IPC-7095C-2013.pdf - 第22页

for the routing requirements for the organic high-density microcircuit board manufacturer . In addition, underﬁll is usually required to accommodate and soften the mismatch between the coef f icient of thermal expansion …

Placing signal pin assignments on the outer rows of an

array package, and using the inner balls for power and

ground will facilitate escape routing. However, the corner

balls of large array packages are more susceptible to

mechanical failure and, therefore, it may be better to use

these for redundant ground connections. The number of

rows of signal I/O that can be routed out will depend on the

desired number of conductor routing layers in the printed

board and the number of conductors that can be routed

between lands and vias.

Figure 3-5 shows examples of conductor and space widths

that will ﬁt between adjacent lands with various pitches

and land diameters. Note that as the ball pitch decreases,

the conductor width and spacing for a given number of

conductors per channel also decreases, and it becomes

more difficult and costly to produce the board.

Using 150 µm conductors and spaces is quite cost effective,

but printed board cost begins to increase signiﬁcantly for

100 µm conductors and spaces. Using an organic intercon-

necting substrate to mount the bare die within a plastic

BGA requires that the mounting lands on the substrate

match the bonding lands on the die.

The bonding lands are typically positioned for wire bond-

ing, since this is the most popular technique. Thermally

conductive adhesive is one of the methods used to attach

the back of the die to the substrate. Depending on the

number of I/O and the lead pitch, multilayer substrate fab-

rication techniques may be used to translate a peripheral

bonding land die, to an area array matrix of bumps, balls,

or columns (see Figure 3-6).

The transition of chip bonding lands that are in an array

format permits the mounting of the die in ﬂip chip conﬁgu-

rations. In this instance, the die is mounted opposite to that

which is wire-bonded and the bumps of the die come into

direct contact with the substrate being used to convert the

die pattern to the BGA pattern. This creates new challenges

Figure 3-5 Conductor Width to Pitch Relationship

Conventional FR-4

125 µm Line

125 µm Space

700 µm Land

Conventional FR-4

125 µm Line

125 µm Space

600 µm Land

High Density FR-4

100 µm Line

100 µm Space

600 µm Land

Next Gen FR-4

60 µm Line

50 µm Space

300 µm Land

Next Gen Microvia

50 µm Line

50 µm Space

50 µm Land

Typical Microvia

75 µm Line

100 µm Space

200 µm Land

0.25 mm Pitch 0.5 mm Pitch 0.75 mm Pitch 1.0 mm Pitch 1.27 mm Pitch

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for the routing requirements for the organic high-density

microcircuit board manufacturer. In addition, underﬁll is

usually required to accommodate and soften the mismatch

between the coefficient of thermal expansion (CTE) of the

chip and the CTE of the organic multilayer board (see

Figure 3-7).

3.1.3 Assembly Equipment Impact Implementing BGA

technology may require some new assembly capability.

Depending upon the type of pick and place systems, a

change in package carrier mechanism may also be required

to transfer packages from matrix tray to the pick position.

Fiducials may also help vision systems recognize the exact

location of the land pattern for the BGA, similar to what is

used for ﬁne-pitch peripheral leaded parts. Large BGA

parts on tape-and-reel will require 44 mm and 56 mm feed-

ers depending on the body size. Use of a forced air convec-

tion oven is preferred. Repair and inspection of BGAs are

rather difficult. Rework machines with paste deposition,

preheat, and vision capability may not be required, but are

very helpful. X-ray and optical inspection (endoscope)

capability for process development is a beneﬁt.

3.1.4 Stencil Requirements The stencil thickness may

need to be reduced when using ﬁner pitch BGA parts. Sten-

cil thickness and land size will determine paste volume,

which is very critical for ceramic BGAs. It is helpful to

have trapezoidal stencil apertures (slightly larger opening

on the bottom than on the top) for better paste release.

Generally, on larger BGA components with 1.25 mm and

1.00 mm pitch, the aperture is large enough that stencil

clogging, print registration and deﬁnition are less of a prob-

lem than with quad ﬂat pack (QFP) components.

Matching solder paste stencil openings to the requirements

of ﬁne-pitch BGAs requires an understanding of the rela-

tionship between the stencil aperture and the size of the

particles in the paste. IPC-7525 provides good descriptions

to help make the appropriate relationship decisions as the

land patterns for attachment become smaller and are closer

to one another.

