IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第70页

14.2.2.3 Thermally Enhanced Matrix A thermally enhanced matrix is a perimeter matrix with contacts added back in the center (see Figure 14-6). 14.2.2.4 Staggered Matrix A staggered matrix is deﬁned by the removal of ever…

electrical performance and will increase the overall pack-

age proﬁle. Also the columns are not as rugged as ball

contact and are susceptible to handling damage.

14.2 General Configuration Issues

14.2.1 Device Outlines

The Grid Array package outlines

detailed in this document are furnished in JEDEC Publica-

tion JEP95. The overall outline speciﬁcation for the array

device allows a great deal of ﬂexibility as far as lead pitch,

contact matrix pattern and construction. The JEDEC stan-

dards allow for die attachment on either side of the inter-

face structure (cavity up or cavity down). Refer to IPC-

7096 for further deﬁnition and interconnect schemes for

BGAs. The example shown in Figure 14-4 illustrates two

225 I/O devices with a common package outline but, with

the variation of contact pitch, a unique matrix format is

provided.

14.2.2 Contact Matrix Options Contacts may be distrib-

uted in a uniform pattern, however the matrix is always

centered about the centerline of the package (see Figure

14-5). Contact depopulation is permitted at the discretion

of the device manufacturer. Contact patterns can usually be

described in the following methods: full even matrix, full

odd matrix, perimeter matrix, or staggered matrix.

14.2.2.1 Full Matrix For a given package size, there are

two full matrix possibilities: even and odd. One of them is

the largest matrix that theoretically could ﬁt on the pack-

age, given the size and pitch of the contacts. The other

matrix is smaller by one row and column (see Figure 14-5).

14.2.2.2 Perimeter Matrix A perimeter matrix is

achieved by removing an array of contacts from the center

of the matrix. Center-depopulation does not affect the cen-

terline of the matrix (see Figure 14-6). In addition, Perim-

eter matrices are usually described by the number of con-

tact perimeters.

IPC-7351-14-04

Figure 14-4 Bottom View of BGA Devices

IPC-7351-14-05

Figure 14-5 One Package Size, Two Full Matrices

February 2005 IPC-7351

14.2.2.3 Thermally Enhanced Matrix A thermally

enhanced matrix is a perimeter matrix with contacts added

back in the center (see Figure 14-6).

14.2.2.4 Staggered Matrix A staggered matrix is deﬁned

by the removal of every other contact in an interstitial pat-

tern. This provides an effective minimum center-to-center

contact spacing of

√

2 x pitch of the full matrix (see Figure

14-7). In order to retain the A1 contact position, the stag-

gered matrix must be developed using a full odd matrix.

14.2.3 Selective Depopulation In addition to depopula-

tion methods which lead to the matrices described above,

contacts may be removed selectively. Selective depopula-

tion can be accomplished in any manner as long as the pat-

tern matrix is not shifted from the center of the package

outline (see Figure 14-8).

14.2.4 Attachment Site Planning The attachment site or

land pattern geometry recommended for BGA devices is

round with the diameter adjusted to meet contact pitch and

size variation. The diameter of the land should be no larger

than the diameter of the land at the package interface and

is typically 20% smaller than the normal diameter speciﬁed

for the ball contact for pitches greater than 1.0 mm and

10% smaller for pitches less than 1.0 mm. Refer to the

manufacturer speciﬁcation before ﬁnalizing land pattern

array and geometry.

14.2.4.1 Copper Defined Land Pattern The land pat-

terns described are deﬁned by the etched copper. Solder

mask clearance should be a minimum of 0.075 mm

[0.00295 in] from the etched copper land. For applications

requiring a clearance that is less than recommended, con-

sult with the printed board supplier.

IPC-7351-14-06

Figure 14-6 Perimeter and Thermally Enhanced Matrices

IPC-7351-14-07

Figure 14-7 Staggered Matrix

IPC-7351-14-08

Figure 14-8 Selective Depopulation

IPC-7351 February 2005

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14.2.4.2 Solder Mask Defined Land Pattern If solder

mask deﬁned patterns are used, then adjust land pattern

diameter accordingly (see 14.4).

14.2.5 Defining Contact Assignment Array contact

identiﬁcation is assigned by the column and row location.

For example, A1 contact position is always at an outside

corner position with alpha characters arranged in a vertical

(row) pattern from top to bottom. Numeric characters are

assigned in a horizontal (column) axis (I, O, Q, S, X and Z

are omitted) (see Figure 14-9).

The designer should note that the A1 position is at the

upper left hand corner when the device is viewed from the

top. Contact pattern is deﬁned when viewed from the bot-

tom. The land pattern provided on the host substrate is

opposite of the contact pattern (with A1 contact position

again at the upper left).

14.3 Handling and Shipping For information on trays

and shipping containers refer to ACH:EIA-481-A,

ACH:EIA-481-3, JEDEC CO-028, and JEDEC CO-029.

14.4 Land Pattern Analysis The following provides an

analysis of tolerance assumptions and result in solder joints

based on the land pattern dimensions shown in Figure 14-9.

The variations that exist in determining these land patterns

include the diameter of the individual ball, the positional

accuracy of the ball in relationship to a true position on the

component and the board, and the manufacturing allow-

ance that can be held for the land on the substrate that

mounts the particular ball. The land pattern of the compo-

nent (where the ball is attached) and the land pattern of the

substrate mounting structure (printed board) should be as

similar as possible. Component manufacturers have made

their determinations that the land pattern of pad on the

component should be less than the ball diameter. They base

their conclusions on the resulting nominal ball diameter

with a slight reduction in the land approximation. Pitch

plays a large role in the determination of what ball diam-

eters can be used in various combinations. Table 14-2

shows the characteristics of those balls that are used with

pitches of 1.5 mm through 1.0 mm, as well as future ball

sizes whose pitches fall between 0.40 mm and 0.25 mm.

14.4.1 Land Approximation In each instance, component

manufacturers and board designers are encouraged to

reduce the land size by some percentage of the nominal

ball diameter. The amount of reduction is based on the

original ball size, which is used to determine the average

land. In determining the relationship between nominal

characteristics, a manufacturing allowance for land size has

been determined to be 0.1 mm between the Maximum

Material Condition (MMC) and Least Material Condition

(LMC). Table 14-3 shows the reduction characteristics, the

nominal land size, and the target land dimensions, as well

as future approximations for ball diameters of 0.25 mm and

below.

14.4.2 Total Variation The total variation of the system

considers three major issues: positioning, ball tolerance,

and substrate tolerance. All three attributes added together

IPC-7351-14-09

Figure 14-9 Device Orientation and Contact A1 Position

Table 14-2 Ball Diameter Sizes (mm)

Nominal Ball

Diameter Total Variation Pitch

0.75 0.90 - 0.65 1.5, 1.27

0.60 0.70 - 0.50 1.0

0.50 0.55 - 0.45 1.0, 0.80

0.45 0.50 - 0.40 1.0, 0.80, 0.75

0.40 0.45 - 0.35 0.80, 0.75, 0.65

0.30 0.35 - 0.25 0.80, 0.75, 0.65, 0.50

0.25 0.28 - 0.22 0.40

0.20 0.22 - 0.18 0.30

0.15 0.17 - 0.13 0.25

February 2005 IPC-7351