IPC-SM-782A 表面安装设计和焊盘设计标准(带BGA).pdf - 第23页

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, …

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 shall be included in the width

dimensions of the lead, termination, or castellation which

are dimensioned as basic at the minimum size.

3.3.2 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 intend to reduce the land size and thus

result in a smaller 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

is a minimum acceptable space between conductors; con-

versely when lands are at their minimum size, the result is

the minimum acceptable land pattern necessary to achieve

reliable solder joints. These thresholds allow for gauging of

the land pattern for go/no-go conditions. The whole con-

cept of the dimensioning system described in this docu-

ment is based on these principles and extends to compo-

nent mounting dimensions, land pattern dimensions,

positioning dimensions etc., so that the requirements may

be examined using optical gauges at any time in the pro-

cess in order to insure compliance with the tolerance analy-

sis.

Figure 3–5 shows the land pattern for an SOIC with gull-

wing leads intended to be a companion to the chip compo-

nent dimensioning concepts shown in Figure 3–4. The

basic dimension is across the outer extremities.

Dimension ‘‘Z’’ is at maximum size, while the inner

extremities (dimension ‘‘G’’) are dimensioned at minimum

size. Unilateral tolerances decreased the basic dimension

for ‘‘Z’’ while increasing the basic ‘‘G’’ dimension. This

action results in a reduced land pattern, thus processing

target values should be as close to the basic ‘‘Z’’ and ‘‘G’’

dimensions as possible. This concept also holds true for the

width (X) of the land dimension which is speciﬁed at maxi-

mum size.

3.3.3 Dimension and Tolerance Analysis

In analyzing

the design of a component/land pattern system, several

things come into play. The size and position tolerances of

the component lead or termination, the tolerances of the

land pattern, the placement accuracy of the man/machine to

center the part to the land pattern, and ﬁnally the amount

of solder area available for a solder joint for 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 = the unilateral proﬁle tolerance(s) for the component

F = The unilateral proﬁle tolerance(s) for the board land

pattern

P = 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,’’ & ‘‘P’’ be added to the mini-

mum dimension of the component length plus the solder

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 tolerances.

This method assumes that all features will not reach their

worst case. The equations for determining chip component

land pattern requirements are as follows:

Z max = L min + 2J

√

+ F

+ P

G min = Smax–2J

–

√

+ F

+ P

X max = W min + 2J

√

+ F

+ P

Where:

Z = Overall length of land pattern

G = Distance between lands of the pattern

X = Width of land pattern

L = Overall length of component

S = Distance between component terminations

W = Width of component

J = Horizontal dimension of solder ﬁllet

= Solder ﬁllet at toe

= Solder ﬁllet at heel

= Solder ﬁllet at side

C = Component tolerances

= Tolerance on component length

= Tolerance on distance between component

terminations

= Tolerance on component width

F = Printed board fabrication (land pattern geometric)

tolerances

P = 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

IPC-SM-782A December 1999

电子技术应用　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ

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) for

chips.

The same concept is true for multiple leaded or leadless

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

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

clearance between a lead, termination, or castellation 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 3–6.

The equation 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

3.3.3.1 Tolerance Analysis

The following tolerance

concepts are used to determine the land patterns for all

electronic components. These concepts are detailed in

Table 3-4, and reﬂect the tolerances on the component, the

tolerances on the land pattern (on the interconnecting sub-

strate), 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 minimum, since the equations in

3.3 address the tolerance of component, board, and place-

ment accuracy tolerances (sum of the squares). The mini-

mum solder joint is increased by the amount that the toler-

ances are not used up.

IPC-782-3-6

Figure 3–6 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

]

▼▼

▼

December 1999 IPC-SM-782A

电子技术应用　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ

If the user of these land patterns desires a more robust pro-

cess condition for placement and soldering equipment,

individual elements of the analysis may be changed to new

and desired dimensional conditions. This includes compo-

nent, board or placement accuracy spread as well as mini-

mum solder joint expectations.

3.3.3.2 Component Dimensions

Illustrations of compo-

nent dimensions beginning in each component section are

accompanied by table of ﬁgures for each of the different

part numbers, as taken from EIA-PDP-100, JEDEC-95 and

other world wide component standards.

