IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第32页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ making calculations, it should be noted that the solder con- tent of most pastes is 50% to 55% by volume (not by weight), depending on particle size. If the amount of solder paste to be deposited is less th…

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SMT printed board. With it, the exact location and volume

of solder paste deposition is precisely controlled. The art-

work for developing the stencil is the component mounting

lands from the outer layers of the board with all other cir-

cuitry deleted. The information provided for the openings

in the stencil is usually indicated as being the same size as

the lands on the board for all components. This information

(or data) may be selectively modiﬁed by the printed board

assembler or process engineer who will deﬁne the speciﬁc

adjustments required to meet speciﬁc solder volume

requirements (see IPC 7525).

The optimum stencil thickness is determined by evaluating

the solder paste requirements for all the components to be

reﬂow soldered. This should be based on study of the mini-

mum and target requirements for SMT solder joints given

in IPC/EIA-J-STD-001.

Ideally, the volume deposited should be the total amount

required to achieve the ‘‘target’’ solder joint condition (see

IPC-A-610), less the solder already available on the land

and termination or lead (the latter can together amount to

10% to 20% of the total and should not be ignored). In

IPC-7351-3-06

Figure 3-6 Component Orientation for Wave-Solder Applications

Preferred Orientation

Defect Prone Orientation

Solder

bridging

Solder

Wave

Solder

Wave

IPC-7351-3-07

Figure 3-7 Alignment of Similar Components

Consistent

Orientation

Pin 1 I.D. Uniform

Spacing

February 2005 IPC-7351

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making calculations, it should be noted that the solder con-

tent of most pastes is 50% to 55% by volume (not by

weight), depending on particle size.

If the amount of solder paste to be deposited is less than

the amount provided by using an aperture at or near the

land area size, a reduced area of print should be placed in

the best position on the land to assure good wetting of the

joint areas. In some cases, this may best be achieved by

reducing the width of the print, in others, the length. For

very ﬁne pitch with inter-land gaps of less than 0.2 mm,

staggering (offsetting) the print at alternate ends of the

lands can reduce the risk of shorts after soldering.

If the amount of solder paste required is more than the

amount available using the geometry provided for the basic

land pattern on the PCB the aperture size in the stencil can

be enlarged to increase solder volume. The amount and

direction of the overhang of the printed solder beyond the

land is dictated by the space available around the land and

the need to avoid shorting and solder bridging if excess

overhang occurs. The tolerances on land position and print-

ing accuracy need to be considered when calculating the

maximum allowable over-print. See IPC-7525 for the

design and fabrication of stencils for surface mount solder

paste application.

3.4.3 Component Stand-off Height for Cleaning The

recommended minimum component stand-off height for

cleaning is affected by the distance across the diagonal of

the component lead pitch.

If a minimum stand-off cannot be achieved, proper clean-

ing under the component may not be possible. In this case,

it is recommended that a no-clean ﬂux be used and/or mask

material should be retained over all exposed via and circuit

patterns located under devices.

3.4.4 Fiducial Marks A ﬁducial mark is a printed art-

work feature created in the same process as the circuit art-

work for optical recognition systems. The ﬁducial and a

circuit pattern artwork must be etched in the same step.

The ﬁducial marks provide common datum points for all

steps in the assembly process. This allows each piece of

equipment used for assembly to accurately locate the cir-

cuit pattern. There are two types of ﬁducial marks.

3.4.4.1 Panel and Global Fiducials Global ﬁducial

marks are used to locate the position of all circuit features

on an individual board. When a multi-image circuit is pro-

cessed in panel form, the global ﬁducials are referred to as

panel ﬁducials (see Figure 3-8).

A minimum of two global ﬁducial marks is required for

correction of offsets (x and y position) and rotational off-

sets (theta position). These should be located diagonally

opposite and as far apart as possible on the circuit or panel.

A minimum of three ﬁducial marks is required for correc-

tion of nonlinear distortions (scaling, stretch and twist).

These should be located in a triangular position as far apart

as possible on the circuit or panel.

3.4.4.2 Local Fiducials Local ﬁducial marks are used to

locate the position of an individual component requiring

more precise placement.

A minimum of two local ﬁducial marks are required for

correction of translational offsets (x and y position) and

rotational offsets (theta position). This can be two marks

located diagonally opposed within or outside the perimeter

of the land pattern (see Figure 3-9).

It is good design practice to locate global or panel ﬁducials

in a three-point grid-based datum system as shown in Fig-

ure 3-10. The ﬁrst ﬁducial is located at the 0-0 location.

The second and third ﬁducials are located in the X and Y

directions from 0-0 in the positive quadrant. The global

ﬁducials should be located on the top and bottom layers of

all printed boards that contain surface mount as well as

through-hole components since even through-hole assem-

bly systems are beginning to utilize vision alignment sys-

tems.

All ﬁne pitch components should have two local ﬁducial

systems designed into the component land pattern to insure

that enough ﬁducials are available every time the compo-

nent is placed, removed and/or replaced on the board. All

ﬁducials should have a soldermask opening large enough to

keep the optical target absolutely free of soldermask. If

soldermask should get onto the optical target, some vision

alignment systems may be compromised due to insufficient

contrast at the target site.

If space is limited, one may be able to share a ﬁducial from

an adjacent component within the location constraint (see

Figure 3-10).

3.4.4.3 Size and Shape of Fiducial The optimum ﬁdu-

cial mark is a solid ﬁlled circle. The preferred diameter of

IPC-7351-3-08

Figure 3-8 Panel/Local Fiducials

Panel Fiducial

▼

Global Fiducial

▼

IPC-7351 February 2005

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IPC-7351-3-09

Figure 3-9 Local Fiducials

Two Corner Fiducial Targets

per FP Device is Preferred

IPC-7351-3-10

Figure 3-10 Fiducial Locations on a Printed Board

Global fiducial targets

Local fiducial targets for Fine Pitch ICs

F1 F3

Sharing of fiducials

among adjacent

components

February 2005 IPC-7351