IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第18页

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 maxi…

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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

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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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tol = tolerance

max = maximum

min = minimum

As an example, the SOIC with 16 leads has the following

limits for the ‘‘L’’ (component length) and ‘‘T’’ (terminal

length) dimensions:

min

= 5.8 mm, L

max

= 6.2 mm

tol

max

-L

min

= 6.2 mm - 5.8 mm = 0.4 mm

min

= 0.4 mm, T

max

= 1.27 mm

tol

max

-T

min

= 1.27 mm - 0.4 mm = 0.87 mm

Therefore, the calculations for ‘‘S’’ minimum and maxi-

mum dimensions are as follows:

min

-2T

max

= 5.8 mm - 2 (1.27 mm) = 3.26 mm

max

-2T

min

= 6.2 mm - 2 (0.4 mm) = 5.40 mm

tol

max

-S

min

= 5.4 mm - 3.26 mm = 2.14 mm

The difference between S

min

and S

max

is 2.14 mm, which

is probably a larger tolerance range than the actual range

within which these components are manufactured. This

worst-case scenario for the tolerance range for ‘‘S’’ can

also be calculated by adding the tolerances for the compo-

nent length and the two terminals:

tol

+2T

tol

= 0.4 mm + 2 (0.87 mm) = 2.14 mm

In order to arrive at a more realistic tolerance range, the

RMS value is calculated using the tolerances on the dimen-

sions involved (‘‘L’’ and ‘‘T’’):

tol

(RMS)=

√

tol

)

+ 2(T

tol

)

√

0.4

+ 2(0.87)

= 1.29 mm

tol

(RMS) is added to S

min

to arrive at a maximum ‘‘S’’

dimension. This technique is used so that a more realistic

max

dimension is used in the land pattern equations for

calculating G

min

(minimum land pattern gap between heel

ﬁllets). In this example, the following calculation is used

for S

max

(RMS)=S

min

+ S

tol

(RMS)

= 3.26 mm + 1.29 mm

= 4.55 mm

3.1.1.1 Solving for Dimension ‘‘Z’’ It should be noted

that there are various options to determine the tolerances

for the component (C), the fabrication allowance (F), and

the placement tolerance (P). In determining the calculations

for the example in Figure 3-3 for the dimension ‘‘Z,’’ one

would note that the component ‘‘SO16’’ has an L

max

equal

to 6.20 mm, and an L

min

equal to 5.80 mm. With the

assumption that ‘‘F’’ is equal to 0.1 mm and ‘‘P’’ is equal

to 0.2 mm, the following conditions would be used for

determining the ‘‘Z’’ dimension:

max

= L

min

+ 2J

√

+ F

+ P

max

= 6.20 mm + 2J

√

0.4

+ 0.1

+ 0.2

In the above example, the two joints should be rounded to

a realistic number. Normally a total Z dimension of 7.0 mm

would be acceptable for a density level B land pattern pro-

viding a 0.4 mm land protrusion at either end of the SO16

component.

3.1.2 Land Tolerancing Proﬁle tolerancing is used for

lands in a manner similar to that of the components. All

tolerances for lands are intended to provide a projected

land pattern with individual lands at maximum size. Uni-

lateral tolerances are intended to reduce the land size and

thus result in a lesser area for solder joint formation. In

order to facilitate companion dimension systems, the land

pattern is dimensioned across outer and inner extremities.

The dimensioning concept in this standard uses limiting

dimensions and geometric tolerancing to describe the

allowable maximum and minimum dimensions of the land

pattern. When lands are at their maximum size, the result

may be a minimum acceptable space between lands; con-

versely when lands are at their minimum size, the result

may be a minimum acceptable land pattern necessary to

achieve the minimum required land protrusion. These

thresholds allow for gauging of the land pattern for

go/no-go conditions. The whole concept of the dimension-

ing system described in this document is based on these

principles and extends to component mounting dimensions,

land pattern dimensions, positioning dimensions, etc., so

that the requirements may be examined using optical

gauges at any time in the process in order to insure com-

pliance with the tolerance analysis (see Table 3-16).

3.1.3 Fabrication Allowances Figure 3-3 shows the land

pattern for an SOIC with gull-wing leads intended to be a

companion to the chip component dimensioning concepts

shown previously in Figure 3-2. The basic ‘‘L’’ dimension

is across the outer extremities of the component lead or

terminal.

For the land pattern, dimension ‘‘Z’’ is at maximum size,

while the inner extremities (dimension ‘‘G’’) are dimen-

sioned at minimum size. Unilateral tolerances decreased

the basic dimension for ‘‘Z’’ while increasing the basic

‘‘G’’ dimension. This action results in a reduced land pat-

tern at Least Material Condition (LMC). Thus, processing

target values should be as close as possible to the basic

‘‘Z’’ and ‘‘G’’ dimensions at Maximum Material Condition

(MMC). This concept also holds true for the width (X) of

the land dimension which is speciﬁed at maximum size.

The variation between the dimensions Z, G, and X are

indicated as a fabrication allowance (F). This fabrication

allowance represents the maximum variation between the

largest land pattern size (MMC) and the least land pattern

size (LMC). This does not include material movement as

described in Table 3-16, which is included in the assembly

tolerancing since machine vision capability revaluates the

true position of the land pattern.

February 2005 IPC-7351