IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第52页

Each surface mount device is aligned using the body cen- ter and a starting orientation for reference. ‘ ‘0’ ’ degree is the basic orientation of the device. Rotational data must be speciﬁed from the ‘ ‘0’ ’ position in …

need to make a photo tool eliminates the potential for mis-

registration. And since there are no photographic steps, a

stencil can be made with excellent positional accuracy and

remake reproducibility. The tolerance on the aperture

dimensions can be held to 7 µm [276 µin], allowing for

printing 0.3 mm [0.0118 in] pitch. This process yields

maximum paste release, resulting in minimal stencil clean-

ing, thereby increasing printing efficiency. Plus, the laser

cutting process inherently creates trapezoidal apertures,

furthering complete paste transfer.

A trapezoidal section aperture is one which has a larger

opening on the contact (board) side of the stencil than on

the squeegee side. The opening on the contact side is typi-

cally 5 µm [197 µin] per side larger than the squeegee side

dimension, depending on the customer’s requirements. This

wall geometry, when further enhanced by electropolishing,

allows for better paste release during the printing process.

The results are more noticeable on ﬁne pitch components.

Depending on the overall array design and given the right

metal thickness selection, chem-etched stencils can per-

form adequately at 0.5mm [0.0197 in] pitch. Their effi-

ciency can be improved with performance enhancing pro-

cesses such as electropolishing and/or trapezoidal section

apertures. For more details in the engineering of the stencil

for speciﬁc soldering requirements, refer to IPC-7525.

7.2.4 Solder Preforms Solder preforms are sometimes

used for through-hole mounted devices as well as SMT

rework or prototype boards. They come in speciﬁc size and

composition, with ﬂux either inside the preforms, or as a

coating or without ﬂux. They may be cost-effective to

avoid wave solder processes if there are only a few leaded

components on the board.

7.3 Component Placement The accuracy requirements

for device placement make it more practical to use roboti-

cally controlled machines for surface mount components

on the board. Selection of the appropriate autoplacement

machine is dictated by the type of components to be placed

and the assembly production rate. Sequential placement

equipment typically utilizes a software controlled X-Y

moving table system. Components are individually placed

on the printed board in succession. Typical cycle times

vary with component size and complexity.

7.3.1 Component Data Transfer Prior to designing the

PCB board in the CAD system, each component is con-

structed in digital form creating an electronic database. The

CAD data is most often used to prepare photo-tool artwork,

PCB fabrication details and assembly instructions but, if

developed in the correct format, it can also be adapted to

manufacturing processes. Direct transfer of CAD data into

automated assembly systems will accelerate production

set-up and reduce overall assembly system programming

time.

When the CAD database for the device is prepared, speciﬁc

physical data for each device can be used to assist assem-

bly machine programming for both component placement

(X-Y coordinate position) and orientation. To facilitate the

X-Y coordinate information, a datum position must be

established on the PW board surface. The recommended

datum ‘‘0’’ for X and Y coordinates ideally, may be one of

the global ﬁducial targets at the lower left or lower right

corner of the board or panel. Surface mount devices are

furnished in tape and reel as well as tube magazine feeders

to accommodate high-speed assembly systems (tray carri-

ers are most often adapted for ﬁne-pitch components).

IPC-7351-7-02

Figure 7-2 Assembly Process Flow for Two-Side Surface Mount with PIH

Two Side SMT / Auto PIH Assembly

Fine Pitch SMT

Placement

Reflow

Solder

Sd. 1 SMT

Placement

Solder Paste

Sd. 2 SMT

Placement

Hand Load

Odd Form PIH

Dispense

Epoxy

Auto Place

PIH

Electrical

Test

Final

Inspection

Clean/

Inspect

Wave

Solder

February 2005 IPC-7351

Each surface mount device is aligned using the body cen-

ter and a starting orientation for reference. ‘‘0’’ degree is

the basic orientation of the device.

Rotational data must be speciﬁed from the ‘‘0’’ position in

a counter-clockwise direction (typically 90°- 180 - 270).

The ‘‘0’’ starting position of the component is signiﬁcant.

Tape and reel and JEDEC tray packaged devices for

example, have an established standard for orientation.

The tape-and-reel packaged devices have a predetermined

orientation that is related to the perforated pattern on one

edge of the embossed tape carrier. The standard orientation

does vary, however, between unique device families.

Passive and active devices are supplied in a tape and reel

format, held and protected within an embossed pocket.

Each device family or package type has a standard orienta-

tion in relation to the perforated indexing pattern at the

tape edge.

