IPC-7095C-2013.pdf - 第51页

bake-out ovens will be required. Bake-out may take between four hours and 48 hours at 125°C or ﬁve days to 68 days at 40°C depending on package thickness and size. In order to remove moisture from BGA components, a rec- …

be characterized by using the Shadow Moiré technique and

should meet the warpage reﬂow speciﬁcation.

The ﬁnal package coplanarity is a very complex combina-

tion of the factors stated at the beginning of this section. A

large percentage (~70%) of the package coplanarity is con-

trolled by factors 1 and 2. The type of solder ball used will

determine how much overall package coplanarity can be

tolerated during the ﬁnal package to board assembly. The

reason why collapsing eutectic solder and tin-alloy based

balls are the most popular is because they can compensate

for larger package coplanarity values.

The JEDEC design guidelines currently deﬁne the ball con-

tact diameter (b) at its maximum diameter, as measured in

a plane parallel to the seating plane, Datum C. The detail

illustrated in Figure 4-30 deﬁnes the proﬁle tolerance zone

controlling coplanarity (ccc) and the limits for parallelism

between the package top surface (bbb) and the seating

plane (Datum C).

Package height is measured from top surface of the pack-

age to the seating plane (where the ball meets the mount-

ing surface of the host printed circuit board). It is impor-

tant that the top surface of the package remains parallel to

the seating plane, accommodating component handling in

test, inspection and assembly. The bilateral tolerance zone

(bbb) for parallelism references the top surface of the pack-

age with respect to Datum C, the seating plane.

4.8.4.1 Example for FBGA Coplanarity Tolerance limits

for coplanarity varies slightly with the increase in ball

diameter. The following shows the Controlled Coplanarity

(ccc) per ball size:

0.30 mm ball = 0.08 mm (ccc)

0.40 mm ball = 0.10 mm (ccc)

0.50 mm ball = 0.12 mm (ccc)

The unilateral proﬁle tolerance zone (ccc) extends upward

from the seating plane. The lowest point of the ball contact

must be within the tolerance zone. Each ball has a toler-

ance zone associated with diameter ‘‘b’’ that is located on

true position with respect to Datums A and B and is per-

pendicular to Datum C. The center of each ball must lie

within the tolerance zone. The positional tolerance for the

ball contact is deﬁned with relationship to package outline

Datums A, B and C, as illustrated in Figure 4-31.

The array of tolerance zones associated with the ball diam-

eter (b), spaced on a basic pitch (e), controls the location

of the balls. The design guideline allows the array to ﬂoat

with respect to the tolerance zone ‘‘ddd’’; however, the

centers of the balls must simultaneously lie within both

tolerance zones. For more detail in measuring the BGA

package, see JEDEC JEP95, Section 4.17 (BGA Package

Measuring and Methodology).

4.8.5 Moisture Sensitivity (Baking, Storage, Handling,

Rebaking)

Moisture sensitivity requirements are deﬁned

by J-STD-020 and J-STD-033. The J-STD-033 provides

information on handling moisture sensitive components.

Components are segregated into eight levels as shown in

Table 4-10. These classes deﬁne how long a component

can be left out on the production ﬂoor once removed from

its sealed shipping bag. Parts exposed to ambient air for

longer than the speciﬁed time must be rebaked prior to use,

to drive out excess absorbed moisture.

Many BGA components are moisture sensitive; particular

attention should be paid to tape ball grid array (TBGA) and

ﬂip chip FBGA components. Ceramic BGA/CGA compo-

nents are generally not moisture sensitive. J-STD-020 lim-

its the amount of time in which the package is permitted to

absorb moisture, but it is recommended that BGAs meet at

least a Level 3 speciﬁcation. Level 5 and Level 6 parts are

particularly undesirable from a manufacturing handling

perspective because they drive additional shop ﬂoor and

component handling controls. In the case of Level 6 parts,

ccc

bbb

IPC-7095c-4-30

Figure 4-30 Establishing BGA Coplanarity Requirement

ddd

eee

b nX

IPC-7095c-4-31

Figure 4-31 Ball Contact Positional Tolerance

Table 4-10 Moisture Classiﬁcation Level and Floor Life

Level

Floor life (out of bag) at factory

ambient ≤30°C/60% RH or as stated

1 Unlimited at ≤30°C/85% RH

2 1 year

2a 4 weeks

3 168 hours

4 72 hours

5 48 hours

5a 24 hours

Mandatory bake before use. After bake, must be

reﬂowed within the time limit speciﬁed on the label

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bake-out ovens will be required. Bake-out may take

between four hours and 48 hours at 125°C or ﬁve days to

68 days at 40°C depending on package thickness and size.

