IPC-7095C-2013.pdf - 第48页

be separated from a noisy ground where all the switching activity takes place. Some applications require several power supplies with dif ferent voltages for each part of the chip. These planes should be distributed evenl…

polyimide ﬁlms are very high temperature limits (~250°C)

and relatively low dielectric constant (~3.5 versus ~4.5 for

FR-4 and ~10.0 for ceramic). In addition, it is very thin and

is much easier to produce the ﬁne line circuit features more

commonly required for high-density area array packages.

On the negative side, the main concern with nonreinforced

or ﬂexible materials has been their dimensional stability.

The reinforcement provides the physical characteristics that

enhance CTE properties in the X-Y axis. The X and Y axes

are the particular segments of the interconnecting product

that affect the stress on the solder joints of the package

when mounted onto the interconnecting product. In addi-

tion, polyimide ﬁlm is generally more expensive than some

of the other reinforced organic base materials and it is rela-

tively hydroscopic. On the other hand, because polyimide

ﬁlms are more ﬂexible, the material will absorb rather than

transfer physical stress.

4.6.2 Properties of Substrate Materials While there are

numerous properties that are speciﬁed and measured rela-

tive to substrate materials, only a few properties are con-

sidered key to the performance of the ﬁnal BGA product.

4.6.2.1 Coefficient of Thermal Expansion (CTE) The

coefficient of thermal expansion is a very important physi-

cal attribute of a BGA substrate. The CTE deﬁnes the rate

of expansion of the material with increase in temperature.

The importance is magniﬁed when there are large differ-

ences in CTE between the BGA package and the circuit

board structure to which it is mounted. When the CTE dif-

ference is large, excessive stress and strain can be placed

on the solder ball connections resulting in accelerated sol-

der joint degradation.

4.6.2.2 Glass Transition Temperature (T

) The glass

transition temperature is the temperature at which the mate-

rial goes through a transformation from a rigid glass-like

state to a rubbery soft state. It is also the point at which the

material begins to lose strength and expands at a much

higher rate (i.e., the CTE increases).

4.6.2.3 Flexural Modulus Flexural modulus is important

as a measure of the stiffness or rigidity of the substrate. The

impact on the BGA is most commonly manifested in the

degree of warpage. This, in turn, can signiﬁcantly impact

board assembly yield if warpage is excessive.

4.6.2.4 Dielectric Properties There are several metrics

that are included under the general heading of dielectric

properties. Dielectric constant, dissipation factor, dielectric

withstanding voltage and surface insulation resistance are

examples of such properties. These properties are impor-

tant; moreover, as computers, portable communication

products, and modules obtain higher processing capability,

signal speed and integrity become paramount. The need for

greater performance capability will be apparent as systems

designed to be run above 200-300 MHz continue to use

FR-4. As processing speeds continue to increase, it is nec-

essary to lower the dielectric constant and also lower the

dissipation factor of the material. The more advanced sub-

strate material systems can provide robust solutions. For

example, cyanate-ester provides signal transmission speeds

of 114 cm/nsec compared to 100 cm/nsec for common

FR-4 epoxy material. Lower dielectric constant (Dk) and

lower dissipation factor (Df) must be considered when

selecting advanced material technologies.

Lower dielectric constant (Dk) beneﬁts include:

• Faster conductor signal speed

• Thinner interconnects for the same conductor geometries

Lower dissipation factor (Df) beneﬁts include:

• Improved signal integrity with high frequencies

• Less signal loss at high frequencies

Table 4-9 shows the different characteristics for some of

the materials used to fabricate substrates for BGA applica-

tions.

4.6.2.5 Moisture Absorption Moisture absorption of

materials used for BGA construction is of great concern.

The ideal material will not retain any moisture. From a

packaging perspective, the concern is predicated on the fact

that moisture can be trapped in the laminate base.

Entrapped moisture can expand and outgas explosively

during assembly causing local delamination, degrading the

reliability of the package.

