IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第38页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ IPC-7351-3-17 Figure 3-17 Conductor Description ▼ Laminate X ▼ Etch Factor = V X An etch factor of 1/1 is usually considered practical. Higher factors may be specified for some applications. Resist ▼ V ▼ ▼ …

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3.4.6.5 Vias as Test Points Via holes, in addition to

being used for connecting surface mounted component

lands to conductor layers, may also be used as test targets

for bed-of-nails type probes and/or rework ports. When a

via is used as a test point it is required that the x-y location

and size of a test land be deﬁned as a secondary ﬁle for test

ﬁxture development.

3.4.7 Standard PCB Fabrication Allowances Manufac-

turing tolerances or standard fabrication allowances (SFA)

exist in all PCB fabrication shops. Virtually every registra-

tion or alignment operation that is performed has some

potential for misregistration. There are approximately 42

basic steps in fabricating a multilayer PCB, several of

which involve operations that require precision in location

and alignment. The tolerance varies according to the

printed board maximum diagonal dimension and must be

included in the land size calculations. The fabricator should

be consulted prior to beginning a design to determine their

SFA. With this SFA value, the designer can proceed

accordingly, preventing tolerances from stacking up and

creating yield and/or production problems.

3.4.7.1 PCB Manufacturing Characteristics Figure

3-17 shows the various characteristics of conductor geom-

etry after etching. End-product drawings and speciﬁcations

should specify only the minimum for conductor spacing;

however, conductor widths should be deﬁned according to

minimum values, where land patterns should be deﬁned as

to their maximum material conditions (MMC). Clear target

values for conductors and land patterns will help the manu-

facturer achieve the desired condition.

3.4.7.2 Conductor Width Tolerances Table 3-17 repre-

sents process tolerances that can be expected with normal

processing. (Speciﬁc process tolerances should be dis-

cussed with the board manufacturer.) The bilateral toler-

ances in Table 3-17 are typical for 0.046 mm [0.00181 in]

copper. For additional copper thickness, a further width

variation can be expected (see Figure 3-17).

3.4.7.3 Conductive Pattern Feature Location Toler-

ance

Table 3-18 is for the tolerance to be applied to the

nominal dimension chosen for the location of the lands,

connector contacts and conductors in relation to the datum

reference. These tolerances include master pattern accu-

racy, material movement, layer registration and ﬁxturing.

3.4.7.4 Annular Ring Control Annular ring is deﬁned as

the amount of land that remains after a hole is drilled

through it. With high-density SMT designs, maintaining

minimum annular requirements has emerged as one of the

most difficult parts of multilayer PCB fabrication in terms

of producibility. Perfect registration will maximize the

annular ring all around the drilled hole.

For example, using a 0.8 mm [0.0315 in] land with a 0.5

mm [0.0197 in] drill will result in a 0.15 mm [0.00591 in]

annular ring under perfect registration conditions. If mis-

registration of 0.15 mm [0.00591 in] occurs in any direc-

tion, the result will be a 0.3 mm [0.012 in] annular ring on

one side of the pad, and no annular ring on the other side.

If misregistration is greater than 0.15 mm [0.00591 in], i.e.,

0.2 mm [0.0079 in], then the drill will actually break out of

the land. If the breakout is in the direction where the con-

ductor connects to the land, the drill will effectively discon-

nect the conductor from the land. The net result is a

scrapped PCB. Since signal conductors intersect the lands

from all directions, any breakout has the potential to ran-

domly disconnect conductors all over the PCB.

Maintaining consistent annular ring control is difficult, but

methods have been developed to insure connectivity

between lands and conductors. Each method is intended to

provide copper material where the conductor enters the

land. The land which has the added material may resemble

a teardrop or keyhole or adopt alternate designs as shown

in Figure 3-18.

3.4.8 Panelization Components can be mounted on indi-

vidual boards or on boards that are organized in a panel

form. Boards or panels that will be moved by automatic

handling equipment or pass through automated operations

(parts placement, soldering, cleaning, etc.) must have spe-

ciﬁc areas kept free of parts or active circuitry. Typically, a

clear area of 3.0 mm [0.012 in] to 5.0 mm [0.0197 in] wide

must be allowed along the sides for the clearance. The

required clearance width is dependent upon the design of

IPC-7351-3-16

Figure 3-16 Vias Under Components

Caution

Solder

Characteristic

Solder

Migration

▼

Wave Solder

▼

February 2005 IPC-7351

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IPC-7351-3-17

Figure 3-17 Conductor Description

▼

LaminateX

▼

Etch Factor =

An etch factor of 1/1 is usually considered practical.