In order to remove moisture from BGA components, a rec-

ommended bake cycle should be established. It should be

noted that the higher reﬂow temperatures required for lead-

free solders may require a more rigorous moisture removal

bake cycle.

4.8.6 Shipping Medium (Tape and Reel, Trays, Tubes)

As with all SMT components, BGA parts should be pack-

aged in Electrostatic Discharge (ESD) packaging which

meets the requirements of the appropriate standards or

speciﬁcations. BGA components should be available in

JEDEC approved matrix trays able to withstand multiple

bake cycles, as many BGAs are moisture sensitive. See

4.8.5 for required bake temperatures and times for the dif-

ferent classes of moisture sensitive components.

It may be preferable to procure high volume parts, e.g.,

SRAM or DRAM devices, in tape-and-reel for faster

assembly cycle times. Component size and moisture sensi-

tivity may dictate that tape and reel is not applicable for

some BGAs. Tape widths up to 56 mm are available, and a

tape ‘‘leader’’ of at least 200 mm should be provided. Cau-

tion must be used when baking parts in tape-and-reel, as

this type of packaging is usually restricted to lower bake

temperatures than matrix trays.

4.8.7 Solder Ball Alloy Component manufacturers

should establish a method to clearly identify the metallurgy

of the solder alloy used for the BGA balls, accompanied by

a part number change. J-STD-609 provides a convenient

method to identify the BGA ball alloy. Users may want to

verify the BGA alloy using screening methods such as

X-Ray Flourescence (XRF).

5 PRINTED BOARDS AND OTHER MOUNTING

STRUCTURES

Printed Boards (PBs) and other similar types of intercon-

nection platforms serve as mounting structures for BGAs

and other components. There is a wide variety of mounting

structures available to fulﬁll various interconnection sub-

strate requirements. These structures employ a wide range

of materials, both organic and inorganic and have a wide

range of physical properties. Materials are normally chosen

based on cost/performance needs of the ﬁnished product.

5.1 Types of Mounting Structures Following is an

examination of some of the more commonly used mount-

ing structure substrates.

5.1.1 Organic Resin Systems Organic substrates are

those that are most commonly used in the construction of

electronic interconnection structures. There is a well-

established worldwide manufacturing base for the organic

type of product. As a result of the large manufacturing

base, the organic materials used to create the structures

have the lowest cost among the competing technologies.

Organic materials have intrinsic beneﬁcial electrical prop-

erties. Most notable is a relatively low dielectric constant

which can be made lower by the choice of resin and rein-

forcement. Rigid organic substrates are commonly rein-

forced with woven glass cloth. Flexible substrates are typi-

cally not reinforced.

5.1.2 Inorganic Structures Inorganic substrates are an

alternative to the organic substrates. They are commonly

refractory materials comprised of sintered metal oxides.

While they are typically brittle, they have some signiﬁcant

beneﬁts not easily obtained with organic substrates.

Chief among the advantages are excellent thermal proper-

ties. Like organic structures there are a number of possible

choices available: ceramic, silicon, and enameled metals.

The dielectric properties of these materials tend to be

higher than organic based materials and, because they are

brittle, they are generally more prone to breakage. Because

of the more limited vendor base for inorganic substrates,

these structures are normally more expensive.

5.1.3 Layering (Multilayer, Sequential or Build-Up)

While single and two metal layer circuits are still common,

multilayer interconnection structures are commonly

required to support the interconnection of BGAs in today’s

high performance electronics. There are several approaches

to creating multilayer circuit product. The traditional mul-

tilayer is created by printing and etching thin layers of

copper clad substrates and laminating them into a mono-

lithic structure which can be drilled and plated so as to

make connection between the layers where required.

