IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第53页

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 solderi…

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

标准分享网 www.bzfxw.com 免费下载

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

perhaps for longer periods, is then required which, in turn,

can lead to damage to components or the board. The fact

that the solder joints may not reach reﬂow temperature is

no guarantee that the component or the board have not

been overheated. Heatsinking effects is a design problem

which must be tackled at the printed board layout stage.

Whenever possible, any component termination which may

not rework, including leaded-through hole type, should be

thermally isolated from any ground plane or integral heat-

sink by a short length of copper conductor.

7.6.2 Dependence on Printed Board Material Type To

ensure minimum damage to the printed board during

rework, base laminate should be a good quality resin and

reinforcement type from a high copper peel strength mate-

rial. High packing density is required. The use of inferior

laminates can easily lead to problems with lands peeling

away during rework. This may result in either scrapping of

complete assemblies or expensive repair of damaged cop-

per area. For boards having high thermal mass such as

middle-core types or those with large area ground planes,

to avoid employing a tool with high heat input rate, the use

of a hot plate to provide background heating is essential.

7.6.3 Dependence on Copper Land and Conductor Lay-

out

The space on a board is at a premium or single con-

ductors must be kept very short. Designers will often route

a conductor between adjacent device land space at a pitch

of the component device being placed. In such cases, con-

ductors should be covered with a soldermask to minimize

the risk of lifting conductors during rework operations.

Routing conductors between lands at 1.0 mm pitch and

below increases the risk of damage to the conductors dur-

ing the rework operation. For more detail regarding devel-

opment, planning and trouble shooting the steps involved

in the process of producing surface mount assemblies, refer

to IPC-S-816.

8 IPC-7352 DISCRETE COMPONENTS

Discrete components are generally purchased in 8 mm and

12 mm wide tape and reel (see Figure 8-1). EIA-481 is the

applicable speciﬁcation for tape and reel. Consult your

manufacturers guide for the packaging availability of your

component.

Parts susceptible to damage by electrostatic discharge shall

be supplied in a manner that prevents such damage. Tape

peel strength shall be 40 grams ± 30 grams. Peel from the

top for the top cover of the tape. Reel materials used in the

construction of the reel shall be easily disposable metal,

chip board, styrene plastic or equivalent. Reels shall not

cause deterioration of the components or their solderability.

Reels must be able to withstand high humidity conditions.

Parts must be capable of withstanding cleaning processes

currently used by board assembly manufacturers. This may

include as a minimum four-minute exposures to solvent

cleaning solutions at 40 °C [104 °F], plus a minimum of a

one-minute exposure to ultrasonic immersion at a fre-

quency of 40 kHz and a power of 100 watts per square

foot. Alkaline systems in use shall also not damage parts or

remove markings.

End terminations should be solder coated with a tin/lead

alloy or a lead free equivalent. The solder should contain

between 58 to 68% tin. Solder may be applied to the ter-

mination by hot dipping or by plating from solution. Plated

solder terminations should be subjected to a postplating

reﬂow operation to fuse the solder. The tin/lead ﬁnish

should be at least 0.0075 mm [0.0003 in] thick.

The termination shall be symmetrical, and shall not have

nodules, lumps, protrusions, etc., that compromise the sym-

metry or dimensional tolerances of the part. The end termi-

nation shall cover the ends of the components, and shall

extend out to the top and bottom of the component.

Solder ﬁnish applied over precious metal electrodes shall

have a diffusion-barrier layer between the electrode metal-

lization and the solder ﬁnish. The barrier layer should be

nickel or an equivalent diffusion barrier, and should be at

least 0.00125 mm [0.00005 in] thick.

For lead free ﬁnishes a combination of tin, silver and cop-

per is the prevalent replacement for the tin/lead ﬁnish. Sol-

derability testing should be applied per IPC-J-STD-002 to

determine attachment capability of the applicable compo-

nent type.

The following sections for each component family provide

information on basic component construction, termination

materials, marking, carrier package format and resistance

to soldering.

8.1 Chip Resistors (RESC) A variety of values exist for

resistors. This section describes the most common types.

IPC-7351-8-01

Figure 8-1 Packaging of Discrete Components

Top Cover

Tape

Sprocket

Hole

Component

Cavity

Embossed Carrier Tape

Sprocket Hole

Embossed

Carrier Tape

Component Cavity

▼

IPC-7351 February 2005

标准分享网 www.bzfxw.com 免费下载