IPC-CM-770D-1996.pdf - 第150页

January

1996

IPC-CM-770

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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Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

bottom of the board. The quantity of preheat needed is

a function of the solvent in the flux. The solvent may be

alcohol, water or blends of these and other solvents.

b) to activate the flux where chemical activity is a function

of temperature.

c) to prevent thermal cracking of components such as chip

capacitors.

d) to reduce thermal shock to the board and components on

entering the solder wave.

e) to prevent warping of the board on entering the wave.

f)

to allow faster conveyor speeds, thereby reducing the

g) to obtain the overall optimum temperature profile

so

as

contact time in the wave.

to minimize soldering defects.

Assemblies are preheated by several different means (e.g.,

radiant infrared heater tubes or panels, hot plates or forced

hot air). Whatever method is used, the preheaters must

have efficient temperature control. Common practice is to

preheat.

27.5.1.3 Wave Soldering

A pump, located in a tank of

liquid solder at a temperature of about 235250°C gener-

ates a positive pressure causing the solder to flow up a

chimney or nozzle where it overflows to form a standing

solder wave with its crest higher than the rim of the solder

tank. It is this solder wave which performs the solder join-

ing of the components to the metallic surfaces on the cir-

cuit board.

The preheated assembly receives the balance of the heat

required to raise the joint areas to the soldering tempera-

ture, causing the liquid solder to wet those areas to be

joined.

The assembly is conveyed, usually up a slope, inclined

between

4

-

7",

till its bottom surface contacts the crest of

the solder wave where the pads, protruding leads, plated

holes and bottom-side surface mounted components are

soldered.

The solder only wets to, or forms joints on, solderable

metallic surfaces. Consequently, no soldering takes place

on the board surface which is non-metallic and poor solder-

ing can occur on any metallic surfaces which are contami-

nated or have poor solderability.

A common wave for soldering traditional boards with leads

in through plated holes is the asymmetrical wave used with

an inclined conveyor. The majority of the solder flowing

from the nozzle flows against the travel direction of the

board. The remaining portion of the solder flows as a

smooth laminar stream of solder in the same direction as

the board direction. This small amount of solder flows over

a weir, often adjustable in vertical position,

so

that the

speed of the solder flowing towards the exit weir is mov-

ing at the same speed as the board and the conveyor. With

the board and the solder moving in substantially the same

direction and at the same speed, the drainage conditions

where the assembly separates from the wave are ideal for

optimum soldering results.

Various wave configurations have been used including the

narrow parabolic wave, the wide wave, the adjustable wide

wave and the hollow jet wave. Some are operated with

horizontal conveyors and others with inclined types.

To eliminate solder bridges sometimes formed as a board

separates from a solder wave, some wave solder machines

may have an air knife fed with hot air to sweep any bridges

from the joint areas before the solder has had a chance to

freeze. When used in a nitrogen wave soldering system,

nitrogen is supplied to the hot knife.

For wave soldering of surface mount assemblies where, in

addition to the usual leaded components, small chip com-

ponents have been glued to the bottom of the board, two

solder waves are sometimes used. The first solder wave is

usually a high, rather narrow wave, made turbulent by

some mechanical means. This is achieved by pumping the

solder through rows of small fixed, or moving holes at the

outlet of the nozzle or by means of a unidirectional hollow

jet wave, with its flow trajectory usually aimed in the same

direction as the board travel direction. This first turbulent

wave is followed by an asymmetrical laminar wave as

described above.

The turbulent action of the first wave causes the solder to

move in and around all the chip components to help ensure

that all solder joints get soldered. Those that are still not

soldered will most probably be soldered when contacting

the second wave. In some designs a vibrating device is

added to produce additional mechanical pressure in the sec-

ond laminar wave to promote hole filling and to further

help reduce solder skips.

With double wave systems, each wave is driven by a sepa-

rate pump for independent wave height control and they

are usually found in the same solder pot. Some machines

have the two waves in separate solder tanks, in which case,

it is possible to control the solder waves at different tem-

peratures.

27.5.2 Atmosphere Controlled Soldering

A great deal

of investigation has been focused on the potential benefits

of using an inert gas in the mass soldering processes.

Research has focused on reactive and protective atmo-

sphere systems. Reactive atmospheres utilize gases (either

oxidizing or reducing in potential) which react with the

surfaces to be soldered and the solder source resulting in a

clean, solderable surface. Protective atmospheres displace

the oxygen in the soldering zone of the mass soldering

processes thus allowing the fluxes to remove the surface

oxides without reoxidation.

