

-

2.25 (both operations).

large particles clog the deposition equipment.

Halide activators in the flux vehicle react with the oxide

film present on all solder powders. In a binary tin-lead sol-

der system, this combines with moisture and carbon diox-

ide to initiate corrosion. The basic carbonate corrosion

products form crusts on top of solder pastes, limiting shelf

life. A system containing silver is more effective in locking

halide in a silver halide form, and this limits the corrosion.

The oxide film thickness is approximately the same on all

sizes of solder particle. This means that for fine particles,

the oxide percentage is higher because of the higher sur-

face aredvolume ratio; consequently, finer particle pastes

deteriorate faster in storage. Refrigeration limits the corro-

sion reactions and increases shelf life.

Corrosion reactions during storage reduce activator avail-

ability which decreases effective solder particle cleaning

during flow- melting and increases the tendency for solder

ball formation. As with conventional wave or reflow sol-

dering operations, the flux used in the paste must meet the

corrosion and insulation resistance test specifications used

with printed board structures. With solder pastes, however,

the term flux also includes additional constituents which

provide the paste properties, and these materials or their

residues may adversely affect the substrate. Therefore, sol-

der pastes should be tested for corrosion and insulation

resistance characteristics in order to meet specified reliabil-

ity requirements.

Reliability requirements and test methods are described in

J-STD-O04 and J-STD-005.

B. Preforms

A major disadvantage of solder paste is the large amount of

flux incorporated with the metal. Common pastes are for-

mulated with 85wt% to 92wt% metal, which equals

45~01% and 55~01% metal at reflow depending on alloy

and flux type. This in turn means about 45% to

55%

of the

liquid phases have to wash out of the joining gap before

solidification or else voids will result. Flux containing pre-

forms usually contain a maximum of 3.5wt% flux, and this

together with the capability for conformity with the joint in

even the most complex joint geometry means smaller lat-

eral flow distances and less flux entrapment, leading to

more sound and continuous joints. Preforms also alleviate

solder balls and spitting.

Preforms are available in a wide variety of shapes and

sizes, from simple solid discs, squares, rings and spheres to

extremely complex configurations. Apart from the simple

shapes mentioned above, a wide variety of “picture frame”

washers and collars are commonly used for electronic sol-

dering. Another form is continuous tape or strip which is

dispensed directly to the bonding site intermediate between

6-11

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services