More recently, however, alternative structures have been

developed to address the higher density and routing diffi-

culties associated with BGAs. These newer structures

employ a variety of different approaches to create suitable

multilayer structures. The new structures are variously

referred to as build-up multilayers, sequential multilayers

and co-laminated multilayers. A key feature of these struc-

tures is their use of very small vias. The term microvias has

been applied to describe these miniscule interconnections.

A typical microvia is less than 150 µm in diameter and has

a capture land (where the via starts) and a smaller target

land (where the via ends). The following examples in Fig-

ure 5-1 show different high density interconnect (HDI)

printed boards (see IPC-2226).

5.2 Properties of Mounting Structures

5.2.1 Resin Systems

There are a number of different

resin systems suitable for use in organic laminate construc-

tion. There is a long and well understood history and years

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of faithful service among the traditional resin systems.

However, to support the move to meet legislated lead free

requirements by the EU, many new resins are being devel-

oped to meet the higher temperature assembly reﬂow

requirements. Test methods have been developed, i.e., Td

(Temperature of decomposition) and T260, T288, T300

(Time to Delaminate), to quantify material properties for

conformance to the new EU requirements. Some of the

new resin systems are classiﬁed to new material slash

sheets such as IPC-4101 /99, /101, /121, /124, /126 and

/129.

5.2.1.1 Epoxy Epoxy is among the organic resins with

the longest history and it is one of the most commonly used

resin systems for PCBs. It offers a good blend of physical,

electrical and processing properties at reasonable cost. The

general properties are provided in Table 5-1. Higher tem-

perature capability epoxy resin systems have been devel-

oped for lead-free applications and are available at a cost

premium.

5.2.1.2 Polyimide Polyimide offers the highest operating

temperature among resin systems in use today. It has been

a favorite for military applications where the potential for

board rework and repair in the ﬁeld with uncontrolled sol-

dering tools is anticipated. Because of its high glass transi-

tion temperature, polyimide provides a safety margin and

potential to reduce damage to the board when uncontrolled

soldering irons are used to remove or replace a component.

The general properties are provided in Table 5-1.

5.2.1.3 Bismaleimide Triazine Bismaleimide triazine or

BT resin is the most popular choice for the construction of

BGA packages because of its combined advantages of high

temperature capability at reasonable cost. The general

properties are provided in Table 5-1.

5.2.2 Reinforcements Reinforcements provide the

dimensional stability and the bulk of the mechanical prop-

erties of the organic substrate laminate. Following are some

of the more commonly used reinforcements.

5.2.2.1 Glass Cloth Glass cloths are the most commonly

used reinforcement for PCB substrates. They are widely

available and are processed with relative ease. The cloths

are available in a number of different thicknesses and

weaves. The chemical make-up of the glass can vary and

can affect the electrical properties. Presently, E type glass

is the most commonly used glass cloth for PCB substrates.

5.2.2.2 Glass Felt Glass felt or nonwoven glass mat has

been commonly used as a reinforcement material for ﬂuo-

roplastic resins and is commonly used in low loss, RF or

microwave applications. It has seen some application in

formable laminates as well.

5.2.2.3 Aramid Cloth Aramid cloth has been used to

reinforce certain laminates. It is unusual in that it has a

negative CTE in the X and Y direction which helps to off-

set the in-plane CTE of the resin. Because of the counter-

acting expansion and contraction, laminate materials of this

combination can match approximately the CTE of ceramic.

However, a drawback of the material is that the aramid has

Z-axis CTE much higher than glass and, in thermal excur-

sions, can fracture nearby resin, leaving micro-cracks along

the surface of the ﬁbers.

IPC-7095c-5-01a,b,c,d

Figure 5-1 Examples of Different Build-Up Constructions

A. Piercing post co-laminated structure - Type IV HDI Construction

B. Sequentially built-up multilayer - Type II HDI Construction

C. Passive substrate with external microvia layers interposer - Type IV HDI Construction

D. Filled via sequentially co-laminated substrate - Coreless Type V HDI Construction

IPC-7095C January 2013

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Not for Resale

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