IPC-7095C-2013.pdf - 第47页

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 c…

4.6.1.2 Epoxy-Glass (FR-4) A ﬁre retardant epoxy-glass

composite can be used for BGA package applications but

the material is most commonly used in the manufacture of

printed circuits. High T

FR-4 laminates (tetra-functional,

multi-functional) have been predominantly used in manu-

facturing multilayer circuit boards; but the material may be

suitable for BGA packaging as well. Recent advances in

the epoxy-resin material formulation have resulted in

greatly improved high temperature performance and rivals

BT in terms of glass transition temperature. Another advan-

tage of using FR-4 resin systems for BGA construction is

that they are more closely matched in CTE to the circuit

board onto which they are mounted. IPC-4101 has under-

gone extensive expansion to meet RoHS compliance and

the requirements of lead-free soldering. The compositions

have been formulated to minimize both the rate of decom-

position and excessive Z axis expansion during soldering

processes (processes that may exceed 260°C). Because

manufacturers use widely varying compositions to manu-

facture epoxy-glass base material, a single slash sheet

speciﬁcation is not practical. Six speciﬁcations that are

RoHS compliant are IPC-4101/99, /101, /121, /124, /126

and /129. There are only slight differences in composition

elements with a T

range between 110°C and 170°C and a

decomposition (T

) range between 310°C and 340°C. All

have a UL ﬂammability rating of V-O (see Table 4-8).

4.6.1.3 Fire retardants for FR-4 In regard to the ﬁre-

retardant used in manufacturing FR-4 composites, the

RoHS directive forbids the use of some bromine com-

pounds, but it does not ban brominated materials used

currently as a ﬂame retardant for glass reinforced base

materials used for substrate fabrication. Bromine-

containing compounds that are outlawed by RoHS are

those that remain as independent molecules within the

polymeric matrix. These include polybrominated diphenyl

ether (PBDE) or polybrominated biphenyl oxide (PBBO)

and polybrominated biphenyls (PBB). Bromine-containing

compounds that are compliant with RoHS include those

that react to become a chemical part of the polymeric

matrix, for example, tetrabromobisphenol A (TBBPA).

Being RoHS compliant does not mean the base material

must be halogen free. Certain brominated ﬂame retardants

including the most popular brominated ﬂame retardant for

FR-4, TBBPA, are accepted by RoHS and decabromodi-

phenyl ether (DBDPE) has been given an exemption by

RoHS.

4.6.1.4 Ceramic Ceramic is the term used for a general

class of substrate based on alumina or aluminum oxide.

The material is one of the ﬁrst used for area array packag-

ing in the form of pin grid arrays and was also the material

ﬁrst used in the construction of the earliest BGA packages.

Ceramic substrates have higher thermal conductivity and,

using a cavity and lid format, can provide hermetic pack-

aging capability. Ceramic substrate material does, however,

have a number of detractors. For example, it is normally

more expensive, more brittle, has a higher dielectric con-

stant (which retards signal propagation speed) and has a

coefficient of thermal expansion much lower than the typi-

cal circuit board structure onto which it is normally

mounted. This last point is a major concern and can limit

the overall package size and the need to maximize the ball

contact size in order to achieve acceptable solder joint reli-

ability of the assembled package.

4.6.1.5 Flexible (Non-Reinforced) Base Films Flexible

base ﬁlms are an increasingly common choice for BGA

construction. The most common base ﬁlm for such con-

structions is polyimide. Polyimide has a number of attrac-

tive attributes, which make it a strong choice for BGA

substrates. Among the positive attributes offered by

Table 4-8 IPC-4101C FR-4 Property Summaries - Speciﬁcation

Sheets Projected to Better Withstand Lead-Free Assembly

Property

IPC-4101B Speciﬁcation Sheets

/99 /101 /121 /124 /126 /129

min (°C) 150 110 110 150 170 170

max (°C) N/A N/A N/A N/A N/A N/A

Td min (°C) 325 310 310 325 340 340

Fillers yes yes no no yes no

Flame Retardant RoHS BR RoHS BR RoHS BR RoHS BR RoHS BR RoHS BR

Flammability V-0 V-0 V-0 V-0 V-0 V-0

Max Z-Axis CTE - alpha 1 60 60 60 60 60 60

Max Z-Axis CTE - alpha 2 300 300 300 300 300 300

Max Z-Axis CTE (50-260°C) 3.5 4.0 4.0 3.5 3.0 3.5

T-260 (minutes) 30 30 30 30 30 30

T-288 (minutes) 5 5 5 5 15 15

T-300 (minutes) AABUS AABUS AABUS AABUS 2 2

UL Max. Operating Temp (°C) AABUS AABUS AABUS AABUS 130 130

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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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Not for Resale

No reproduction or networking permitted without license from IHS

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