IPC-7095C-2013.pdf - 第38页

require peak reﬂow temperatures from 240-260°C, depend- ing on the overall mass of the assembly being processed. Depending on the speciﬁc alloy composition speciﬁed, the liquidus point of the solder ball can vary by as m…

collapse during the assembly operation; therefore, the

coplanarity requirement is not as tight as a high tempera-

ture (302°C melting point) solder balls in which the balls

do not collapse during the assembly operation.

4.4 Component Packaging Style Considerations The

JEDEC Design Guidelines for BGA do not deﬁne speciﬁc

materials or method of assembly. The base material will

vary from one supplier to another depending on applica-

tion. The base structure is most likely a reinforced organic

laminate, a nonreinforced polyimide ﬁlm or ceramic.

Ceramic-based BGA packages are often supplied with non-

collapsing solder balls made of high temperature solder

(90% lead, 10% tin) with a melting point of 302°C. The

ball size will vary with the pitch and package size. Larger

packages will usually have larger ball sizes to improve

reliability. Since the ceramic package is relatively very ﬂat

and the tolerances in ball diameters are very narrow, the

coplanarity requirements can be relatively narrow as well.

The supplier of the ceramic-based package will generally

furnish recommendations for selecting a suitable solder

paste composition for board level assembly.

In an effort to develop new attachment methods, a patent

was ﬁled in the late 1990’s that uses a polymer-coated ball

as the interconnection media. The metallic sphere is con-

ductive and may be made of copper, silver, gold, solder,

etc. and then coated with the conductive polymer. The

method of coating is left up to the ball producer and may

be of different thickness as required for the application.

This novel interconnect method was developed in order to

address the different CTE of a ceramic interposer and the

organic board. Several technical papers have been pub-

lished on the system that heats the interfaces and how the

conductive polymer joins the land on the interposer and the

board. Temperature cycling data was obtained from both

conventional and polymer core balls tested on ceramic

packages mounted on an FR-4 board. An electrical simula-

tion was also performed to compare the polymer-core ball

with a conventional solder ball. Figure 4-6 shows the con-

cept of the process.

Laminate and polyimide ﬁlm-based BGAs are very differ-

ent, however. The laminate based package is essentially

made of circuit board material with a high temperature (T

)

rating. A high T

rated resin system adopted by several

companies for BGA package applications is bismaleimide

triazine (BT). Reinforced polyimides and polyimide ﬁlms

have an even higher temperature rating and are also in

wide use for both BGAs and FBGAs.

4.4.1 Solder Ball Alloy Solder ball composition is

deﬁned by the overall PCA technology, e.g., tin/lead or

lead-free, and the package type, e.g., ceramic or laminate

substrate. The alloy composition selected for ball contacts

on reinforced laminate and polyimide ﬁlm based BGA

packages can vary a great deal. Many are furnished with a

tin/lead eutectic solder having a melting (liquidus) point of

183°C (or 179°C for eutectic solder with 2% silver). The

ball contacts are commonly applied to the package sub-

strate using only ﬂux and a reﬂow soldering temperature of

215-220°C to complete the joining process.

4.4.1.1 Tin/Lead Technology Alloys Although not in

wide use for most commercial applications, tin/lead alloys

can be adapted for both solder ball and solder joining mate-

rials. One common alloy composition is the Sn63Pb37

eutectic, with a liquidus temperature of 183°C. The eutec-

tic silver containing alloy composition, Sn62Pn36Ag2, has

a liquidus temperature of 179°C and is an acceptable alter-

native to eutectic Sn63Pb37. Ceramic BGAs, when used in

the tin/lead technology, often employ a high-lead alloy of

Sn10Pb90 composition to provide the required solder joint

reliability. This alloy does not melt during the tin/lead

reﬂow process, but when attached to the PCB with eutectic

Sn63Pb37, provides a reliable interconnect.

4.4.1.2 Lead-free Technology Alloys In compliance

with the RoHS directive (Restriction of the Use of Certain

Hazardous Substances in Electrical and Electronic Equip-

ment), companies manufacturing electrical and electronic

assemblies and electrical components must use Pb-free

(lead free) materials for both ball contacts and package to

substrate joining. Although a number of Pb-free alloy

compositions are available, a majority of manufacturers

have adopted tin as the primary element and include

silver or a combination of silver and copper to the tin

alloy. Typical Pb-Free alloy compositions include

Sn96.5Ag3.0Cu0.5 (SAC305), Sn95.5Ag3.8Cu0.7

(SAC387) or Sn95.5Ag4.0Cu0.5 (SAC405). These solders

have liquidus temperatures in the range of 217-227°C and

IPC-7095c-4-6

Figure 4-6 Polymer Coated Sphere Interconnection

Copper

Land

Solder

Mask

Package

Substrate

Interconnection

Substrate

Copper

Land

Polymer

Core

Solder

January 2013 IPC-7095C

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require peak reﬂow temperatures from 240-260°C, depend-

ing on the overall mass of the assembly being processed.