4.6.2.6 Flatness Requirements Flatness requirements

for BGA substrates must be maintained to assure that the

components will not be excessively warped or bowed after

package assembly. Such conditions could make testing and

the assembly to the next level difficult. The package assem-

bly process will likely improve some of the negative effects

once the die is attached, especially if the die is of substan-

tial size relative to the package outline. The recommended

ﬂatness criteria for BGA packaging should not exceed

0.3%.

4.7 BGA Package Design Considerations In addition to

the die design rules, the substrate designer must understand

both thermal and electrical performance issues. BGA pack-

age designers must consider manufacturability issues as

well: substrate fabrication, ﬁrst and second level assembly

yield, and ﬁnished package reliability.

4.7.1 Power and Ground Planes In-package power and

ground distribution must be planned in advance. For some

high-speed applications, entire circuit layers are required

for power and ground distribution. Ground and voltage

planes are also used when controlled impedance transmis-

sion lines are required. In addition, a quiet ground needs to

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be separated from a noisy ground where all the switching

activity takes place. Some applications require several

power supplies with different voltages for each part of the

chip. These planes should be distributed evenly on the

BGA package substrate to minimize component warpage.

For applications requiring a solid power or ground plane, a

minimum of a four-layer substrate is required. The four

layer substrates will also exhibit lower thermal resistance

and higher power dissipation compared to two-layer pack-

ages. In thermally enhanced BGAs where a copper heat

sink is incorporated inside the package, the heat sink is

commonly used as a ground plane. By connecting ground

lines through vias to the heatsink, the heatsink becomes an

active current-carrying ground plane.

4.7.2 Signal Integrity There are three major BGA pack-

age design considerations that affect signal integrity:

1. Reﬂections due to discontinuation in the characteristic

impedance lines.

2. Cross-talk between adjacent lines generated from the

coupled noise between an active line and the quiet line.

3. Switching noise generated from multiple outputs switch-

ing simultaneously, which is commonly known as ΔI

noise or SSO noise. Multiple simultaneously switching

outputs (SSOs) require the power and supply lines to

have a lower effective inductance (L

eff

ΔI Noise = L

eff

in millivolts

The effective inductance in a BGA package depends on the

number and placement of power and ground pins in rela-

tion to the power and ground pads on the die. By proper

pin assignments for power and ground pins and commer-

cially available signal integrity tools, L

eff

as well as the ΔI

noise can be minimized.

4.7.3 Heat Spreader Incorporation Inside the Package

A heat spreader may be incorporated inside the package

when the chip power exceeds the maximum power dissipa-

tion that can be supported by the package substrate. Due to

lower thermal conductivity of most laminate material, the

heat generated by the IC is carried through the copper con-

ductors, plated through-hole vias, and the solder balls. By

furnishing a copper plane or section under the die mount-

ing area, a heat spreader is incorporated into the package.

It is important that the package design is as thermally bal-

anced as possible to avoid excessive warpage during tem-

perature changes. In ceramic-based BGA the heat spread-

ing can be achieved by replacing the low conductivity

alumina-based ceramic material by high conductivity

copper-tungsten materials having a thermal expansion coef-

ﬁcient matched to alumina substrate materials.

4.8 BGA Package Acceptance Criteria and Shipping

Format

There are several issues related to the acceptance

criteria for BGA packages. These include having a process

control strategy during qualiﬁcation and production where

sampling plans are used to deﬁne the level of nonconfor-

mance. The major issues are:

• Missing balls (4.8.1)

• Voids in solder balls (4.8.2)

• Solder ball attach integrity (4.8.3)

• Package and ball coplanarity (4.8.4)

• Moisture sensitivity (4.8.5)

• Shipping medium (4.8.6)

• Solder ball alloy (lead vs. lead-free) (4.8.7)

• Ball size and shape

• Module ﬂatness/package bow

• Presence of contamination

• CSAM (C-mode scanning acoustic microscopy) for

delamination

4.8.1 Missing Balls Missing or damaged ball contacts

are not acceptable on incoming BGA components. Figure

4-27 provides an illustration of balls missing from the

BGA package.