Higher factors may be specified for some applications.

Resist

▼

"B" (DCW)

"E" Outgrowth

▼

"C" Undercut

▼

"A"

(MCW)*

Overhang

Panel Plating (Liquid Resist)

▼

(OCW)*

▼

"D"

"B" (DCW)*

"E" Outgrowth

▼

(MCW)*

"A"

▼

Pattern Plating (Liquid Resist)

▼

"B" (DCW)

(MCW)*

"C" Undercut

& Overhang

Panel Plating (Dry Film Resist)

▼

"A"

▼

"D"

▼

"A"

(MCW)*

Overhang

Pattern Plating (Dry Film Resist)

▼

(OCW)*

"B" (DCW)*

▼

"E" Outgrowth

▼

"C" Undercut

▼

"B" (DCW)*

▼

"C" Undercut

▼

Thin Clad & Pattern Plating (Dry Film Resist)

A = MCW (Minimum Conductor Width) B = DCW (Design Conductor Width) D = OCW (Overall Conductor Width)

Table 3-17 Conductor Width Tolerances,

0.046 mm [0.00181 in] Copper, mm [in]

Feature

Producibility

Level A

Producibility

Level B

Producibility

Level C

Without

plating

± 0.06 mm

[± 0.00236 in]

± 0.04 mm

[± 0.00157 in]

± 0.015 mm

[± 0.0005906 in]

With

plating

± 0.10 mm

[± 0.00393 in]

± 0.08 mm

[± 0.00314 in]

± 0.05 mm

[± 0.0197 in]

Table 3-18 Feature Location Accuracy (units: mm [in])

Greatest

Board/ X,Y

Dimension

Producibility

Level A

Producibility

Level B

Producibility

Level C

Up to 300

[11.81]

0.30

[0.012]

0.20

[0.00787]

0.10

[0.00394]

Up to 450

[17.72]

0.35

[0.0138]

0.25

[0.00984]

0.15

[0.00591]

Up to 600

[23.62]

0.40

[0.0157]

0.30

[0.012]

0.20

[0.00787]

IPC-7351-3-18

Figure 3-18 Examples of Modiﬁed Landscapes

Corner Entry

Filleting

Key Holing

IPC-7351 February 2005

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the board handling and ﬁxturing equipment. These dimen-

sions should be obtained from the process equipment

manufacturer before board or panel design (see Figure

3-19).

Special tooling and ﬁxturing holes are generally located

within the edge clearance areas. The clearance areas are

needed to avoid interference with board handling ﬁxtures,

guidance rails and alignment tools.

For accurate ﬁxturing, two or more nonplated holes are

located in the corners of the board to provide accurate

mechanical registration on board transfer equipment. Board

handling holes (typically 3.2 mm [0.126 in]) may also be

located in the clearance areas. These holes may be used by

automated board handling equipment or for test ﬁxture

alignment. Speciﬁc panel size should be obtained from the

equipment manufacturer or process engineer.

3.4.8.1 Board Size and Panel Construction In order to

fully utilize the automation technology associated with

surface mount components, a designer should consider how

a printed board or P&I structure will be fabricated,

assembled and tested. Each of these processes, because of

the particular equipment used, may require ﬁxturing, which

will affect or dictate certain facets of the board layout.

Tooling holes, panel size, component orientation and clear-

ance areas (both component and conductor) on the primary

and secondary sides of the board are all equipment and

process dependent.

To produce a cost-effective layout through optimum base

material utilization, a designer should consult with the

board manufacturer to determine optimum panel size. The

board should be designed to utilize the manufacturer’s sug-

gested usable area. Smaller boards can be ganged or nested

in a uniform panel format to simplify ﬁxturing and reduce

excessive handling during assembly. Panel layout is typi-

cally deﬁned by an assembly process specialist or the

manufacturing service provider.

IPC-7351-3-19

Figure 3-19 Typical Copper Glass Laminate Panel

▼

The keepout zone defined in this illustration is typical for in-line assembly automation using reflow and wave solder processes.

▼

Typical Finished Panel

for Automated SMT

Assembly Equipment

Showing two

Printed Board Assemblies

▼

300mm

[12.0 in]

▼

3.0mm

[0.12 in]

▼

5.0 mm

[2.0 in]

▼

10 mm [0.40 in]

▼

5 mm [0.20 in]

▼▼

▼

3.0 mm

[0.12 in]

Keep Clear

▼

Primary Component Side

Secondary Component Side

▼

12.0 mm

max.

[0.500 in]

5mm

[2.0 in] min.

DIRECTION OF FLOW

KEEPOUT ZONE

▼

February 2005 IPC-7351