The majority of the atmosphere soldering systems on the

market use protective gases. Nitrogen is the most prevalent

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COPYRIGHT Association Connecting Electronics Industries

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January

1996

IPC-CM-770

gas used. An oxygen concentration less than

100

ppm has

been found to provide excellent soldering results. Nitrogen

consumption varies depending on the length of machine,

soldering work load, and type of entrylexit interlock sys-

tem employed. The use of atmosphere soldering systems

can produce the following benefits:

-

improved soldering yields

-

reduced flux consumption

-

improved solder joint appearance

-

improved solder joint geometries

-

improved flux residue removability

-

reduced machine maintenance

27.5.3 Reflow Soldering

Several different processes for

flow melting solder pastes involve either vapor-phase

reflow soldering, conduction-reflow belt soldering, infrared

heating, forced hot gas convection, wave soldering, heated

bar soldering, etc.

27.5.3.1 Vapor-Phase Soldering

Although each of the

above techniques can produce acceptable results, vapor-

phase soldering is an extremely consistent soldering pro-

cess because the soldering temperature is maintained con-

stantly at the boiling point of the primary liquid. Also,

vapor-phase soldering can solder both sides of a printed

board structure simultaneously.

The process uses a perfluorinated liquid which is heated to

its boiling point, creating a saturated vapor zone. At atmo-

spheric pressure, the temperature of the saturated vapor is

the same as the boiling liquid. Typically, a perfluorinated

fluid with a working temperature of 215°C is used for

eutectic grade solder. Fluids with other boiling tempera-

tures are available for lower or higher solder compositions,

in multistep soldering, or for use with pure tin.

The soldering process moves the populated printed board

structure into the vapor zone using either a vertically oper-

ating baton system or a conveyorized in-line system. In

either configuration, reflow cycle times of 30 to

60

seconds

are common.

The printed board structures come out of the soldering pro-

cess uniformly soldered and dry. The uniformity results

from the inert, oxygen-free conditions in the vapor and the

precise maximum temperature limitations of the process.

27.5.3.2 Conduction-Reflow Belt Soldering

A

conduction-reflow belt system uses conduction to transfer

heat through the printed board structure. This is accom-

plished by a series of heated platens located under a

continuously-moving belt.

The platen temperature and belt speed are individually con-

trolled

so

that the reflow profile can be varied for different

types of printed board structures, which may have different

melting temperatures. Since heat is conducted through the

board, a thermal gradient exists across the printed board

structure. The cooling rate can also be varied by increasing

or decreasing the flow of air over the belt as the assemblies

travel off the end of the final platen.

Because the conduction of the heat through the printed

board structure depends on the contact of the structure to

the belt, a weighted fixture is usually used to apply pres-

sure to multiple points on the structure. This pressure is

applied from the primary side of the printed board structure

and is designed not to interfere with the chip carriers on

that side. Universal fixtures are usually designed for mul-

tiple printed board structures.

A typical schedule for a three-stage conduction reflow sol-

dering belt system is as follows:

Ist platen

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

50°C

2nd platen

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

200°C

3rd platen

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

230°C

belt speed

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

10-11

ips

air flow

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

as required

The belt speed should be adjusted to allow the solder to

reflow for a total of 30 to

60

seconds. In addition, adjust

the air flow

so

that the solder, when solidified, is shiny and

not grayish in color.

27.5.3.3 Oven Heating

Electrically heated air circulation

ovens are suitable for batch operations, provided the whole

assembly can withstand the soldering temperature, inert or

reducing atmospheres can be used if necessary.

27.5.3.4 Furnace Heating

For continuous operation, gas

or electrical conveyorized furnaces may be used and these

also allow use of controlled atmospheres. Smaller units

require careful matching of temperature settings with ther-

mal pad of product if close control of temperature profiles

is required.

27.5.3.5 Heater Bar Reflow Soldering

Heater bar reflow

soldering is generally used to attach leaded surface mount

devices to printed wiring board conductive land areas. Heat

is supplied via electrical resistance heating of the heater bar

tip. The tip temperature is controlled by a variable current

feedback loop with a thermocouple on the heat bar tip.

A.

Pulsed Heat Cycle

The heat cycle can be either pulsed

or constant. For a pulsed heat cycle system the following

events occur:

1.

Heater bars are aligned over leads to be soldered.

2. Heater bars descend on top of the leads exerting

approximately

0.1

to 0.2 pounds per lead being sol-

dered.

3. Power supplies preset for time at temperature are acti-

vated and send variable current to the heater bar tips to

reflow soldering the leads to the printed board structure.

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Licensed by Information Handling Services