4.8.2 Voids in Solder Balls Voids in solder balls should

be based on incoming acceptance criteria or post-assembly

solder joint acceptance criteria. There is a fundamental dif-

ference in whether voids are present in solder balls prior to

assembly or afterwards.

Voids typical of that shown in Figure 4-28 may or may not

dissipate during board level assembly processing. Voids

Table 4-9 Typical Properties of Common

Dielectric Materials for BGA Package Substrates

Property

Material

High

Performance Epoxy

Bismaleimide

Triazine/Epoxy Polyimide Cyanate Ester

Dielectric Constant (Neat Resin) 3.4 2.9 3.5 - 3.7 2.8

Electric Strength (x 10

V/mm) [x 10

V/in] 70.9 [1.8] 47.2 [1.2] 70.9 [1.8] 65.0 [1.65]

Volume Resistivity (x 10

D-cm) 4.9 4.0 2.1 1.0

Water Absorption (wt%) 0.3 1.3 1.3 0.8

Dissipation Factor 0.012 0.015 0.01 0.004

Note: Also see Table 5-1

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found in solder balls at incoming inspection indicate

reduced solder volumes in the solder balls. This will trans-

late to smaller collapsed solder ball stand-off heights and

thus may reduce reliability.

For voids found during post-assembly inspection, reliabil-

ity of the solder joint may be compromised if a void is

excessive due to the reduction of solder cross-section

and/or surface bonding area both at the interposer and the

product board. It will be necessary to establish an accept-

able level of voiding so that the product can meet customer

expectations, has a useful working life and meets product

reliability requirements.

4.8.3 Solder Ball Attach Integrity One of the factors the

success of the BGA package depends upon is reliable sol-

der ball attachment. The solder balls need to be attached

within acceptable dimensional tolerances. Their height and

width after attach need to be within speciﬁed and/or

acceptable limits. Most important of all, they need to form

a proper metallurgical bond with the lands on the substrate.

All the solder balls need to see a temperature proﬁle which

ensures wetting necessary for optimum connection. If the

joint does not wet, then the solder ball is not properly

attached for necessary mechanical and electrical intercon-

nect. Such balls may fall off during shipping, transportation

and handling, or may fail or act intermittently during elec-

trical testing. Figure 4-29 shows the surfaces of the solder

ball and the land. The left picture shows the ﬂattened bot-

tom of a solder ball and the right picture shows the pad on

an FBGA where a ball was supposed to be attached.

If the attach process temperature is inadequate to form a

proper metallurgical bond, only a small amount of force is

needed to dislodge the solder ball from the pad since it was

held in place only by mechanical adhesion.

Solder ball attach integrity can be evaluated through solder

ball shear. Manual and automated shearing and shear force

recording instruments are available for that purpose. For

shear test it is not only the magnitude of the shear force

that is important, but also the failure mode. Failures at the

solder interface, indicated by ripping soldering pads out of

the resin matrix of the BGA or the lands of the PCB, are

positive indications of good wetting. For good attachment,

there should not be any unwetted areas.

4.8.4 Package and Ball Coplanarity Package coplanar-

ity is the result of all of the following factors:

1. Package thickness, pitch, and thermal requirements

2. Substrate design, material, and manufacturing processes

3. Number of devices (silicon) - size and thickness

4. Number of passives - size and thickness

5. Assembly materials and manufacturing processes

Coplanarity that is currently speciﬁed at room temperature

does not always ensure that the component will have a

proper joint formation at SMT. The package behavior dur-

ing reﬂow (dynamic warpage) could result in good or bad

contact with the solder during reﬂow regardless of room

temperature coplanarity. The dynamic warpage of the pack-

age depends on the package CTE (package size, die size,

package material). The dynamic warpage signature could

Figure 4-27 Example of Missing Balls on a BGA

Component

Figure 4-28 Example of Voids in Eutectic Solder Balls at

Incoming Inspection

Figure 4-29 Examples of Solder Ball/Land Surface

Conditions

Flattened bottom of

a solder ball

Pad on an FBGA where it was

supposed to be attached

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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