Overmolded Epoxy

BT Substrate

Wire Bonds

Die Attach

Solder Balls

(Sn63Pb37)

Silicon Die

IPC-7095c-3-6

Figure 3-6 Plastic Ball Grid Array, Chip Wire Bonded

IPC-7095c-3-7

Figure 3-7 Ball Grid Array, Flip Chip Bonded

High Melt Temperature Solder

or Z-Axis Interconnection Material

Soldermask

Epoxy Underfill

Copper Circuitry

and Plated Vias

Thermal Vias

Solder Balls

IC chip

High Performance

Laminate Material

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3.1.5 Inspection Requirements As with any surface

mount part, BGAs should not be moved after component

placement because this may smear the paste and cause sol-

der bridges. Even if some misalignment is visually

observed, many parts will self-align during reﬂow if their

terminations are no more than 50% off the land. If a BGA

has a gross misalignment problem, it should be removed

before reﬂow and reworked later. Though it may not be

practical for high volume production, using X-ray or opti-

cal inspection (endoscope) to inspect failures before

removing the part may be desirable.

3.1.6 Test Test strategies need to be developed before

using BGAs. The solder joints cannot be probed and test

points are required. It may be difficult to incorporate

enough test points to adequately test all solder joints. Some

alternative test strategies may be needed such as designing

BGA components with boundary scan capability designed

into them reducing the number of I/O fanout probe points.

In an effort to improve test capability, some BGA compo-

nents had test points designed right on the top of the pack-

age. This was not a good solution since it put pressure on

the BGA components and the joints.

BGAs into products, it is important to address not just the

technical issues, but also the possible business implica-

tions. It is very likely that time-to-market will be adversely

impacted if both products and the technology are devel-

oped simultaneously. It is a good idea to ﬁrst develop and

validate the technology before implementing it on a real

product. Otherwise, if any problem develops in the product

or technology, the deadline for product shipment will be

missed. To assure time-to-market readiness, BGA imple-

mentation methodology and process steps should be ana-

lyzed.

3.3 Methodology Several factors must be considered

when making design decisions in an attempt to balance

size, cost, and functionality. In addition to these consider-

ations, the product must also be able to perform reliably

over the expected lifespan of the product in its intended

environment. Package selection may be inﬂuenced by these

reliability factors and environmental conditions such as

temperature vibration, shock, and humidity.

3.4 Process Step Analysis There are several available

paths to utilizing BGAs effectively. The length of each path

depends on what design and assembly facilities a company

presently has, and how quickly they can be made ready for

production. The following is an example of one approach:

1. Select a list of candidate products for BGAs.

2. Develop an equipment list based on the projected vol-

ume needs. If sufficient in-house expertise does not

exist, it may be desirable to use a reputable training

center or consultant to save cost and time.

3. Organize a team representing design, production, test,

quality, and purchasing. This team is responsible for

component and equipment selections and review.

4. Develop a comprehensive BGA design guide that

stresses manufacturability. Use existing standards where

possible.

5. Design the candidate products starting with the conver-

sions of existing products using ﬁne pitch components.

6. Determine the need for lead-free products including the

alloy used on the part as well as the surface ﬁnish

needed on the mounting substrate.

7. Conduct rigorous assembly and test reviews. Carefully

monitor component purchasing to assure that compo-

nents have the speciﬁed package, shipping method, met-

allization, solderability, and orientation in the shipping

containers.

8. Develop comprehensive workmanship standards and a

process control system that is statistically sound.

9. Design the remaining candidate products.

With the major emphasis on using parts that meet both

customer requirements and conform to new environmental

regulations, many customers are requiring reports listing all

materials used at the component level and also for com-

pleted assemblies. To help facilitate this, IPC has devel-

oped IPC-1751 and IPC-1752 on Materials Declaration,

and have encouraged software providers to make tools

available that meet the requirements of those standards.

The requirement to establish a formal declaration system

has been in place since the automotive industry was chal-

lenged by the ‘‘End-of-Life’’ European directives.

To show an example of the breadth of the variation in

material properties that may occur in products, Table 3-3

shows a list of materials that might be used as a surface

ﬁnish or a material that was added to the assembly as the

second level interconnection.

3.5 BGA Limitations and Issues Even though BGA tech-

nology has moved into the mainstream, there are still some

decisions that need to be considered. These are business

and technical issues that must be resolved. The areas of

special concern are:

• Visual inspection

• Moisture sensitivity

• Rework

• Cost

• Availability

• Voids in BGA

• Open joint (BGA or PoP BGA)

• Head-on-pillow phenomenon

• Standards and their adoption

• Reliability concerns

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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