IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第36页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ 3.4.6.3 Vias Under Components During wave soldering of the assembly , ﬂux may potentially become trapped under zero clearance devices. If the assembly is to be wave soldered, via holes underneath zero clear…

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IPC-7351-3-12

Figure 3-12 Use of Vias in High Component Density Printed Circuit Boards

Surface Mount

Componant Land Pattern

Internal Connection

Plated via for interface

between the outer circuit

layer and subsurface layer

or layers of the board.

External Surface

Routed Circuits

Internal Subsurface

Route Circuit Layer

IPC-7351-3-13

Figure 3-13 Conductor Routing Capability Test Pattern

2 Channel 2 Channel 3 Channel 3 Channel 4 Channel 5 Channel

February 2005 IPC-7351

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3.4.6.3 Vias Under Components During wave soldering

of the assembly, ﬂux may potentially become trapped

under zero clearance devices. If the assembly is to be wave

soldered, via holes underneath zero clearance components

on the primary side should be avoided on boards unless

vias are tented with soldermask. Untented via holes may be

located underneath zero clearance surface mount packages

in reﬂow soldered surface mount assemblies that will not

be exposed to wave soldering (see Figure 3-16).

3.4.6.4 Vias Within Lands Via holes within the surface

mount component attachment lands are permitted only if

the via is plated closed, ﬁlled or plugged and capped

(plated) in such a way as to prevent solder migration dur-

ing the component attachment process. A blind or plated

closed micro-via in the land is typically acceptable for sol-

der attachment of surface mount components. Refer to

IPC-2226 for minimum annular ring requirements for

microvias.

IPC-7351-3-14

Figure 3-14 Land Pattern to Via Relationship

▼

Square land

0.50 mm

[0.020 in] min

Chip Components and Feed Through Via Holes

0.50 mm

[0.020 in] min

Good Design

Poor Design

Feed through lands flush against contact area

▼

Too close

to contact

area

IPC-7351-3-15

Figure 3-15 Examples of Via Positioning Concepts

Side 2

Vias

Side 1

Vias

Side 1

Vias

Side 2

Vias

Chip Component

PLCC

SOIC

IPC-7351 February 2005

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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