IPC-7351-2005_表面贴装焊盘布局设计和标准通用要求.pdf - 第72页

result in a worst case analysis, however as with other land patterns in the standard, a statistical average is determined by using the RMS (root, mean, square) value. T able 14-4 shows the total variation in the system f…

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

result in a worst case analysis, however as with other land

patterns in the standard, a statistical average is determined

by using the RMS (root, mean, square) value. Table 14-4

shows the total variation in the system for each of the four

ball sizes identiﬁed in the standard.

It should be noted that the target value for lands on the

substrate of the component or the board should be at Maxi-

mum Material Condition. The variation from the Maximum

Material Condition indicates that ball-to-land misalignment

is achieved by taking the maximum land size and subtract-

ing the variation. The resulting dimension would indicate

the amount of attachment area that would result from a

system where all conditions are at a negative instance. For

lands that are solder mask-deﬁned, the land size should be

increased by the amount of encroachment of the solder

mask. As an example, if the requirement is that solder

mask should be on the land by 0.05 mm, then the maxi-

mum land size should be increased by 0.1 mm. It should be

noted that for solder mask-deﬁned lands, since the land size

increases, the opportunity to route conductors between

lands is impacted by reducing the available area for con-

ductor width and spacing.

14.4.3 Future Ball Conditions Although not required for

the BGAs shown in the present release of IPC-7351, pro-

jected future ball sizes of 0.40 mm pitch and below are

shown in Table 14-2. Their projected land size approxima-

tions for ball diameters of 0.25 mm and below are shown

in Table 14-3.

14.4.4 Land Pattern Calculator The land pattern calcu-

lations for BGAs are based on ball size. As a result of ball

variation and component conditions, Table 14-5 shows the

land pattern calculator headings needed to describe the

variations in the system. This data is usually described at

the Maximum Material Condition for nonsolder mask-

deﬁned lands and is dimension ‘‘X’’ in the IPC-7351

datasheets for BGA land patterns.

15 IPC-7359 NO LEAD COMPONENTS (QFN, SON, LCC)

15.1 LCC

A leadless chip carrier is a ceramic package

with integral surface-metallized terminations. Leadless

Types A, B, and D chip carriers have a chamfered index

corner that is larger than that of Type C. Another difference

between the A, B, and D types and Type C is the feature in

the other three corners. The types A, B, and D, were

designed for socket applications and printed wiring inter-

connections. The Type C is primarily intended for direct

attachment through reﬂow soldering. This application dif-

ference is the main reason for their mechanical differences.

These packages mount in different orientations, depending

on type, mounting structure and preferred thermal orienta-

tion.

Table 14-3 Land Approximation (mm)

Nominal Ball

Diameter Reduction

Nominal

Land

Diameter

Land

Variation

0.75 25% 0.55 0.60 - 0.50

0.60 25% 0.45 0.50 - 0.40

0.50 20% 0.40 0.45 - 0.35

0.45 20% 0.35 0.40 - 0.30

0.40 20% 0.30 0.35 - 0.25

0.30 20% 0.25 0.25 - 0.20

0.25 20% 0.20 0.20 - 0.17

0.20 20% 0.15 0.15 - 0.12

0.15 20% 0.10 0.10 - 0.08

Table 14-4 BGA Variation Attributes (mm)

Nominal

Ball Size

Positional

Allowance

Ball

Tolerance

Substrate

Tolerance

Variation

RMS

Value

0.75 0.1 dia. DTP 0.25 0.10 0.25

0.60 0.1 dia. DTP 0.20 0.10 0.24

0.50 0.1 dia. DTP 0.10 0.10 0.17

0.45 0.1 dia. DTP 0.10 0.10 0.17

Table 14-5 Land-to-Ball Calculations for Current and Future BGA Packages (mm)

Land Size

Location

Allowance

Ball

Variation

PCB

Fabrication

Allowance

Ball Size

Reduction

from

Nominal

Variation

AllowanceMMC LMC Nominal MMC LMC

0.60 0.50 0.10 0.25 0.10 0.75 0.90 0.65 25% 0.25

0.50 0.40 0.10 0.20 0.10 0.60 0.70 0.50 25% 0.20

0.45 0.35 0.10 0.10 0.10 0.50 0.55 0.45 20% 0.17

0.40 0.30 0.10 0.10 0.10 0.45 0.50 0.40 20% 0.17

0.35 0.25 0.10 0.10 0.10 0.40 0.45 0.35 20% 0.17

0.25 0.20 0.05 0.10 0.05 0.30 0.35 0.25 20% 0.15

0.20 0.17 0.05 0.06 0.03 0.25 0.28 0.22 20% 0.08

0.15 0.12 0.05 0.04 0.03 0.20 0.22 0.18 20% 0.07

0.10 0.08 0.05 0.04 0.02 0.17 0.17 0.13 20% 0.07

IPC-7351 February 2005

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Leadless Type A is intended for lid-down mounting in a

socket, which places the primary heat-dissipating surface

away from the mounting surface for more effective cooling

in air-cooled systems.

