IPC-CM-770D-1996.pdf - 第152页

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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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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January

1996

IPC-CM-770

This process uses a screen (usually stainless steel) held taut

in a rigid frame. A master artwork stencil is then laid on

top of the screen and a photographic emulsion is printed

onto the screen using the artwork stencil. The emulsion is

then exposed and developed to obtain a hard finished pat-

tern. The artwork is removed, and then solder paste placed

across the entire screen. The solder paste can only pass

through the screen where there is no hardened photo-

graphic emulsion. The quality and accuracy of screen print-

ing depends upon the size of the screen mesh, the solder

paste properties (particle size and shape, rheology, compo-

sition), the screen frame tooling, and the printing process

itself (squeegee pressure and angle, etc.)

The screen material is usually 304 stainless steel, and

frame sizes very considerably. Generally, fine mesh sizes or

small mesh openings are used when screening very thin

deposits, (e.g., for up to

0.002

inches use

250

to

150

mesh,

for 0.003 inches to

0.005

inches use

150

to

105

mesh, and

for greater solder screened thicknesses use

80

mesh

screens). A

0.005

inches increase in both the

X

and

Y

directions of the stencil pattern openings may be necessary

to compensate for the stainless steel mesh. Additional

allowances may be necessary for very thick solder paste

deposits.

To facilitate passage of solder paste through the screen, the

mean particle size of the solder paste should be about one-

third the mesh size. For example, for a mean mesh opening

of 0.0065 inches, the mean particle size of the solder paste

should be about

0.0020

inches.

The screen should be positioned approximately 0.013

inches to 0.063 inches above and parallel to the printed

board structure. The structure should be located in exact

registration with the screen image and held in place with a

reliable tooling pin setup, double-sided adhesive tape, or by

some other reliable means.

27.5.4.1.2 Stencil Printing

Pattern metal-foil stencils

avoid the registration and allowance problems of a screen

mesh, while increasing the solder paste volume deposited

and improving the uniformity of deposition. Stencils may

be used on conventional screen printing equipment, pro-

vided the equipment is adjusted for contact printing. Use

sufficient squeegee pressure to wipe the metal foil without

“dishing-out’’ paste from the etched pattern. Stencil print-

ing can deposit solder paste up to

0.015

inches thick with

center-to-center land spacings as small as

0.007

inches.

The fabrication of metal-foil stencils by chemical etching

becomes increasingly more difficult for opening/aspect

ratios (stencil opening widthhtencil thickness) less than

2.

In addition, for aspect ratios less than

2,

paste begins to

cling to the side walls of the stencil and irregular paste

deposits may result. The stickiness of the flux vehicle can

also cause this.

The openings in the stencil should be at least as large as the

outlines for the surface attachment lands. Since melted sol-

der paste tends to pull towards wettable printed board

structure lands, the stencil openings may be larger than the

lands in order to provide additional solder volume. How-

ever, leave sufficient space between deposits to prevent sol-

der bridges.

27.5.4.1.3 Dispensing

In the dispensing process paste

from a supply cartridge is pneumatically or mechanically

forced through small-diameter tubes or orifices to cause

small discrete deposits of paste to be placed at interconnec-

tion sites. Solder paste dispensing is best suited to repair,

production volume applications, model fabrication, and in

mixed technology, where surface mounted chip carriers are

added after the wave-soldered through-hole mounted com-

ponents. Multipoint dispensers should be used for the local

deposition of paste at a complete chip carrier site, whereas

single site dispensers are best suited to the local repair of

individual joints.

27.5.5 Quality Assurance

Prior to soldering, unsoldered

assemblies should be examined to ascertain that no damage

has occurred during transit or handling, and that compo-

nent mounting is in accordance with the appropriate

requirements.

When, for any reason, a component lead is terminated

so

that the lead is allowed to stress the soldered joint, the

working of the joint has been observed to result in the gen-

eration of cracks which tend to relieve the stresses. In its

ultimate condition such a connection, with extended cracks

and discontinuities, might result in a fractured joint.

These connections are called “disturbed joints.

“

In the past

they were called “cold solder joints.

“

The consensus is that

cracking is usually caused by mechanical stress on the joint

which may be induced either mechanically or thermally.

Mechanical or thermal conditions causing cracking can be

created by component mounting impacts such as design

established mismatches or component lead forming.

Some examples are:

A component (such as a power transistor or module)

mounted flat on the surface of a board with non-plated

holes. In this case, the differential in coefficient of expan-

sion between the board material and the lead material acts

directly on the solder joint.

Unsupported holes much larger than the component leads

causing the solder fillet to thin-out on one side of the hole

to

0.1

mm or less. This situation is particularly noticeable

when clinched leads are used.

The use of eyelets in interfacial holes causing a thermal

coefficient mismatch between the board material and

eyelet.

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