IPC-7095C-2013.pdf - 第37页

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. …

4.3.5 Package-on-Package BGA The package-on-

package (PoP) innovations are proving to be preferred for

a broad range of SiP applications. User companies have

realized that complex mixed-technology functions can be

produced with higher yield and more economically if the

semiconductors are individually prepackaged and tested

before joining. The most common solution for PoP appli-

cations utilizes package sections designed around the exist-

ing JEDEC FBGA standard array packaging format (see

Figure 4-5). Stacking prepackaged die has less risk because

the individual packages are fully tested before conversion

to the stacked PoP format.

Whether or not to join one package to the other before or

during the board level assembly process is a decision that

may be inﬂuenced by the requirement for in-process con-

ﬁguration ﬂexibility. For example, the base package can be

furnished by vendor ‘‘A’’ while the memory sections of the

stack are supplied by vendor ‘‘B,’’ ‘‘C’’ or ‘‘D.’’ After all,

memory functions are available from a greater number of

sources and the testing for memory is somewhat special-

ized. Furthermore, the concern of ownership of total qual-

ity and reliability can be alleviated. The logic device sup-

plier is responsible for the logic, the memory

manufacturers are responsible for the respective memory,

and the board assembler is responsible for only the surface

mount attachment of the two. This alternative has two ben-

eﬁts. It allows the user to specify multiple variations (dif-

ferent memory functions, data rate and so on) as well as

accommodating secondary sources of supply.

4.3.6 Coplanarity A critical issue in surface mount

packages is the limits for coplanarity of the contacts. The

coplanarity requirements in a BGA package are very differ-

ent from other lead-frame packaged surface mount compo-

nents. Coplanarity for any BGA is the distance of compo-

nent contact surface above a common seating plane. Thus

noncoplanarity, a simpliﬁed term, is the maximum distance

between the lowest and the highest contact when the pack-

age rests on a perfectly ﬂat surface. This deﬁnition repre-

sents a package sitting on a PC board on at least three

locations.

Coplanarity tolerance deﬁnes the distance from the seating

plane to the highest point of the package. This dimension

includes the standoff height, package body thickness and

(if present) lid thickness. The measurement criteria do not

include attached features such as heat sinks or other com-

ponents. An integral heat-slug, however, is not considered

an attached feature. If the package happens to be laminate

substrate based BGA, additional coplanarity issues can be

expected due to problems associated with adapting larger

substrates and maintaining ﬂatness within the tolerances.

This, in part, is the reason why the plastic BGA (PBGA)

package coplanarity requirement is established at 150 µm.

Most suppliers would like the allowable BGA coplanarity

limit to be around 200 µm but the users would prefer the

maximum to be no greater than 100 µm (see 4.6.2.6 and

4.8.4). There are different coplanarity requirements for dif-

ferent types of BGAs. Table 4-6 shows a sampling of

JEDEC registered BGA package outlines.

The coplanarity values may vary from JEDEC outline-to-

outline because of the ball metallurgy. In low temperature,

eutectic (183°C melting point) solder balls, the balls

Table 4-5 Land-to-Ball Calculations for Current and Future BGA Packages (mm)

Land Size

Location

Allowance

Ball

Variation

Ball Size

% Reduction

from Nom.

Variation

AllowanceMMC LMC Nominal MMC LMC

0.60 0.50 0.10 0.25 0.75 0.90 0.65 25% 0.25

0.50 0.40 0.10 0.20 0.60 0.70 0.50 25% 0.20

0.45 0.35 0.10 0.10 0.50 0.55 0.45 20% 0.17

0.40 0.30 0.10 0.10 0.45 0.50 0.40 20% 0.17

0.35 0.25 0.10 0.10 0.40 0.45 0.35 20% 0.17

0.25 0.20 0.05 0.10 0.30 0.35 0.25 20% 0.15

0.20 0.17 0.05 0.06 0.25 0.28 0.22 20% 0.08

0.15 0.12 0.05 0.04 0.20 0.22 0.18 15% 0.07

0.10 0.08 0.05 0.04 0.15 0.17 0.13 15% 0.07

IPC-7095c-4-5

Figure 4-5 JEDEC Standard Format for Package-on-

Package Components Source: JEDEC Publication 95-4.22

= 0.65 mm

0.50 mm

0.65 mm

0.80 mm

0.50 mm

Table 4-6 Examples of JEDEC Registered BGA Outlines

Registered Outline Package Type Coplanarity

MO-151 Plastic BGA 0.20 mm

MO-156/MO-157 Ceramic BGA 0.15 mm

MO-195 Fine Pitch BGA 0.08 mm

IPC-7095C January 2013

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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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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