Type C is a ceramic package similar to leadless Type B

except for corner conﬁguration. The 1.27 mm center fam-

ily, which includes both leadless and leaded devices, is

designed to mount on a common mounting pattern. They

may be directly attached to the mounting structure, or can

be plugged into sockets. One basic restriction is that there

shall be no terminals in the corners of the package. There

are a number of common sizes.

End terminations should be solder coated with a tin/lead

alloy or a lead free equivalent. The solder should contain

between 58 to 68% tin. Solder may be applied to the ter-

mination by hot dipping or by plating from solution. Plated

solder terminations should be subjected to a postplating

reﬂow operation to fuse the solder. The tin/lead ﬁnish

should be at least 0.0075 mm [0.0003 in] thick.

The termination shall be symmetrical, and shall not have

nodules lumps, protrusions, etc., that compromise the sym-

metry or dimensional tolerances of the part. The end termi-

nation shall cover the ends of the components, and shall

extend out to the top and bottom of the component.

Solder ﬁnish applied over precious metal electrodes shall

have a diffusion-barrier layer between the electrode metal-

lization and the solder ﬁnish. The barrier layer should be

nickel or an equivalent diffusion barrier, and should be at

least 0.00125 mm [0.00005 in] thick.

For lead free ﬁnishes a combination of tin, silver and cop-

per is the prevalent replacement for the tin/lead ﬁnish. Sol-

derability testing should be applied per IPC/EIA-J-STD-

001 to determine attachment capability of the applicable

component type.

15.1.1 Marking All parts shall be marked with a part

number and ‘‘Pin 1’’ location. ‘‘Pin 1’’ location may be

molded into the plastic body.

15.1.2 Carrier Package Format Tube carriers are pre-

ferred for best handling.

15.1.3 Process Considerations LCCs are usually pro-

cessed using standard solder reﬂow processes. Parts should

be capable of withstanding ten cycles through a standard

reﬂow system operating at 215° C [419 °F]. Each cycle

shall consist of 60 seconds exposure at 215° C [419 °F].

See IPC/JEDEC J-STD-020 for appropriate reﬂow cycles

and proﬁles when using lead free solders.

15.2 Quad Flat No-Lead (QFN) The Quad Flat No-Lead

(QFN) package is a near CSP plastic encapsulated package

with a copper leadframe substrate. This is a leadless pack-

age where electrical contact to the PCB is made by solder-

ing the lands on the bottom surface of the package to the

PCB, instead of the conventional formed perimeter leads.

The exposed die attach paddle on the bottom efficiently

conducts heat to the PCB and provides a stable ground

through down bonds or electrical connections through con-

ductive die attach material. The design of the QFN pack-

age also allows for ﬂexibility. Its enhanced electrical per-

formance enables the standard 2 GHz frequency to be

increased up to 10 GHz with some design considerations

(see Figures 15-1 and 15-2).

Solderable areas of the mating surfaces on the end termi-

nations (package bottom side) should be solder coated with

a tin/lead alloy or a lead free equivalent. Ends/sides of the

terminals are not designed as a solderable surface and are

not required to be plated, as this is a physical impossibility

with many current manufacturing processes. Solder may be

applied to the termination by hot dipping or by plating

from solution. Plated solder terminations should be sub-

jected to a postplating reﬂow operation to fuse the solder.

The tin/lead ﬁnish should be at least 0.0075 mm [0.0003

in] thick.

The termination shall be symmetrical, and shall not have

nodules lumps, protrusions, etc., that compromise the sym-

metry or dimensional tolerances of the part. The end termi-

nation shall cover the ends of the components, and shall

extend out to the top and bottom of the component.

Solder ﬁnish applied over precious metal electrodes shall

have a diffusion-barrier layer between the electrode metal-

lization and the solder ﬁnish. The barrier layer should be

nickel or an equivalent diffusion barrier, and should be at

least 0.00125 mm [0.00005 in] thick.

For lead free ﬁnishes a combination of tin, silver and cop-

per is the prevalent replacement for the tin/lead ﬁnish. Sol-

derability testing should be applied per IPC-J-STD-002 to

determine attachment capability of the applicable compo-

nent type.

IPC-7351-15-01

Figure 15-1 Quad Flat No-Lead (QFN) Construction

February 2005 IPC-7351