IPC-CM-770D-1996 - 第151页

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

6-14

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Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

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

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

4.

After the soldering cycle time at temperature is com-

pleted, the power supplies are deactivated and airjets,

directed at the heater bars, are activated. Meanwhile, the

heater bars maintain their contact pressure on the leads

for a specified dwell time allowing the leads to cool

undisturbed.

5.

After the solder has solidified the heater bars are

retracted and aligned over the next set of leads to be

reflow soldered.

B.

Constant Heat Cycle

For a constant heat cycle system the following events

occur:

Heater bar tips are heated to and maintained at a prede-

termined temperature setting.

Heater bars then contact the leads for a specified amount

of time, reflow soldering the leads.

The heater bars are then removed allowing the joints to

cool undisturbed.

Both methods have their different applications but require

the common set up of tinned leads and a predetermined

amount of solder paste on the conductive land area.

When using a pulsed heat cycle system, take care to prop-

erly form the leads. Otherwise, the pressure of the heater

bars will induce mechanical stress into the joints because

the leads are forcibly held in place during cooling. This can

cause solder joints to crack during bum-in, thermal and

vibrational cycling.

27.5.3.6 Laser Reflow Soldering

This process uses a

laser beam to supply the heat necessary for reflow solder-

ing. Presently, the continuous wave and YAG laser is pre-

ferred, although some companies have demonstrated suc-

cess utilizing a carbon dioxide or eximer laser. The process

is non-contact unless tooling specifically designed to apply

pressure is used.

Other advantages are the extremely fast soldering and

localized heat cycles. A typical joint is reflowed in less than

one second. This reduces the amount of heat conducted

away and therefore localizes the heat applied to the solder

joint area.

Both leaded and leadless components can be reflowed with

this process. It is recommended that when reflow soldering

leadless components, the laser beam should be oriented at

approximately

45"

to the soldering surface and directed at

the solder to solder interface, as compared to leaded

devices, where the beam is at

90"

to the solder surface and

is directed at the component lead.

The reflow soldering process typically involves mounting

the populated printed board structure to a computer con-

trolled motion system; loading a computer program which

automatically directs the laser to each joint and controls the

power level and time duration of the laser at each joint

verify the initial joint location, usually with a closed circuit

TV system or a He-Ne spotting laser; placing the system

into the automatic mode, thus allowing the system to

reflow solder all programmed joints.

A laser soldering system properly set up and programmed

will yield very high consistent high quality solder joints.

However, consistent processing (i.e., lead forming, lead tin-

ning, amount of solder on the land, and component mount-

ing) before reflow soldering is an integral part of this pro-

cess and requires strict controls.

27.5.3.7 Infra-Red Soldering

Unfocused infra-red lamps

can rapidly heat the printed board structures to soldering

temperatures, although features such as component color

and surface texture influence the rate of heating and the

final temperature. Polished aluminum reflectors may be

used to minimize local heating. Focused infra-red heating

provides an efficient means of even more rapid heating,

provided the distribution of preforms lends itself to local-

ized patterns of heating. A throughput of 2dmin or more

is possible.

27.5.3.8 Heat Guns

Hot air guns can be both effective

and versatile for preform soldering. A variety of nozzle

shapes are available to localize the heated area and masks

or covers can be used to protect sensitive components. If an

inert or reducing atmosphere is required this can be pro-

vided by connecting the intake impeller to a suitable sup-

ply of gas.

27.5.3.9 Induction Soldering

This can be used to con-

fine heating to local areas of a panel or assembly, particu-

larly when unusual shapes or spots are involved. Fast

throughput is possible using coils uniquely designed for

reflow of the specific areas of interest. Clad coils prevent

inadvertent electrical contact with components.

27.5.4 Solder Paste Specific Considerations

Solder

paste reflow, currently a common method of surface

mounting assembly, involves essentially the following pro-

cess sequence:

-

Solder Paste Deposition

-

Component Placement (see Sections 22)

-

BakingKuring of Solder Paste

-

Soldering

27.5.4.1 Solder Deposition

These pastes can be applied

to substrates using three basic methods: screen printing,

stenciling and dispensing.

27.5.4.1.1 Screen Printing

In order to ease the passage

of solder paste through the screen for a complete printed

image, the mean particle size of the solder paste should be

about one-third the mesh size.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services