Depending on the speciﬁc alloy composition speciﬁed, the

liquidus point of the solder ball can vary by as much as

10°C (see Table 4-7).

Within the SnAgCu system, the addition of other alloying

elements has also been suggested; however, these additions

can affect solder under-cooling, the formation of various

intermetallics, unusual matrix properties and changes in

solder microstructure. Great care must to be taken when

introducing new alloys into BGAs and new alloyed BGAs

into the assembly process. Outside the SnAgCu system,

various alloy systems containing Bi, In, Sb or Zn have also

been developed. The underlying metallurgy of each system

differs, which drives the physical properties and mechani-

cal behavior; thus, assembly process parameters and solder

joint reliability.

4.4.2 Ball Attach Process The package substrate is typi-

cally fabricated in a strip format containing multiple pack-

ages (see Figure 4-7). Ball contact placement is performed

after they have gone through wire bonding and plastic

molding or encapsulation process steps. Both automated

and semi-automated ball placement is being utilized for

volume BGA assembly. Alloy spheres of the desired size

(as shown in Table 4-1) are attached either by gang place-

ment machines, individually placed using automated sys-

tems, or dispensed in mass with a stencil-like ﬁxture. For

companies doing development or for low-volume place-

ment, simple template ﬁxtures can be provided for precise

ball positioning. The overall ball attachment process, how-

ever, is the same. To begin, liquid or ‘‘tacky’’ paste ﬂux is

dispensed or printed onto the contact pattern. The ﬂux

holds the balls in place during reﬂow soldering. Reﬂow

soldering of the ball to the substrate is often performed in

a nitrogen gas environment. The nitrogen gas environment

helps provide consistent ball quality and keeps the surface

from oxidizing during reﬂow. However, nitrogen gas may

not be necessary for reﬂow attachment of the package to

the PCB. The eutectic solder balls provide a ‘‘controlled

collapse’’ that, during reﬂow soldering, promotes self-

alignment (compensating for some misplacement during

assembly).

There are some issues to consider when using plastic BGA

packages. The ﬁrst is moisture sensitivity. Moisture sensi-

tive components around other components being removed

and replaced during rework must be dried in advance

and/or protected from the heat of the rework activity. If the

plastic cased BGA is to be re-used then special care will be

required to re-ball the BGA after the package is detached

from the board assembly. Moisture sensitivity is not the

case with the high melting point noncollapsing balls used

on ceramic based BGA; also the balls do not melt during

rework.

The plastic BGA package is also susceptible to warpage

when exposed to temperatures required for solder attach-

ment. The edges of the package tend to lift up or curve

down during reﬂow soldering and can severely disturb or

interrupt the electrical interface between package and board

assembly. The larger packages are even more susceptible to

warpage than the smaller packages. The package warpage

is caused by CTE mismatch between the substrate struc-

ture, the mold compound and the silicon die inside. This

problem may become more acute when the die is large, or

when the BGA has a heat spreader.

4.4.3 Ceramic Ball Grid Array The internal connections

in the ceramic-based package can be either wire-bond or

ﬂip-chip. Figure 4-8 shows ﬂip-chip bonding inside the

package. The package can be furnished with the die

mounted on the top surface of the substrate (cavity up) or

with the die mounted to the substrate’s lower surface (cav-

ity down). The solder balls generally used for ceramic

packages typical of that illustrated in Figure 4-9 are a high

temperature alloy composition (90% lead and 10% tin)

Table 4-7 Pb-Free Alloy Variations

Composition Alloy

Liquidus

Temperature

Reﬂow

Temperature

Sn96.5/Ag3.5 Tin/Silver 221°C 240-250°C

Sn99.3/Cu0.7 Tin/Copper 227°C 245-255°C

Sn93.6/Ag4.7/

Cu1.7

Tin/Silver/

Copper*

216°C 237-247°C

Sn95/Ag4.0/

Cu1

Tin/Silver/

Copper*

218°C 238-248°C

Sn96.5/Ag3.0/

Cu0.5

Tin/Silver/

Copper*

218°C 238-248°C

Sn95.5/Ag4.0/

Cu0.5

Tin/Silver/

Copper

218°C 238-248°C

Sn95.5/Ag3.8/

Cu0.7

Tin/Silver/

Copper

218°C 238-248°C

Sn96.3/Ag3.2/

Cu0.5

Tin/Silver/

Copper

218°C 238-248°C

Sn95.75/Ag3.5/

Cu0.75

Tin/Silver/

Copper*

218°C 238-248°C

* May be covered by patents

IPC-7095c-4-7

Figure 4-7 Plastic Ball Grid Array (BGA) Package

Die

Package

Substrate

Package

Substrate

Wire Bond

IPC-7095C January 2013

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with a melting point of 302°C. The ball attachment alloy,

however, may be a eutectic solder (Sn63Pb37).

