IPC-7095C-2013.pdf - 第46页

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 g FR-4 laminates (tetra-func…

alignment resulting in either a bridge or an open. Another

common defect type is opens that fall into the head and

pillow category. The primary cause of these open defects is

warping and is most often seen in the corner balls of the

socket. Besides having a socket that remains relatively ﬂat

during reﬂow, there are several steps that can be taken to

mitigate the effect.

Processor sockets have an opening in the center to allow

for capacitors on the bottom side of the component.

Because the socket is covered by the pick and place cap,

this creates somewhat of a dead air space during reﬂow

causing the solder balls closest to the cavity to be cooler

than those on the outer rows. It is, therefore, important that

close attention is paid to the reﬂow proﬁle minimizing the

temperature delta between the inner and outer solder balls.

Lowering the temperature delta will help minimize the

socket warp during reﬂow. Another step to reduce opens is

to increase the solder paste volume particularly in the areas

having the issue. It is not always practical to increase the

stencil thickness, so increasing the size of the stencil aper-

ture may be an alternative. As always, it is best to follow

the solder paste manufacturer’s recommendation when cre-

ating a reﬂow proﬁle; but making sure the maximum tem-

perature, soak time, and time above liquidus are not at the

lower end can also help avoid head and pillow opens.

4.6 BGA Construction Materials

4.6.1 Types of Substrate Materials

A number of differ-

ent materials are used in the construction of BGAs. The

material choice is predicated on a number of different fac-

tors including cost, use environment, reliability require-

ments, etc. The material choice is also dependent on the

processes used in the manufacture of the BGA and the

complexity of the design required to redistribute the chip

I/O to area array format. Base materials are selected not

only by their electrical characteristics, but also their

mechanical properties. Most component manufacturers

require that the material used to redistribute the I/Os meet

a stress test identiﬁed in the JEDEC standard, JESD22, Test

Method A102B. The test consists of an exposure in a pres-

sure vessel for 168 hours. This severe accelerated stress

test permits the use of only the most robust materials for

the substrate interposer.

4.6.1.1 Bismaleimide Triazine-Glass (BT) Bismaleimide

triazine resins used in combination with glass fabric rein-

forcements are a common choice for the fabrication of sub-

strates used in BGA packages. The material is available

from a number of sources and provides good thermal per-

formance (based on a relatively high glass transition tem-

perature). In addition, the electrical properties of BT resin

(IPC-4101/30 with a T

range of 170-220°C) are suitable

for a great number of IC package applications.

IPC-7095c-4-25

Figure 4-25 LGA Contact Pin

Figure 4-26 LGA Socket With and Without Pick and Place

Cover

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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

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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