IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第17页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ formation of the acceptable solder joint. Component manu- facturers usually provide dimensions for their parts with a nominal size and then put a tolerance on that nominal dimension. In order to facilitate …

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2.4 International Electrotechnical Commission

IEC-61188 Printed Boards and Printed Board Assemblies -

Design and Use

2.5 Joint Electron Device Engineering Council (JEDEC)

Publication 95 JEDEC Registered and Standard Outlines

for Solid State Products

3 DESIGN REQUIREMENTS

3.1 Dimensioning Systems

This section describes a set

of dimensional criteria for components, land patterns, posi-

tional accuracy of the component placement capability and

the opportunity to create a certain size solder joint com-

mensurate with reliability or product performance analysis.

Proﬁle tolerances are used in the dimensioning system to

deﬁne the size range between maximum and minimum

component/lead dimensions without ambiguity. The proﬁle

tolerance is intended to control both size and position of

the land. Figure 3-1 shows the proﬁle tolerancing method.

The use of the proﬁle dimensioning system requires an

understanding of the concepts. The use of a set of require-

ments are adopted and invoke the following rules, unless

otherwise modiﬁed.

a. All dimensions are basic (nominal).

b. Limits of size control form as well as size.

c. Perfect form is required at maximum dimensions.

d. Datum references and position tolerances apply at maxi-

mum dimensions, and are dependent on feature size.

e. Position dimensions originate from maximum dimen-

sions.

f. Tolerances and their datum references other than size

and position apply regardless of feature size (RFS).

The dimensioning concepts used for this system of analy-

sis consider the assembly/attachment requirements as their

major goal. Speciﬁcation (data) sheets for components or

dimensions for land patterns on boards may use different

dimensioning concepts, however, the goal is to combine all

concepts into a single system. Users are encouraged to

establish the appropriate relationship between their dimen-

sioning system(s) and the proﬁle dimensioning system and

analysis concepts described herein to allow for ease of tai-

loring these concepts for robust process performance. As

an example, if the tolerance used for positioning is larger

than the machine tolerance used in production, a single

dimensional change could modify the land pattern.

3.1.1 Component Tolerancing The component manu-

facturers and industry standards organizations are respon-

sible for the dimensioning and tolerancing of electronic

components (see 3.1.5.2). The basic dimensions and toler-

ance limits published in the speciﬁcations have been con-

verted to a functional equivalent using the proﬁle toleranc-

ing method with all components shown with their basic

dimensions as limit dimensions (maximum or minimum

size). Proﬁle tolerances are unilateral, and are described to

reﬂect the best condition for solder joint formation.

The concept for component dimension evaluations is based

on evaluating the surfaces of the component termination

and component lead or contact that are involved in the

4. www.iec.ch

5. www.jedec.org

IPC-7351-3-01

Figure 3-1 Proﬁle Tolerancing Method

0.7

1.27

▼

2.5

MMC

▼

0.1

MMC

Places

IPC-7351 February 2005

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ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ

formation of the acceptable solder joint. Component manu-

facturers usually provide dimensions for their parts with a

nominal size and then put a tolerance on that nominal

dimension. In order to facilitate the dimensioning system,

these dimensions and their associated tolerances are con-

verted to minimum and maximum size.

As an example, capacitor C3216 has a manufactured nomi-

nal dimension for its length of 3.2 mm. The tolerance

described by the manufacturer is ± 0.2 mm. Thus, the mini-

mum dimension of ‘‘L’’ is 3.0 mm with a unilateral toler-

ance of 0.4 mm, resulting in its maximum dimension being

3.4 mm.

Figure 3-2 shows the characteristics for the 3216 capacitor.

Item A in Figure 3-2 shows the component manufacturer’s

dimensions for the length of the capacitor. Item B in Fig-

ure 3-2 shows the component length at its minimum size in

the converted dimensions of the new system using proﬁle

tolerancing. Item C in Figure 3-2 shows the land pattern at

its maximum size. These conditions provide for an opti-

mum toe ﬁllet. For optimum heel ﬁllet, the component

basic dimensions are at the maximum and the land pattern

is at its minimum.

