IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第50页

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 th…

6.1.1 Categories In general, a PCB structure will ﬁt into

one of four basic categories of construction: organic base

material, nonorganic base material, supporting plane, and

constraining core.

6.1.2 Thermal Expansion Mismatch A primary concern

when using low expansion surface mount parts is the ther-

mal expansion mismatch between the leadless part and the

PCB structure. This mismatch will fracture solder joint

interconnections if the assembly is subjected to thermal

shock, thermal cycling, power cycling and high operating

temperatures. The number of fatigue cycles before solder

joint failure depends on the thermal expansion mismatch

between the part and the PCB structure, the temperature

range over which the assembly must operate, the solder

joint thickness, the size of the part and the power cycling.

For example, power cycling may cause an undesirable ther-

mal expansion mismatch if a signiﬁcant temperature differ-

ence exists between a device or package and the PB struc-

ture.

6.2 Organic-Base Material Organic-base materials work

best with leaded chip carriers. With leadless chip carriers

and some BGA packaging, the thermal expansion mis-

match between package and substrate can cause problems.

Also, ﬂatness, rigidity, and thermal conductivity require-

ments may limit their use. Finally, attention should be paid

to package size, I/O count, thermal cycling stability, maxi-

mum operating temperature and solder joint compliance.

6.3 Nonorganic Base Materials Nonorganic ceramic

base materials typically used with thick- or thin-ﬁlm tech-

nology, although more costly, are suited for leaded and

leadless chip carrier designs. Suppliers can incorporate

thick- or thin-ﬁlm resistors directly on the ceramic struc-

ture and buried capacitor layers that increase density and

improve reliability. However, repairability of the PCB

structure is limited. Ceramic materials, usually alumina,

appear ideal for PCB structure with leadless ceramic chip

carriers because of their relatively high thermal conductiv-

ity. Unfortunately, the structure is limited to approximately

100 mm square. Ceramic PCB structures have three pri-

mary applications: ceramic hybrid circuits, ceramic multi-

chip modules (MCM-L) and ceramic printed boards.

6.4 Alternative PCB Structures

6.4.1 Supporting-Plane PCB Structures

Supporting

metallic or nonmetallic planes can be used with conven-

tional printed boards or with custom processing to enhance

PCB properties. Depending on the results desired, the sup-

porting plane can be electrically functional or not and can

also serve as a structure stiffener, heatsink and/or CTE con-

straint.

6.4.2 High-Density PCB Technology High-density,

sequentially processed, multilayer PCB structures are avail-

able in a wide variety of organic dielectrics. Using thinner

copper foils for fabrication the board manufacturer can pro-

vide very narrow conductor and spacing features and by

implementing smaller mechanical drills, laser ablation,

photo-lithography or plasma processes, smaller blind

and/or buried vias can be provided for layer-to-layer inter-

connections.

The major advantage of this system is that the vias can be

as small as 0.10 mm [0.00394 in] or less and conductor

widths can range from below 0.12 mm [0.00472 in] for

high interconnection density. Thus, some applications can

be satisﬁed with fewer signal layers while providing addi-

tional layers for power and ground. Refer to IPC-2226 for

more detailed design guidelines for high density PCB.

6.4.3 Constraining Core Structures As with supporting

plane, one or more supporting metallic or nonmetallic

planes can serve as a stiffener, heatsink, and/or CTE con-

straint in constraining core structures.

6.4.4 Porcelainized Metal (Metal Core) Structures An

integral core of low-expansion metal (for example, copper-

clad Invar) can reduce the CTE of porcelainized metal

structures so that it closely matches the CTE of the ceramic

chip carrier. Also, the structure size is virtually unlimited.

However, the low melting point of the porcelain requires

low-ﬁring-temperature conductor, dielectric and resistor

inks.

7 ASSEMBLY CONSIDERATION FOR SURFACE MOUNT

TECHNOLOGY (SMT)

The smaller size of surface mount components and the

option of mounting them on one or both sides of the pack-

aging and interconnecting structure reduces board real

estate signiﬁcantly. The type of SMT assembly is basically

determined by the type of surface mount components to be

used; see 7.1 for a description of types and classes.

7.1 SMT Assembly Process Sequence The SMT assem-

blies are soldered by reﬂow (infrared, hot air convection,

laser, conduction, vapor phase, and/or wave soldering pro-

cesses) depending upon the mix of surface mount and

through-hole mount components. The process sequence for

one-sided SMT is shown in Figure 7-1. Solder paste is

applied, components are placed, the assembly is reﬂow sol-

dered and cleaned. For two-sided SMT assemblies, the

board is turned over and the process sequence just

described is repeated. The assembly process for two-sided

SMT is simply a sequential combination of SMT processes,

however, component weight vs. surface tension should be

calculated to determine if heavy components will require

additional reinforcement prior to the second reﬂow solder-

ing process.

February 2005 IPC-7351

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