Orientation as well as polarity of a device must be deﬁned

in the CAD database if the output transferred to assembly

systems is to be reliable. Resistors and capacitor devices

are common in orientation and have no deﬁned polarity. As

the designer develops the component database, numbers

are typically assigned to each end of the device to accom-

modate circuit routing and maintain orientation of value

marking or polarity. Tantalum capacitors, diodes, ICs and

other polarized components, for example, have unique ori-

entation in relation to tape feed systems. Consider the rela-

tionship of the device orientation within the tape cavity to

perforation at the tape carrier material edge.

7.4 Soldering Processes Like the selection of auto-

mated placement machines, the soldering process selection

depends upon the type of components to be soldered and

whether or not they will be used in combination with

leaded parts. For example, if all components are surface

mount types, reﬂow method (vapor phase, hot air convec-

tion or infrared) may be desirable. However, for through-

hole and surface mount combinations, in mixed technology,

a combination of wave soldering and reﬂow soldering may

be used. No process is best for all soldering tasks. In addi-

tion, the number of soldering processes discussed in the

following text are by no means complete.

7.4.1 Wave Soldering Wave soldering is an economical

method of soldering mass terminations. There are ﬁve to

six main process variables that must be controlled in the

wave soldering process: ﬂuxing, preheat, conveyor speed,

conveyor incline, solder temperature, and possibly cooling

rate.

In preheat, allowance in the conveyer system must be made

for the thermal expansion of the board during preheating

and soldering to prevent board warpage.

In ﬂuxing, ﬂux density, activity and ﬂux foam/ﬂux spray/

ﬂux wave height must be closely monitored. A system must

be in place to determine when the ﬂux activity has deterio-

rated and when the old ﬂux must be replaced and the new

ﬂux added.

Speed is the time sequence and duration of all of the steps

in soldering. By controlling the speed, more uniform and

better joints result. In controlling the conveyer speed, pre-

heating a packaging and interconnecting assembly in two

or three stages minimizes the thermal shock damage to the

assembly and improves its service life. Uniform preheating

is achieved by developing a solder schedule that speciﬁes

preheat settings and conveyer speed for each type of board.

The solder wave is an important variable. Wave geometry

is especially important for preventing icicles and bridges

and for the proper soldering of surface mounted compo-

nents. Wave geometries include uni-directional and

bi-directional; single and double; rough, smooth and dead

zone; oil intermix, dry, and bubbled, and with or without a

hot air knife. Special solder waves just for surface mounted

components are also available.

The concern generally expressed in wave soldering of sur-

face mount devices is damage to the components when

they go through the soldering wave at 260 °C [500 °F]. The

maximum shift in tolerance of resistors and capacitors is

generally found to be 0.2%. This is a negligible amount

considering the part tolerance of commonly used compo-

nents is 5% to 20%. The components generally spend about

three seconds in the wave but they are designed to with-

stand soldering temperatures of 260 °C [500 °F] for up to

ten seconds.

In wave soldering, outgassing and solder skips are two

other main concerns. The outgassing or gas evolution

occurs on the trailing terminations of chip resistors and

capacitors. It is believed to be caused by insufficient drying

of ﬂux and can be corrected by raising the packaging and

interconnecting assembly preheat temperature or time. The

other concern, solder skips, is caused by the shadow effect

of the part body on the trailing terminations. Orienting the

part in such a way that both terminations are soldered

simultaneously solves most shadow effect problems. Some

manufacturers use an extra land to serve as a ‘‘solder

thief’’ for active components.

The most common method for solving both outgassing and

shadow effect is by switching to the dual wave system

where the ﬁrst wave is turbulent and the second wave is

laminar. The turbulent wave serves to provide an adequate

amount of solder across the surface of the packaging and

interconnecting structure in order to help eliminate outgas-

sing and solder skips. The laminar wave is used to help

eliminate icicles and bridging.

7.4.2 Vapor Phase (VP) Soldering Vapor phase solder-

ing, also known as condensation soldering, uses the latent

heat of vaporization of an inert liquid for soldering. The

IPC-7351 February 2005

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latent heat is released as the vapor condenses on the part to

be soldered. The soldering temperature is constant and is

controlled by the type of ﬂuid.

