IPC-7095C-2013.pdf - 第45页

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…

increases beyond the T

point, the connector will tend to

either bow toward the board, or bow away from the board

(‘‘warp’’). The actual behavior is a function of the connec-

tor geometry, connector material, and the surface tension of

the connector balls to the substrate. Also included in this

analysis are the connector coplanarity requirements for suc-

cessful soldering. The material properties behavior during

reﬂow and the overall connector size will dictate the con-

nector ball coplanarity requirements. Typically, the copla-

narity requirements of BGA connectors are more stringent

than those of BGA IC packages, due in part to their larger

size.

4.5.3 BGA Materials and Socket Types BGA sockets

are designed to provide the interface between the processor

or other components and the circuit board. These sockets

are primarily made using a glass polymer material that can

withstand a reﬂow of up to 265°C. One advantage of the

glass polymer material is that it has a CTE approximately

the same as the circuit board. The interface between the

socket and circuit board utilizes BGA technology, but there

are two different designs depending upon the component

interface of the package they need to mate with. The ﬁrst

is for Pin Grid Array (PGA) components, the other is for

Land Grid Array components. Sockets for PGA compo-

nents are typically of a zero insertion force type (ZIF) and

utilize a cam nut that moves a cover-plate that the pins

drop through, forward pushing the pins into the contacts

and providing the load needed to make electrical contact

(see Figure 4-23 and Figure 4-24).

LGA sockets on the other hand use a contact bent at a pre-

cise angle allowing them to contact the land on the pack-

age. The package must have a downward load applied in

order to push the PGA Pin down against the connector

contacts. A loading mechanism is thus required and may be

built into the socket body, or in the case of an Independent

Loading Mechanism (ILM), it is installed after the socket

has been reﬂowed to the circuit board. The loading mecha-

nism utilizes a lever that when closed applies the necessary

downward force on the component. Both LGA and PGA

socket designs utilize a pick and place cover that snaps

over the socket. These covers serve a dual role of both

protecting the contacts and providing a ﬂat surface for the

placement equipment nozzle to pick them up (see Figure

4-25 and Figure 4-26).

4.5.4 Attachment Considerations for BGA Sockets BGA

sockets have many similar requirements for successful

attachment as those of other BGA components. It is very

important to maintain proper ball coplanarity and control

socket warp before and during reﬂow. BGA socket han-

dling is even more critical than other BGA components due

to the fact that the solder ball is attached to a paddle and

not directly to a component body or substrate. If bumped,

the paddle can easily be bent, moving the solder ball out of

IPC-7095c-4-23

Figure 4-23 PGA Socket Pins

Socket

contact

PGA Pin

Figure 4-24 PGA Socket With and Without Pick and Place

Cover

IPC-7095C January 2013

Provided by IHS under license with IPC

Not for Resale

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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

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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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

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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