EIA-PDP-100 is a catalogue listing of outline drawings

illustrating the dimensions of supplier registered passive

components; JEDEC-95 as the outlining document for

solid state products. At times, the component tolerances or

component gauge requirements do not necessarily reﬂect

the exact tolerance on a manufacturers data sheet. Usually,

this occurs when industry component speciﬁcation ranges

are so broad that they defy good surface mount design

principles. When this action is taken, the tolerance is indi-

cated with an asterisk (*).

Component dimensions are provided according to the con-

cepts of maximum and least materials condition (MMC and

LMC). Both conditions are presented in the tables. The

component manufacturer may not always dimension his

component in accordance with the limits shown in the

tables, however, these limits may be used as a gauge for

go/no-go acceptance of the component. The LMC dimen-

sions of the ﬁgure are those that have been used in the

equations described in 3.3 for determining the recom-

mended land pattern.

Dimensions that have had their tolerance spread reduced

are so indicated in the tables. Parts that are available with

shape characteristics or tolerance limits that fall outside the

recommended norms require land patterns that must be

altered slightly from those presented.

Users of these specialized parts are encouraged to develop

their own land patterns which then become unique to a

speciﬁc component vendor part. A dimensioning system

with speciﬁc equations has been provided to facilitate

unique land pattern development or enhance process usage.

3.3.3.3 Land Pattern Dimensions

Land pattern dimen-

sions are provided according to the concepts of maximum

material conditions (MMC). Sometimes a dimension is pre-

sented as a minimum distance. This occurs when deﬁning

a space(s) that exists between lands at MMC. The printed

board manufacturer may not always inspect his board in

accordance with the limit concepts shown in the table.

However, these limits may be used as a gauge for go/no-go

acceptance of the printed board land pattern. The dimen-

sions shown in each table are those that have been used in

the equations described in 3.3 for determining the recom-

mended land patterns.

All land patterns are designed to be transparent to the sol-

dering process to be used in manufacturing. This reduces

the number of land sizes in the component library, and is

less confusing for the designer, but may not be the most

robust for the placement or the soldering process.

3.3.3.4 Land Pattern Registration

Each land pattern has

received a registration number. The RLP (Registered Land

Pattern) number is a three digit number with a set of num-

bers assigned to land patterns for a particular family of

components. The original number assigned to a particular

component, uses that analysis shown for the speciﬁc sec-

tion (sections 8 through 16). The analyses assume certain

tolerances for board fabrication, placement machine accu-

racy and minimum desired solder joint. Changes in the

assumptions will result in a revision letter to the number.

Thus the ﬁrst change to RLP 106 would be identiﬁed as

RLP 106A, the second change would be RLP 106B etc.

The letters x,y&zarereserved for user modiﬁcations to

the standard land pattern. If a company wishes to change

the approved standard land pattern the user would identify

his customized version as RLP 106X, RLP 106Y or RLP

106Z. It should be recognized that X, Y & Z type RLP’s

reﬂect unique land patterns and will differ company to

company. The number reservations are shown in Table 3-5.

Not every RLP is assigned in Sections 8 through 16, how-

ever all three digit number from 100 to 999 are reserved as

IPC standard RLP numbers. Users of the system may

enhance their computer libraries by tailoring the IPC RLP’s

(adding X, Y or Z), using numbers 001 to 099 or using four

digit numbers to describe their custom standard land pat-

terns. The letter ‘‘W’’ may be used to signify a special user

pattern signifying that it is for wave soldered attachment

application (i.e. RLP 102 deﬁnes as RLP 102W.)

Table 3–4 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 electrodes 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 ‘‘F’’

tolerance in the equations.

Positional Accuracy 0.1 – 0.2 mm DTP

Toe Fillet 0.4 – 0.6 mm

Heel Fillet 0.0 – 0.2 mm

Side Fillet Width –0.02 – 0.02 mm

IPC-SM-782A December 1999

电子技术应用　　　　　ｗｗｗ．ＣｈｉｎａＡＥＴ．ｃｏｍ