IPC-7095C-2013.pdf - 第52页

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 re…

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

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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

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

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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

5.2.2.4 Aramid Paper The supply of nonwoven aramid

paper has diminished due to a lack of manufacturing

sources. Aramid papers have been effectively used in a

number of multilayer applications. They have most of the

beneﬁts of the aramid cloth with more process latitude.

They are often used for thin core layers at or near the sur-

faces of the printed boards to better control CTE. Because

the aramid is organic, it has the added advantage of being

more easily processed by laser ablation and can also be

processed using plasma etching for making holes. The

organic nature of the material also helps to keep the dielec-

tric constant low.

5.2.3 Laminate Material Properties There are a number

of different material properties that are important in the

selection of a laminate for BGA substrate manufacture.

5.2.3.1 Thermal Expansion Thermal expansion is usu-

ally characterized in terms of changes to the x-y plane,

which is controlled primarily by the reinforcement of the

material. The x-y expansion will have the greatest effect on

surface mounted components and their reliability. Thermal

expansion also occurs in the z-axis at a rate signiﬁcantly

larger than in the x-y plane, particularly at temperatures

above the T

. The z-axis expansion will have its greatest

effect on plated-through hole and via reliability.

Table 5-1 shows the conditions for various reinforced resin

types. All thermal expansion is measured in parts/million/

change in temperature (°C).

5.2.3.2 Glass Transition Temperature Glass transition

temperature is that property of the material where the rein-

forcement and the resin systems transition from a linear

coefficient of thermal expansion and expand at a much

higher rate. The glass transition temperature (T

) indicates

a temperature range within which resins change molecular

structure from a glassy to an amorphous state; these differ-

ent molecular structures result in very different physical

properties. This occurs when the resin system exceeds its

cured polymer state. It is usually an expansion in the Z-axis

of the material and the temperature stated expands at a

faster rate, although still linear (mm/mm of thickness).

Table 5-1 shows some of the characteristics of the condi-

tions for glass transition temperature of various material

types. Figure 5-2 illustrates the concept graphically and

shows how different resins might perform.

Table 5-1 Environmental Properties of Common Dielectric Materials

Environmental Property

Material

FR-4

(Epoxy

E-glass)

Multi-

Functional

Epoxy

High

Performance

Epoxy

Bismaleimide

Triazine/

Epoxy Polyimide

Cyanate

Ester

Coefficient of Thermal Expansion,

xy-plane, CTE(xy) (ppm/°C)

16-19 14-18 14-18 ~15 8-18 ~15

Coefficient of Thermal Expansion,

z-axis below T

, CTE(z,<T

) (ppm/°C)

50 - 85 44 - 80 ~44 ~70 35 - 70 ~81

Coefficient of Thermal Expansion

z-axis above T

1, CTE(z,>T

) (ppm/°C)

240 - 390 240 - 390 240 - 390 220 - 370 200 - 350 210 - 360

Thermal Expansion z-axis,

TE(50-260°C) (%)

3.0 - 4.5 2.5 - 4.0 2.0 - 3.5 2.0 - 3.5 1.8 - 3.0 2.0 - 3.3

Glass Transition Temperature

(°C)

110 - 140 130 - 160 165 - 190 175 - 200 220 - 280 180 - 260

Decomposition Temperature

Td (5%) (°C)

310 - 330 320 - 350 330 - 400 ~334 ~376 ~376

Soldering Temperature Impact Index

STII

170 - 205 200 - 220 215 - 260 TBD TBD TBD

Flexural Modulus (GPa)

Fill

Warp

18.6

12.0

18.6

20.7

19.3

22.0

20.7

24.1

26.9

28.9

20.7

22.0

Tensile Strength (MPa)

Fill

Warp

413

482

413

448

413

524

393

427

482

551

345

413

Water Absorption (wt%) 0.5 0.1 0.3 1.3 1.3 0.8

1. CTE (z,<T

) is also known as Alpha 1, and CTE(z, >T

) as Alpha 2. Contact supplier for speciﬁc values of the other materials.

2. The glass transition temperature can be measured by three different methods (TMA, DSC, DMA). Of these the values obtained by TMA are the most

pertinent for the purpose of assessing reliability issues. A very rough relationship between the results of these three methods is T

(TMA) ≈T

(DSC) -10°C

≈T

(DMA) -20°C. Contact supplier for speciﬁc values of other materials.

3. The decomposition temperature can be measured to two different values of weight loss, Td (2%) and Td (5%). Td (5%) is more commonly used, but Td (2%)

is becoming popular because of its greater usefulness. Contact supplier for speciﬁc values of other materials.

4. Soldering Temperature Impact Index, STII, which is deﬁned as STII = T

/2 + Td/2 — (TE%(50 to 260°C) x 10).

5. Fill - yarns that are woven in a crosswise direction of the fabric.

6. Warp (cloth) - yarns that are woven in a lengthwise direction of the fabric.

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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