IPC-SM-782A 表面安装设计和焊盘设计标准(带BGA).pdf - 第19页

the length of the capacitor . Figure 3–4b shows the compo- nent length at its minimum size in the converted dimen- sions of the new system using proﬁle tolerancing. Figure 3–4c shows the land pattern at its maximum size.…

Proﬁle tolerances are unilateral, and are described to reﬂect

the best condition for solder joint formation usually at

minimum component size. As the proﬁle tolerance moves

in this unilateral direction toward maximum component

size, the opportunity for a robust solder joint decreases.

The concept for component dimension evaluations is based

on evaluating the surfaces of the component termination

and component lead that are involved in the formation of

the acceptable solder joint. Component manufacturers usu-

ally provide dimensions for their parts with a nominal size

and then put a tolerance on that nominal dimension. Inor-

der to facilitate the dimensioning system, these dimensions

and their associated tolerances are converted to minimum

and maximum size.

As an example, capacitor C1206 has a manufactured nomi-

nal dimension for its length (L) 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 3–4 shows the characteristics for the C1206. Figure

3–4a shows the component manufacturers dimensions for

IPC-782-3-2

Figure 3–2 Lead-form (or terminal-shape) examples

BUTT "C" BEND

SOLDER LUG FLAT

GULL WING HIGH-CURRENT CABLE

"J" BEND NO LEAD

PIN/PEG WRAPAROUND

"S" BEND THROUGH-HOLE

"J" INVERTED WIRE

BODY OF PACKAGE LAND STRUCTURE

IPC-SM-782A December 1999

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the length of the capacitor. Figure 3–4b shows the compo-

nent length at its minimum size in the converted dimen-

sions of the new system using proﬁle tolerancing. Figure

3–4c shows the land pattern at its maximum size. These

conditions provide for an optimum toe ﬁllet. For optimum

heel ﬁllet the component basic dimensions are at the maxi-

mum 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 solder

ﬁllet formation; 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. The inner

(heel-to-heel) dimensions are not provided in industry stan-

dards or manufacturer’s speciﬁcations, and are more diffi-

cult to determine, not only because of the form of the lead,

Table 3–3 Lead-Form (or Terminal-Shape) Suffixes

Code Form/Shape Description (see Figure A2)

B Butt A noncompliant lead intended for attachment perpendicular to the land structure.

C ‘‘C’’ bend A ‘‘C’’ shaped noncompliant lead bent down and under the body of the package.

D Solder lug A lug terminal on the package

F Flat A noncompliant, nonformed ﬂat lead that extends away form the body of the package.

G Gull wing A compliant lead bent down from the body of the package with a foot at the end pointing

away from the package.

H High-current cable A lug terminal at the end of a ﬂexible lead.

J ‘‘J’’ bend A ‘‘J’’ shaped compliant or noncompliant lead bent down and back under the body of the

package.

N No lead Metallized terminal pads located on the body of the package.

P Pin/Peg A tempered lead extending from the body of the package and intended for attachment to a

plated through-hole in the land structure.

R Wraparound A metallized noncompliant terminal wrapped around the package body.

S ‘‘S’’ bend An ‘‘S’’ shaped compliant lead bent under the body of the package.

T Through-hole A terminal with ﬂat or V-shaped cross section intended for attachment to a plated

through-hole in the land structure.

U ‘‘J’’ inverted A ‘‘J’’ shaped compliant or noncompliant lead bent down form the doby of the package with

the curved end pointing away from the package.

W Wire An untempered wire lead extending from the body of the package.

X Other A lead form or terminal shape other than those deﬁned.

IPC-782-3-3

Figure 3–3 Proﬁle tolerancing examples

0.7

1.27

▼

2.5

MMC

▼

0.1

MMC

Places

December 1999 IPC-SM-782A

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termination, or castellation; but also because the inner

dimensions must be derived by subtracting the sum of the

dimensions of the leads (with all their inherent tolerances)

from the overall dimensions of the part.

Figure 3–5a shows the concept for the manufacturers

dimensions and tolerances for a gull-wing SOIC. Figure

3–5b shows the converted dimensions to be considered in

the overall mounting system requirements. Figure 3–5c

shows the land pattern dimensions. The basic dimensions

deﬁne the minimum length as measured across the two

outer extremities. Tolerances increase this dimension to the

maximum width, reducing toe ﬁllet opportunity. The inner

dimensions between heel ﬁllets on opposing sides are the

most important. Inner dimensions are derived by:

a. Establishing the maximum width of the component as

measured from lead termination to lead termination.

(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 (b) from

the maximum overall component length of (a) 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

(a) and (b) 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 in (c.); tolerances on the overall component,

plus the tolerances for the lead on each end. Since not

all three tolerances are considered at their worse case, a

recommended method for determining the statistical

impact is to summarize the squares of the tolerances 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

As an example, the SOIC with 16 leads has the follow-

ing limits for the ‘‘L’’ (component length) and ‘‘T’’ (ter-

minal length) dimensions:

min

= 5.8 mm, L

max

= 6.2 mm

tol

max

–L

min

= 6.2 – 5.8 = 0.4 mm

min

= 0.4 mm, T

max

= 1.27 mm

tol

max

–T

min

= 1.27 – 0.4 = 0.87 mm

IPC-782-3-4

Figure 3–4 Example of C1206 capacitor dimensioning for optimum solder ﬁllet conditions

3.2 ± 0.2

▼

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.

IPC-SM-782A December 1999

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