IPC-CM-770D-1996 - 第147页

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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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

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IPC-CM-770

Januaty

1996

strip and individual preforms are chains where each unit is

broken off as it is dispensed.

The assembly sequence using preforms can be very simple.

The placement method of the preforms between component

parts or component and substrate is often the single most

unique feature of a preform process. The preform materials

have to be fixed or located between the components of the

joint with sufficient accuracy to insure that on melting the

metal contacts both parts of the joint

so

that they wet, and

surface tension forces can act to draw them into perfect

alignment. After this, the only remaining stage is to reflow

melt the preform. Solder preforms may be located by plac-

ing them over a lead or projection in the chip carrier, or by

positioning them in a groove or notch on the substrate.

Another alternative is to use auxiliary jigs and plates to

immobilize preforms until the reflow operation. Regardless

of the placement method, the preforms may be handled

through simple manual transfer, vibrator units, tumble

plates, or two part alignment plates.

In general, flux cannot be used to tack preforms in place

since the flux layer melts before the metal, allowing the

solid preform to float and move over the substrate. The

same applies to using partial flux reflow from flux filled

and flux coated preforms.

Metal coated preforms, however, can be affixed by partial

reflow, provided there is sufficient difference in melting

point between the cladding alloy and the core. Reflow of

the cladding layer can be used to attach the preform to one

side of the joint, which can then be fully assembled and

reflowed at the higher temperature to complete the bonding

operation.

The vapor-phase process is particularly suited for this if

alloy melting points allow the use of different reflow fluids.

A reflow fluid with a boiling point of 215°C would create

a molten surface zone to attach the preform. Subsequent

use of a reflow fluid with a boiling point of 250°C would

reflow the whole composite assembly.

27.4 Component Retention

Component retention is

described in detail in section 21.4.

27.5 Machine Soldering Processes

Soldering is defined

as a joining process with the use of heat (below 400°C) and

a non-ferrous filler metal that has a melting temperature

below that of the base metals to be joined. Soldering is a

practical technique for producing mechanically sound and

electrically reliable interconnections. Different soldering

processes exist, suited for a variety of electronics assembly

applications and technologies. The following sections

describe the specific details of the most common processes

used for mass production soldering of electronics

assemblies.

27.5.1 Wave Soldering

The large percentage of through-

hole board mounted components are mass wave soldered.

Small surface mount components such as passive chip

capacitors and resistors as well as SOTS (small outline tran-

sistors), bonded to the bottom side of the board with an

adhesive are also wave soldered extensively.

Wave soldering involves the following sequence of

operations:

Fluxing

Preheating

Soldering

27.5.1.1 Fluxing

A flux is applied to the bottom surface

of the electronics assembly as the first step after loading it

into the conveyor system of the wave soldering machine.

The flux also coats the component leads and some flux

enters the plated holes in the board. Flux is required to

remove oxides from the metallic surfaces to be soldered.

Flux also promotes solder wetting, helps the solder to rise

in plated holes, leaves solder joints with a smooth and

oxide-free surface and can prevent solder balls, solder sliv-

ers and webbing from remaining on the board surface after

soldering.

One commonly used method for applying a flux in a wave

soldering machine is foam fluxing. Other methods are

spray fluxing, wave fluxing and brush fluxing. Fluxers are

fabricated from materials which are compatible with the

chemical nature of fluxes. For rosin fluxes, the fluxer is

often fabricated from stainless steel or polyvinylchlo-

ride(PVC), a chemically resistant plastic material. For

water soluble acid fluxes which are more corrosive, the

units are either made of PVC or titanium.

A.

Foam Fluxing

Many wave soldering systems utilize foam fluxers. This is

the simplest method for applying flux. The foam is gener-

ated by feeding low-pressure compressed air into a porous

diffuser stone or a porous plastic diffuser or filter tube

which is immersed into the liquid flux in the fluxer bath.

The pores in the diffuser tube range from

50-60

micron

pore size for regular fluxes down to

10

microns for low

solids fluxes. The fine foam bubbles of flux are generated

by the air coming out of the pores. The flux rises in a cen-

trally mounted chimney nozzle which guides the foam

upward till it overflows the top edge of the chimney. As the

assembly is conveyed over the foam fluxer chimney, the

bottom of the board touches the foam head and becomes

coated with flux. A short distance after the exit end of the

fluxer an air knife is usually used, made from a tube with

small diameter holes drilled in a row along its length. Low

pressure air is directed upward at a slight angle toward the

entry end of the machine to remove excess flux and drip-

pings without disturbing the components. A well designed

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