IPC-SM-782A-表面贴装焊盘图形设计标准.pdf.pdf - 第30页

grid based placement rules of the THT boards. This ulti- mately results in components being randomly placed, and vias being even more randomly placed across the board. T wo problems created by random component placement …

the design is memory, general logic, or analog, it is recom-

mended that all component orientations are such that pin

one orientation is the same.

3.6.1.4 Grid-Based Component Placement SMT com-

ponent placement and orientation is generally more difficult

than THT printed boards for two reasons: higher compo-

nent densities, and the ability to put components on both

sides of the board. For THT designs, the component leads

are on 2.54 mm [0.100 in] centers and, assuming 1.3 mm

[0.065 in] lands, the spacing between lands would be 1.2

mm. In high density SMT designs, however, the spacing

between lands is often less, down to 0.63 mm [0.025 in]

and smaller. Grid based component placement (0.100 inch

grid is standard with THT) is complicated by the large

variety of land sizes associated with the SMT component

packages now becoming available. Most SMT designs

being done today have abandoned the 2.54 mm [0.100 in]

IPC-782-3-8

Figure 3–8 Recommended minimum land-to-land clearances

Maximize View

Angle

for

Solder

Joint

Inspection

2.5 mm (0.100 in) Minimum

▼

1.25 mm (0.050 in)

1.5 mm (0. 060 in.)

0.63 mm (0.025 in.)

1.0 mm (0.040 in.)

1.0 mm

(0.040 in.)

▼

0.63 mm

(0.025 in.)

1.25 mm

(0.050 in)

1.5 mm (0.060 in.)

▼

DIP

IPC-782-3-9

Figure 3–9 Component orientation for wave solder applications

Wave Solder For SMT

Prefered IC orientation

Rubber pads will reduce

solder bridging

Typical solder

bridge locations

▼

Non Prefered IC orientation

December 1999 IPC-SM-782A

grid based placement rules of the THT boards. This ulti-

mately results in components being randomly placed, and

vias being even more randomly placed across the board.

Two problems created by random component placement

are a loss of uniform grid based test node accessibility and

a loss of logical, predictable routing channels on all layers

(possibly driving layer counts). In addition the accepted

international grid identiﬁed in IPC-1902 states that for new

designs the grid should be 0.5 mm, with a further subdivi-

sion being 0.05 mm. One solution to the problem is to

build CAD libraries with all component lands connected to

vias on 0.05 mm centers (or greater, based on design) to be

used for testing, routing, and rework ports. Then when

doing the component placement on the CAD system, sim-

ply place the components so that there is a minimum space

of 0.5 mm between lands, then snap the vias of the compo-

nent being placed out to the next 1.0 mm grid point. With

this procedure, all of the components should have between

0.4 mm and 0.6 mm (or an average of 0.5 mm) spacing

between the lands. From the assembly point of view, it is

easier to process a PCB which has the component centroids

on a 1.0 mm grid, with approximately equal spacing

between all of the lands across the board in both directions.

3.6.1.5 Single vs Double Sided Boards The term single

or double sided referred to one or two conductor layers on

a printed board prior to the advent of Surface Mount. Now,

however, the term single side refers to components

mounted on one side (Type 1 assembly), and the term

double refers to components mounted on both sides (Type

2 assembly) of the board. It has been observed that many

SMT designers, especially novices, are too quick to place

components on the secondary side of the PB, forcing the

assembly process to be executed twice instead of once.

Designers should concentrate on getting all components on

the primary side of the board whenever possible without

creating component spacing violations. This will result in a

lower assembly cost. If double sided placement is deﬁ-

nitely required, then grid-based placement, although more

difficult, is even more crucial for accurate ﬁnal component

placement, circuit routability, and testability. Double sided

boards using conventional SMT design rules often require

double sided, or clamshell test ﬁxtures that are 3 to 5 times

the cost of single sided test ﬁxtures. Grid based component

placements have been known to improve nodal accessibil-

ity as well as eliminate the need for dual sided testing.

3.6.1.6 Solder Stencil Design The solder stencil is the

primary vehicle by which solder paste is applied to the

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

of solder paste deposition is precisely controlled. The art-

work for the stencil generally consists of the component

mounting lands from the outer layers of the board with all

other circuitry deleted. The openings in the stencil should

be the same size as the lands on the board for all compo-

nents. The printed board assembler may alter the stencil

IPC-782-3-10

Figure 3–10 Alignment of similar components

Consistent

Orientation

PIN One I. D.

Uniform

Spacing

▼

PREFERRED LAYOUT FOR SMT

IPC-SM-782A December 1999

opening size prior to manufacturing the stencil to change

the volume of solder paste which is deposited on the lands.

3.6.1.7 Component Stand Off Height for Cleaning The

minimum component stand off height for cleaning is based

on the distance across the diagonal of the component. This

dimension implies a component surface area that may trap

contamination if care is not exercised. Table 3–7 shows the

relationship for recommended component standoff dis-

tances.

If the 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.

3.6.1.8 Fiducial Marks A Fiducial Mark is a printed art-

work feature which is created in the same process as the

circuit artwork. The ﬁducial and a circuit pattern artwork

must be etched in the same step.

The Fiducial Marks provide common measurable 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. These

are:

A. Global Fiducials

Fiducial marks used to locate the position of all circuit

features on an individual board. When a multi image

circuit is processed in panel form, the Global Fiducials

are referred to as Panel Fiducials when present for the

panel. (See Figures 3–11/3–12.)

B. Local Fiducials

Fiducial marks used to locate the position of an indi-

vidual component requiring more precise placement.

(See Figure 3–11.)

A minimum of two global ﬁducial marks is required for

correction of translational offsets (x and y position) and

rotational offsets (theta position). These should be located

diagonally opposed and as far apart as possible on the cir-

cuit or panel.

A minimum of three ﬁducial marks is required for correc-

tion of non linear distortions (scaling, stretch and twist).

These should be located in a triangular position as far apart

as possible on the circuit or panel.

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 the perimeter of the land

pattern.

If space is limited, a minimum of one local ﬁducial mark

may be used to correct translational offsets (x and y posi-

tion). The single ﬁducial should be located inside the

perimeter of the land pattern with a preference for the cen-

ter.

The minimum size for local, global or panel ﬁducials is 1.0

mm. Some companies have chosen a larger ﬁducial (up to

1.5 mm) for panel ﬁducials. It is a good practice to keep all

ﬁducials the same size.

Table 3–7 Component Stand Off

Component

Diagonal

Component

Surface Area

Component Stand

Off

< = 50 mm < = 2500 mm

> = 0.5 mm

<=25mm <=625mm

> = 0.3 mm

<=12mm <=144mm

> = 0.2 mm

<=6mm <=36mm

> = 0.1 mm

<=3mm <=9mm

> = 0.05 mm

IPC-782-3-11

Figure 3–11 Local/global ﬁducials

Global fiducial

Local fiducial

▼

IPC-782-3-12

Figure 3-12 Panel/global ﬁducials

Panel fiducial

▼

Global fiducial

▼

December 1999 IPC-SM-782A