Similar concepts are applied to leaded surface mount parts.

The critical dimensional characteristics identiﬁed are those

that relate to the formation of the toe and heel solder ﬁllet.

For components with gull-wing leads, the basic dimensions

apply across the outer extremities of the part for toe land

projection; and within the inside of the formed radius of

opposing leads for heel solder ﬁllet formation.

The outer dimensions of leaded or even leadless chip car-

riers are usually easy to determine since these are readily

available from the component manufacturer or standards

organization. The inner (heel-to-heel) dimensions are not

provided in industry standards or manufacturers’ speciﬁca-

tions and are more difficult to determine, not only because

of the form of the lead, termination, or castellation but also

because the inner dimensions must be derived by subtract-

ing the sum of the dimensions of the leads (with all their

inherent tolerances) from the overall dimensions of the

part.

Item A in Figure 3-3 shows the concept for the manufac-

turer’s dimensions and tolerances for a gull-wing SOIC.

Item B in Figure 3-3 shows the converted dimensions to be

considered in the overall mounting system requirements.

IPC-7351-3-02

Figure 3-2 Example of 3216 (1206) Capacitor Dimensioning for Optimum Solder Fillet Condition

3.2 ± 0.2 mm

▼

3.0 LMC

▼

0.2

▼

Maximum

Component Size

▼

MMC

▼

0.05

Manufacturers dimensions

and tolerances (maximum

length of part is 3.4 mm).

Part shown with length at

"least material condition,"

and profile tolerance to

indicate maximum range of

component length at 3.4 mm.

Land pattern with dimension Z

at "maximum material condition."

Profile tolerance of part (0.2 X 2),

plus profile tolerance of land pattern

(0.05 X 2) plus placement accuracy

(0.1 diameter of true position) are

considered in determining the

proper dimension for Z , plus

the desired toe fillet.

February 2005 IPC-7351

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ

Item C in Figure 3-3 shows the land pattern dimensions.

The basic dimensions deﬁne the minimum length as mea-

sured across the two outer extremities. As component tol-

erances for ‘‘L’’ increase the maximum length, the oppor-

tunity for the toe ﬁllet is subsequently reduced.

The inner dimensions between heel ﬁllets on opposing

sides are the most important. Inner dimensions are derived

by:

a. Establishing the maximum outline of the component as

measured from lead termination end to lead termination

end. (This dimension is shown as ‘‘L,’’ and is provided

by the manufacturer).

b. Establishing the minimum amount of the lead length as

measured across the ‘‘footprint’’ (from heel to toe for

gull-wing leads). (This dimension is ‘‘T,’’ and is pro-

vided by the manufacturer).

c. Subtracting twice the minimum lead length of (T) from

the maximum overall component length of (L) to arrive

at the maximum length inside the leads across the length

of the component (the inner dimension between oppos-

ing heel ﬁllets). Including the tolerances on dimensions

(L) and subtracting the maximum dimension of T x 2

will yield the minimum dimension between opposing

heels. This signiﬁes worst- case tolerance analysis.

d. Three sets of tolerances are involved in the analysis

described within three tolerances on the overall compo-

nent, plus the tolerances for the lead on each end. Since

not all three tolerances are considered at their worst

case, a recommended method for determining the statis-

tical impact is to summarize the squares of the toler-

ances and take the square root of their sum as the RMS

(root-mean-square) tolerance difference.

For example,

RMS tolerance accumulation =

√

tol

)

+ 2(T

tol

)

Where:

tol

max

-L

min

tol

max

-T

min

IPC-7351-3-03

Figure 3-3 Proﬁle Dimensioning of Gull-Wing Leaded SOIC

▼

MMC

▼

0.05

Places

Fabrication

tolerance

equals 0.1 mm.

Manufacturers dimensions and tolerances

converted to profile dimensions, with S

at "maximum material condition."

Note: If S is not provided by the component

manufacturer it may be determined by subtracting

T terminal dimensions from the length.

S = L - 2 T

Manufacturing dimensions of SOIC's.

MMC MMC LMC

IPC-7351 February 2005

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