IPC-CM-770D-1996.pdf - 第148页

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

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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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foam fluxer is equipped with the necessary pressure regu-

lators, gauges, valves and flow meters to control the pro-

cess. Other important controls sometimes found are auto-

matic specific gravity controllers and methods for

automatically maintaining the correct liquid level above the

foaming tube.

Foam fluxers can be used where the length of the compo-

nent leads, protruding from the bottom of the board do not

exceed 6 mm.

B. Spray Fluxing

Spray fluxing is widely used for the application of low-

solids fluxes because of the ability to better control the flux

deposit for no-clean applications. While spray fluxers have

existed for years, they were not popular because the sticky

rosin fluxes commonly used at that time, blocked the

nozzles of the spray units, the interior of the soldering

machine was hard to keep clean and there was always a

risk of fire. The use of low solids fluxes, with a solids con-

tent in the range of

1-3%

often contain no rosin. This

reduces blocked nozzles and decreases the maintenance

problems. Nitrogen is often used to direct the flux spray

onto the assembly.

Spray fluxing is used when lead lengths preclude the use of

other fluxing methods or when fine control of the amount

of flux applied to the assembly is required. Use of spray

fluxes is popular to increase control of the application of

the low residue fluxes.

Spray fluxers, in general are more expensive than all other

fluxers used in wave soldering systems.

One spray fluxing methods consists of rotating a fine stain-

less steel screen drum in liquid flux with compressed air or

nitrogen jets inside the drum. The flux fills the mesh of the

screen and the upward pointing jets blow the flux from the

holes in the mesh onto the bottom of the assembly passing

above. The amount of flux transferred is a function of the

mesh size and is controlled by the speed of the rotating

drum and the pressure of jets. As with a foam fluxer, the

need to control the operating parameters is essential,

including frequent measurement of the solids content of the

flux remaining in the flux tank.

Other spray fluxing methods include direct spray jets and

nozzles. These are popular when using a low solids flux

because there is no need to periodically measure and cor-

rect the solids content of the flux as with all other fluxing

techniques.

Some designs have one or more fixed spray nozzles below

the board and spray the flux upward, usually assisted or

driven by compressed air or inert nitrogen which reduces

the risk of fire.

Other designs have one or more spray nozzles or jets which

move back and forth, spraying the flux deposit onto the

bottom of the assembly from below the conveyor. These

nozzles are similar to paint spray nozzles, or they may be

lower cost simple jets. Other designs use ultrasonic energy

to form a mist of flux which is then directed upwards with

a stream or jets of nitrogen. The liquid flux flow and/or gas

flow rate, energy applied, and speed of the traversing

nozzle relative to the conveyor speed are among some of

the essential parameters controlled for consistent applica-

tion of a uniform deposit of flux.

As with a foam fluxer, an air knife is sometimes integrated

after the exit end of some spray fluxers to spread out the

flux deposit.

“Inkjet” spray technology is also used in some spray fluxer

applications. This technique provides precise application of

the flux with no other spray since no air or nitrogen gas is

used. Hole fill with this technology has been demonstrated

to be excellent.

C.

Wave Fluxing

Wave fluxing can be used when the leads protruding below

the bottom of the board exceed 6mm and the flux contains

a high percentage of solids, usually rosin, or when the flux

cannot be applied with a foam fluxer or a spray unit. A

wave fluxer is more expensive than a foam fluxer but less

than a spray unit. The materials must be compatible with

the flux and are similar to those used to fabricate a foam

fluxer.

The liquid flux is pumped up a nozzle to form a symmetri-

cal standing wave of flux. The wave is parabolic in profile

and flows equally in both directions, returning the flux by

gravity to the main tank. The bottom of the assembly is

coated with liquid flux as it contacts the crest of the wave

and as the protruding leads pass through the wave, the

washing action combined with capillary forces promotes

flux rising in the plate holes. For process control, the wave

height is adjustable. As the production day proceeds, the

solvent in the flux will evaporate, but at a slower rate than

from a foam fluxer. Periodic specific gravity checks or an

automatic specific gravity control unit are recommended.

An air knife is available after the exit of the wave fluxer to

remove excess flux and drippings from the bottom of the

assembly.

D.

Brush Fluxing.

Brush fluxing should only be used where special require-

ments preclude other fluxing methods. This method is lim-

ited to assemblies where the components or their leads are

fixed in some way that they cannot be dislodged or moved

by the brush. In brush fluxing, a rotating cylindrical brush

transfers flux to the bottom of the board from either a

foaming flux head or a static bath of liquid flux.

27.5.1.2 Preheating

Preheating the board has several

functions:

a) to evaporate the solvents from the flux deposit on the

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