IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第20页

ｗｗｗ．ｂｚｆｘｗ．ｃｏｍ 3.1.4 Assembly Tolerancing Another part of the equa- tion is the assembly variation deﬁned by the letter ‘ ‘P .’ ’ This variation represents the location of the component in relation to its true position as…

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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

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3.1.4 Assembly Tolerancing Another part of the equa-

tion is the assembly variation deﬁned by the letter ‘‘P.’’

This variation represents the location of the component in

relation to its true position as deﬁned by the design. The

term diameter of true position (DTP) is used to describe

this variation and is a single number that can be used in the

dimensional tolerance analysis.

As an example, for establishing the target heel protrusion

dimensions of the example shown in Figure 3-3, the fol-

lowing conditions would be true:

Where:

J is 0.5 mm (target heel ﬁllet).

CisS

tol

(RMS) = 1.29 mm (see previous calculations

from component dimensions).

F is 0.1 mm (assumed fabrication tolerance).

P is 0.05 mm (assumed assembly equipment placement

tolerance).

Therefore:

min

= 4.55 mm − 2(0.5 mm)−

√

(1.29)

+(0.1)

+(0.05)

= 2.25 mm

Another major condition for multiple-leaded components

that must be considered in land pattern design is lead, ter-

mination, or castellation pitch. The pitch describes the

basic dimension of the spacing of one component lead ter-

mination or castellation to its adjacent counterpart(s). No

tolerance is assigned to pitch in the proﬁle dimensioning

concept. Differences in pitch are included in the width

dimensions of the lead, termination, or castellation which

are dimensioned as basic at the minimum size.

3.1.5 Dimension and Tolerance Analysis In analysing

the design of a component/land pattern system, several

things come into play, including the size and position tol-

erances of the component lead or termination, the toler-

ances of the land pattern, and the placement accuracy of

the man/machine to center the part to the land pattern. The

result is the land area available for a solder joint that pro-

vides a proper formation of a toe, heel, or side ﬁllet.

System equations have been developed for chip compo-

nents and multiple leaded parts. These concepts assume

that the target values of parts and land patterns are maxi-

mized to reﬂect solder joint formation (i.e., outer dimen-

sions of components at minimum size with outer dimen-

sions of land patterns at maximum size). The equations use

the following symbols:

C is the unilateral proﬁle tolerance(s) for the component.

F is the unilateral proﬁle tolerance(s) for the board land.

pattern

P is the diameter of true position placement accuracy to

the center of the land pattern.

With the assumption that a particular solder joint or solder

volume is desired for every component, some methods use

the worst-case criteria for determining a dimension. This

would require that ‘‘C,’’ ‘‘F,’’ and ‘‘P’’ be added to the

minimum dimension of the component length plus the sol-

der joint requirements, in order to determine the maximum

dimension of the outer land pattern.

Experience shows that the worst-case analysis is not

always necessary; therefore statistical methods are used by

taking the square root of the sum of the squares of the tol-

erances. This method assumes that all features will not

reach their worst case. The equations for determining com-

ponent land pattern requirements are as follows:

max

= L

min

+ 2J

√

+ F

+ P

min

= S

max

− 2J

−

√

+ F

+ P

max

= W

min

+ 2J

√

+ F

+ P

Where:

Z is the overall length of land pattern.

G is the distance between lands of the pattern.

X is the width of land pattern.

L is the overall length of component.

S is the distance between component terminations.

Wis the width of the lead or termination.

J is the desired dimension of solder ﬁllet or land

protrusion.

is the solder ﬁllet or land protrusion at toe.

is the solder ﬁllet or land protrusion at heel.

is the solder ﬁllet or land protrusion at side.

C is the component tolerances.

is the tolerance on component length.

is the tolerance on distance between component

terminations.

is the tolerance on the lead width.

F is the printed board fabrication (land pattern geometric)

tolerances.

P is the part placement tolerance (placement equipment

accuracy).

The formula (the square root of the sum of the squares) is

identical for both toe and heel solder joint formation (dif-

ferent tolerances are used, however). However, the desired

solder joint dimension and the square root of the sum of the

squares are added for outer land pattern dimensions and

subtracted for inner land pattern dimensions. The result

provides the ﬁnal land pattern dimensions Z, G, and X.

The same concept is true for chip, multiple leaded or lead-

less components. Additionally, pitch with lead-to-land

overlap (M) can be evaluated as well as the space (N) to

reﬂect the clearance between a lead, termination, or castel-

lation and the adjacent land(s). These latter values are not

used in the equations to determine the land pattern sizes,

but may be used to limit lead-to-adjacent land proximity

and to adjust lead-to-land overlap (see Figure 3-4).

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The equations for determining if the clearance ‘‘N’’ or the

attachment overlap ‘‘M’’ are sufficient is as follows:

M =

[

W + X

]

−

√

+ F

+ P

N = E-

[

W + X

]

√

+ F

+ P

3.1.5.1 Tolerance and Solder Joint Analysis The fol-

lowing tolerance concepts are used to determine the land

patterns for electronic components. These concepts are

detailed in Table 3-1 and reﬂect the tolerances on the com-

ponent, the tolerances on the land pattern (on the intercon-

necting substrate), and the accuracy of the equipment used

for placing components.

Solder joint minimums are shown for toe, heel and side ﬁl-

lets. These conditions are minimums, since the equations in

3.1 address the component, board, and placement accuracy

tolerances (sum of the squares). The minimum solder joint

or land protrusion is increased by the amount that the tol-

erance variation does not use up. The courtyard excess is

added to the maximum dimension that the land pattern or

component occupies. The courtyard excess number is

added to each side of the dimension in question. It is

intended that this addition provides sufficient room for

electrical and physical clearance between components

and/or land patterns. Since the total of all the number cal-

culations may not result in a reasonable numerical equiva-

lent, a suggested round-off (up or down) feature has been

added to the tables to identify a rounding up value for the

ﬁnal number to be used in the design.

IPC-7351-3-04

Figure 3-4 Pitch for Multiple Leaded Components

0.63 Pitch

0.3 - 0.2

▼

Pitch

▼

Leads (W)

Lands (X)

Pitch

▼

M =

W+X

Note: Positional tolerance takes angularity into account

▼

N = E -

[

W+X

]

▼▼

▼

Table 3-1 Tolerance Analysis

Elements for Chip Devices

Tolerance

Element Detailed Description

Component

Tolerance

The difference between the MMC and the

LMC of each component dimension, length,

width and distance between terminations or

leads. This number is the ‘‘C’’ tolerance in

the equations.

Board

Tolerance

The difference between the MMC and the

LMC of each land pattern dimension. This

number is the ‘‘F’’ tolerance in the equations.

Positional

Accuracy

Positional accuracy is deﬁned as diameter

of true position (DTP). This is the variation

of the part centroid related to the land pattern

theoretical center (includes feature location

tolerance from Table 3-17).

Toe Fillet The land protrusion beyond the lead or

termination extremities (see Tables 3-2

through 3-14).

Heel Fillet The land protrusion beyond the internal lead

or termination dimensions (see Tables 3-2

through 3-14).

Side Fillet

Width

The land protrusion to either side of the lead

or termination (see Tables 3-2 through 3-14).

February 2005 IPC-7351