IPC-7095C-2013.pdf - 第44页

increases beyond the T g 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…

The name coined to describe this unique contact proﬁle is

‘‘µPILR™,’’ providing signiﬁcantly smaller contact feature

and ﬁner pitch variations of 0.40 typical of the examples

shown in Figure 4-20.

When comparing the µPILR contact proﬁle to the more

common solder ball connections, the contact geometry is

signiﬁcantly smaller in both diameter and height, enabling

a much lower ﬁnished package proﬁle. The solid copper

core contact illustrated in Figure 4-21 is slightly tapered in

shape and coated with a nickel/gold (ENIG) alloy that is

compatible with either eutectic or lead-free soldering pro-

cesses.

Circuit board assembly of the µPILR packaged device is

very typical of most surface mount processes beginning

with solder paste printing, pick-and-place and reﬂow

soldering. The stencil successfully used for printing is a

100 µm thick stainless steel foil with laser ablated 270 µm

square apertures. Because of the very small stencil aper-

ture, the recommended solder paste used for this appli-

cation is a Type 5 powder size. Precise placement of the

0.40 mm pitch components is critical as well. When plac-

ing ultra-ﬁne pitch array devices, machine placement accu-

racy should be in the range of ± 20 µm to assure a reason-

ably uniform solder ﬁllet at all contact sites.

4.5 BGA Connectors and Sockets

4.5.1 Material Considerations for BGA Connectors

The

BGA connector shown in Figure 4-22 is designed to pro-

vide a relatively low proﬁle horizontal or parallel interface

between two circuit assemblies. The material engineered

for this application has been developed to withstand the

reﬂow soldering temperatures associated with surface

mount assembly and furnish a reliable interconnect in the

varying environmental conditions typical of the product’s

end use.

It is important to understand the material properties of the

connector system. During the life of the product, the circuit

board assembly will undergo many varying thermal cycles.

These thermal cycles will cause material expansion/

contraction to the assembly components, including the

BGA connector. Therefore, material selection for BGA con-

nectors is signiﬁcant due to the thermal interaction of the

connector to the PCB substrate. Speciﬁcally, matching

coefficient of thermal expansion (CTE) between the BGA

connector material and the PCB substrate material are

important to the reliability of the package and the solder

joints. A close CTE match ensures that the thermally

induced stresses are minimized.

4.5.2 Attachment Considerations for BGA Connectors

Several items need to be addressed concerning the place-

ment and soldering of BGA connectors to a PCB substrate.

Some BGA connector designs do not lend themselves to a

vacuum pick-up using standard SMT nozzles. In this case,

two options are available:

1) Mechanical chuck pick-up using a custom nozzle

2) Design the BGA connector with a cap or other tempo-

rary surface so a standard vacuum nozzle can be used

Both options can be successful in production, and the best

option is highly dependent on the connector design.

Depending on the connector material, reﬂow proﬁles have

to be examined and compared to the T

temperature of the

connector material. When the temperature of the connector

IPC-7095c-4-20

Figure 4-20 Semiconductors Packaged with µPILR Sub-

strate

Figure 4-21 Solder Interface Between µPILR-Conﬁgured

Substrate Interposer and Circuit Board

Figure 4-22 BGA Connector

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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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

No reproduction or networking permitted without license from IHS

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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

--`,,```,,,,````-`-`,,`,,`,`,,`---