IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第51页

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 process sequence for surface mount with through-hole

or pin-in-hole (PIH) component technology is shown in

Figure 7-2. Adhesive is applied and the surface mounted

components placed. The adhesive is then cured, and the

board is inverted to receive the through-hole component

leads automatically or by hand insertion. After lead clinch-

ing (if required), and with the through-hole components on

top and the surface mount components beneath, the board

is typically wave soldered. An alternative sequence is to

reverse the initial stages i.e., insert (and clinch) the

through-hole components before attaching the surface

mounted components and then wave soldering.

Finally the assembly may be cleaned, inspected, repaired if

necessary, and tested, though not necessarily in that order.

7.2 Substrate Preparation

7.2.1 Adhesive Application In wave soldering surface

mount components, selection and application of adhesive

plays a critical role. With too much adhesive, the adhesive

ﬂows onto lands resulting in poor solder ﬁllets. Too little

adhesive will fail to accomplish its objective of holding

parts to the bottom of the board during wave soldering.

7.2.2 Conductive Adhesive Some applications for SMT

attachment use conductive adhesive as the attachment

material. Unlike solder paste which is redistributed when

reﬂowed, conductive adhesives must be properly controlled

to ensure joint strength. Also, component placement must

be controlled in order to prevent excessive adhesive

squeeze-out, and possible shorts to adjacent lands.

7.2.3 Solder Paste Application Solder paste plays an

important role in reﬂow soldering. The paste tacks the

component before reﬂow. It contains ﬂux, solvent, sus-

pending agent, and alloy of the desired composition. Sol-

der paste is applied on the lands before component place-

ment either by screening, stenciling, or syringe. Screens are

made from stainless steel or polyester wire mesh, and sten-

cils are etched stainless steel, brass, and other stable alloys.

Stencils are preferred for high-volume applications. They

are more durable than screens, easier to align, and can be

used to apply a thicker layer of solder paste, and, where

narrow, point apertures are required for example, for ﬁne-

pitch lands. Electroformed stencils may be required for

very small components such as 0201 capacitors and resis-

tors.

The goal of the technology that’s employed to make the

stencil is to ensure that this transfer is as efficient and com-

plete as possible. There are several post processes that

enhance the stencil’s performance, including electropolish-

ing and trapezoidal section apertures that are created with

laser cut technology

7.2.3.1 Laser Cut Stencil Development Laser cut sten-

cils are produced directly from the customer’s original Ger-

ber ﬁle or GenCAM or IPC-2581 data. Eliminating the

IPC-7351-7-01

Figure 7-1 Typical Process Flow for Full Surface Mount Type 1b and 2b Surface Mount Technology

Basic Assembly Process for SMT

Solder

Printing

Assembly

Test

Package

and Ship

Inspection

Reflow

Solder Process

Clean

and

Inspection

Device

Placement

When cleaning is required.

Test may include functional, in-circuit, burn-in as well as post assembly programming.

IPC-7351 February 2005

标准分享网 www.bzfxw.com 免费下载

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

标准分享网 www.bzfxw.com 免费下载