Unlike wave, IR, convection and laser soldering, vapor

phase soldering does not require control of the heat input

to the solder joints or to the board. It heats independently

of the part geometry, heats uniformly, and does not exceed

the ﬂuid boiling temperature. This process is also suitable

for soldering odd-shaped parts, ﬂexible circuits, and pins

and connectors, as well as for reﬂow of tin-lead electro-

plate and surface mount packages. Since heating is by con-

densation, the rate of temperature rise depends on the mass

of the part. Therefore, the leads on the package in contact

with the packaging and interconnecting structure heat up

faster than the component body. This may lead to wicking

of the solder up the lead. Before exposing the loaded

assembly to VP reﬂow process, preheating the assembly is

highly recommended to avoid thermal shock to compo-

nents and the PCB.

7.4.3 IR Reflow Soldering

In infrared (IR) reﬂow soldering, the radiant or convective

energy is used to heat the assembly. There are basically two

types of IR reﬂow methods, either focused (radiant) or

nonfocused (convective). The latter is proving more desir-

able for SMT. The focused IR radiates heat directly on the

parts and may unevenly heat assemblies. The heat input on

the part may also be color-dependent. In nonfocused or

diffused IR, the heating medium can be air or an inert gas

or simply the convection energy. A gradual heating of the

assembly is necessary to drive off volatiles from the solder

paste. After an appropriate time in preheat, the assembly is

raised to the reﬂow temperature for soldering and then

cooled.

7.4.4 Hot Air/Gas Convection Soldering The reﬂow

process affects soldering by transporting the boards through

a stream of heated gas (e.g., air, nitrogen). Heat is trans-

ferred to the components and board by conduction from the

gas. Because the boards do not receive signiﬁcant direct

radiation from the heating source, convection soldering

avoids the shadowing problems that can occur with infra-

red soldering machines, especially short wavelength (lamp)

versions. This enables more uniform heating and a higher

component density on the board compared to other mass

reﬂow soldering methods. The gas temperature controls the

maximum temperature that can be seen by the assembly.

Use of a nitrogen atmosphere permits better thermal cou-

pling between the circulating gas and the component termi-

nations. In addition to improved wetting, the process win-

dow for double-sided reﬂow is enlarged, and lower

activated solder paste ﬂux can be used.

7.4.5 Laser Reflow Soldering Laser soldering comple-

ments other mass soldering processes rather than replacing

them and, as with in-line reﬂow soldering, lends itself well

to automation. It is faster than hand soldering but not as

fast as wave, vapor, IR soldering or hot air convection.

Heat-sensitive components that may be damaged in reﬂow

processes can be soldered by laser. Process problems

include thermal damage to surrounding areas and solder

balls.

7.4.6 Conduction Reflow Soldering Conduction reﬂow

affects soldering through the transference of heat from

beneath the PCB. This can have advantages with high mass

components, temperature sensitive components and metal

backed assemblies. In comparison with other solder pro-

cesses, the slightly slower heating and cooling ramp times

caused by heat spreading through the PCB substrates can

provide a reduction of thermal shock and improved resis-

tance against rapid cooling issues such as tombstoning.

Though in-line conduction reﬂow ovens are available, the

most common use of conductive reﬂow is in ‘‘hot plate’’

rework systems.

For more detail regarding reﬂow soldering refer to IPC-

7530.

7.5 Cleaning Flux requiring solvent cleaning—synthetic

or rosin-based ﬂuxes are generally known as synthetic acti-

vated (SA), synthetic mildly activated (SMA), rosin acti-

vated (RA) or rosin mildly activated (RMA). Stabilized

halogenated hydrocarbon/alcohol azeotropes are the pre-

ferred solvents for removal of synthetic and rosin-based

ﬂux residues.

7.6 Repair/Rework The repair/rework of surface mount

assemblies requires special care in design and practice.

Because of the small land geometries, heat applied to the

board should be minimized. There are various tools avail-

able for removing components. Resistance heating twee-

zers are usually used for removing surface mounted com-

ponents. Various types of hot air/gas and IR systems are

also used for removing surface mounted components. One

of the main issues when using hot air/gas devices is pre-

venting damage to adjacent components. Refer to IPC-

7711/21. There are four basic requirements for a successful

rework; good printed board design layout, selection of the

correct rework equipment or tools, sufficient manual skill,

and adequate training.

Successful removal of large multi-leaded integrated circuit

packages involves the use of hot gas or heated electrode

tools. Sufficient clearance around the package to permit the

re-work is essential. Clearance should be provided com-

pletely around the device as identiﬁed in the standards as

the ‘‘courtyard manufacturing zone.’’

7.6.1 Heatsink Effects Large ground planes or heatsinks

will conduct heat away from the component being

reworked if present in a printed board substrate. Extra heat,

February 2005 IPC-7351