Although polymer encapsulation or over-molding is widely

used to encase the die area, some ceramic-based BGA

packaged devices are hermetic (do not absorb moisture).

Since solder balls often adapted for these products have a

high melting point, they do not collapse during rework.

The disadvantage of the ceramic-based BGA is that its high

thermal mass will be slightly different than the plastic

packaged ICs and can make solder reﬂow proﬁle develop-

ment more difficult. Because of the CTE mismatch between

the ceramic-based package and the circuit board, the solder

joints may need to be physically reinforced. Following the

soldering and cleaning process, it is a common practice to

apply an epoxy based underﬁll between ceramic substrate

interposer and organic PCB materials.

4.4.4 Ceramic Column Grid Arrays Solder column con-

tacts typical of that illustrated in Figure 4-10 are used for

larger ceramic-based packages (32 mm to 45 mm). The

package resembles the earlier pin-grid-array but with closer

contact pitch and more fragile leads (columns). The column

contact diameter is approximately 0.5 mm with its length

varying from 1.25 mm to 2.0 mm. The columns are

attached to the package either by eutectic (Sn63Pb37) sol-

der or they are cast in place using 90% lead and 10% tin.

The longer columns typically increase solder joint reliabil-

ity by absorbing a great deal of the thermal expansion mis-

match between the ceramic package and the board.

Longer columns, on the other-hand, may reduce electrical

performance and will increase the overall package height

proﬁle. The columns are not as rugged as ball contacts and

are susceptible to handling damage.

4.4.5 Tape Ball Grid Arrays A tape (polyimide ﬁlm)

based ball grid array, illustrated in Figure 4-11, can furnish

a lower overall proﬁle package. The low dielectric polyim-

ide ﬁlm can be furnished with one or two metal layers for

high density in-package circuit routing.

For polyimide interposer-based BGAs, typical of that illus-

trated, the CTE mismatch is not an issue since the die

attach adhesive and substrate ﬂexibility will take up strains

within the package structure. Tape-based BGAs can adapt

ﬂip-chip, wire-bond or lead-bond to achieve die-to-

substrate interconnection. The single metal layer tape sub-

strate is typically used for low cost and low lead count

package applications and the two metal layer tape for

higher lead count or performance driven applications.

An additional layer of copper, for example, can provide an

efficient ground return, signiﬁcantly lowering inductance

and reducing the effects of switching noise. The ground

plane effect impacts noise level reduction as well, but the

number of current sinks within the ground plane will also

inﬂuence inductance levels. The two metal layer substrate

compared in Figure 4-12 not only furnishes better electri-

cal performance, it also provides a signiﬁcant improvement

in in-package circuit routability.

Circuit routing of the single metal material is limited to the

narrow gap of dielectric between ball contact attachment

sites. As the contact pitch reduces down to 0.50 mm, the

space between contact features is reduced to 75 µm, pro-

viding the possibility of routing only a single circuit trace.

This factor limits the use of the single metal layer to a nar-

row band of low I/O package applications.

4.4.6 Multiple Die Packaging Portable and wireless

electronics represent the most aggressive growth area for

high-density package technology. In both circuit board fab-

rication and IC packaging, the technology for compressing

even the most sophisticated electronic functions into a

smaller and lighter ﬁnished product continues to evolve.

Portable or handheld electronics are a natural target. Digi-

tal cameras and camcorders, for example, must consider

ease of use, lighter weight and performance. Cellular

phones, pagers, personal communicators, palm top comput-

ers, industrial and automotive electronics, personal GPS,

medical and diagnostic products, are all viable candidates

for more efficient device miniaturization.

IPC-7095c-4-8

Figure 4-8 Cross-Section of a Thermally Enhanced

Ceramic Ball Grid Array (CBGA) Package

Metal Alloy

Heat Spreader

Thermal Interface

Material (TIM)

Flip Chip

Mounted

Semiconductor

Chip

Ceramic Alumina

Substrate Base

Solder Balls

IPC-7095c-4-9

Figure 4-9 Ceramic Ball Grid Array (CBGA) Package with

Molded Polymer Encapsulation

Encapsulant

Solder Ball

(Lead Free)

Ceramic

Substrate

(Double Side)

Bonding

Wire

Chip

January 2013 IPC-7095C

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