IPC-SM-782A 表面安装设计和焊盘设计标准(带BGA).pdf - 第28页

3.6 Design Rules During the component selection phase of a design, manufacturing engineering should be consulted regarding any components outside the scope of this docu- ment. The printed board design principles are a st…

3.5.3 Service Life

The design service life, N, can vary

signiﬁcantly for the use categories in Table 3–6. The design

service lives can range from less than one year, barely

exceeding the warranty period for consumer products, to

20 years or more for telecommunications equipment and

commercial aircraft. For some military applications the ser-

vice life is measured in thousands of hours.

3.5.4 Acceptable Cumulative Failure Probability

The

acceptable cumulative failure probability, F(N), at the end

of the design service life, N, can vary signiﬁcantly depend-

ing on the speciﬁc purpose of the product, the complexity

(number and mix of components) of the product, and per-

haps the design service life. F(N) values could range from

1 ppm for products whose failure has critical conse-

quences, e.g., cardiac pacemakers, to perhaps 10,000 ppm

(1%) for consumer products or products which provide

redundancy or ‘‘limp-home capability’’ in case of electrical

system failure. (See IPC-SM-785).

Table 3–6 Worst-Case Environments and Appropriate Equivalent Accelerated Testing

USE CATEGORY

WORST-CASE USE ENVIRONMENT ACCELERATED TESTING

Tmin

°C

Tmax

°C

∆T

(1)

°C

hrs

Cycles/

year

Typical

Years

Service

Approx.

Accept.

Failure

Risk, %

Tmin

°C

Tmax

°C

∆T

(2)

°C

min

1) CONSUMER 0 +60 35 12 365 1-3 1 +25 +100 75 15

2) COMPUTERS +15 +60 20 2 1460 5 0.1 +25 +100 75 15

3) TELECOM - 40 +85 35 12 365 7-20 0.01 0 +100 100 15

4) COMMERCIAL

AIRCRAFT

-55 +95 20 12 365 20 0.001 0 +100 100 15

5) INDUSTRIAL &

AUTOMOTIVE

PASSENGER

COMPARTMENT

-55 +95 20

&40

&60

&80

185

100

10 0.1 0 +100 100 15

& COLD

(3)

6) MILITARY

GROUND &

SHIP

-55 +95 40

&60

100

265

10 0.1 0 +100 100 15

& COLD

(3)

7) SPACE leo

geo

-55 +95 3

to 100

8760

365

5-30 0.001 0 +100 100 15

& COLD

(3)

8) MILITARY

AVIONICS a

-55 +95 40

&20

365

10 0.01 0 +100 100 15

& COLD

(3)

9) AUTOMOTIVE

UNDER HOOD

-55 +125 60

&100

&140

1000

300

5 0.1 0 +100 100 15

& COLD

(3)

& LARGE ∆T

(4)

& = in addition

1) ∆T represents the maximum temperature swing, but does not include power dissipation effects; for power dissipation calculate ∆T;

power dissipation can make pure temperature cycling accelerated testing signiﬁcantly inaccurate. It should be noted that the cyclic

temperature range, ∆T, is not the difference between the possible minimum, T

MIN

, and maximum, T

MAX

, operational temperature

extremes; ∆T is typically signiﬁcantly less.

2) All accelerated test cycles shall have temperature ramps <20°C/minute and dwell times at temperature extremes shall be 15 min-

utes measured on the test boards. This will give ~24 test cycles/day.

3) The failure/damage mechanism for solder changes at lower temperatures; for assemblies seeing signiﬁcant cold environment

operations, additional ‘‘COLD’’ cycling, from perhaps –40 to 0°C, with dwell times long enough for temperature equilibration and for

a number of cycles equal to the ‘‘COLD’’ °C operational cycles in actual use is recommended.

4) The failure/damage mechanism for solder is different for large cyclic temperature swings traversing the stress-to-strain –20 to

+20°C transition region; for assemblies seeing such cycles in operation, additional appropriate ‘‘LARGE ∆T’’ testing with cycles

similar in nature and number to actual use is recommended.

December 1999 IPC-SM-782A

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3.6 Design Rules

During the component selection phase

of a design, manufacturing engineering should be consulted

regarding any components outside the scope of this docu-

ment.

The printed board design principles are a statement of cur-

rent test and manufacturing capabilities. Exceeding or

changing these capabilities requires concurrence of all par-

ticipants in the process including manufacturing, engineer-

ing and test technology.

Involving test and manufacturing early in the design helps

to move a quality product quickly into production. Figure

3–7 shows a list of concurrent engineering team partici-

pants that should be involved.

3.6.1 Component Spacing

3.6.1.1 Component Considerations

The land pattern

design information discussed so far is important for reli-

ability of surface mount assemblies. However, the designer

should not lose sight of manufacturability, testability and

repairability of SMT assemblies. A minimum interpackage

spacing is required to satisfy all these manufacturing

requirements. There is no limit on maximum interpackage

spacing; the more the better. Some designs require that sur-

face mount components are positioned as tightly as pos-

sible. Based on experience, the examples shown in Figure

3–8 meet manufacturability requirements.

The land to land spacing between adjacent components

should be 1.25 mm [0.050 in] clear space all around the

edges of printed boards if boards are tested off the connec-

tor or 2.5 mm [0.100 in] minimum if vacuum seal for test-

ing is used. The requirements speciﬁed herein are recom-

mended minimums excluding conductor geometry

tolerances.

3.6.1.2 Wave Solder Component Orientation

All polar-

ized surface mount components should be placed in the

same orientation, when possible. On any printed board

assembly where the secondary side is to be wave soldered,

the preferred orientation of devices on that side is

described and shown in Figure 3–9. The preferred orienta-

tion is used in order to optimize the resulting solder joint

quality as the assembly exits the solder wave.

• All passive components shall be parallel to each other.

• All SOICs shall be perpendicular to the long axis of

passive components.

• The longer axis of SOICs and of passive components

shall be perpendicular to each other.

• The long axis of passive components shall be perpen-

dicular to the direction of travel of the board along

the conveyer of the wave solder machine.

3.6.1.3 Component Placement

Similar types of compo-

nents should be aligned on the board in the same orienta-

tion for ease of component placement, inspection, and sol-

dering. Also, similar component types should be grouped

together whenever possible, with the net list or connectiv-

ity and circuit performance requirements ultimately driving

the placements. See Figure 3-10. In memory boards, for

example, all of the memory chips are placed in a clearly

deﬁned matrix with pin one orientation the same direction

for all components. This is a good design practice to carry

out on logic designs where there are many similar compo-

nent types with different logic functions in each package.

On the other hand, analog designs often require a large

variety of component types making it understandably diffi-

cult to group similar components together. Regardless if

IPC-782-3-7

Figure 3–7 Simpliﬁed electronic development organization

▼

Admin-

istrative

control

Chain of

command

Operational

control

Working

level

PROGRAM

OFFICE

System

Manage-

ment

Project

Task

MANAGEMENT

ENGINEERING

MANAGEMENT

System

Engin-

eering

Elec-

tronic

Design

MANUFACTURING

MANAGEMENT

Product

Design

Fabri-

cation

Assem-

bly

Testing

Project

Task

Project

Task

Project

Task

Project

Task

Project

Task

Project

Task

Deliverable

Hardware

IPC-SM-782A December 1999

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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

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