IPC-7095C-2013.pdf - 第53页

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 ef fectively used in a number of multilayer applications. They have most of the bene…

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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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.

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Laminates of different resins often have different glass

transition temperatures resulting in different capabilities in

high temperature applications. Lead free with its high tem-

perature processing will require higher performance lami-

nates. These are normally a higher cost.

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) ≈

(DSC) -10°C ≈ T

(DMA) -20°C.

5.2.3.3 Moisture Absorption Most organic materials are

hygroscopic to some degree and soak up moisture at differ-

ent rates; some do so relatively rapidly. This moisture

absorption changes the electrical properties of the material,

such as loss tangent and the processing characteristics of

the material, as outgassing can result in blisters. It can also

impact physical dimensions and the laminate’s weight.

Thus a simple way to determine that the material has

absorbed moisture is to note the increase in weight under

deﬁned moisture exposure conditions. Table 5-1 shows the

water absorption rate by weight for the various materials

highlighted in this section. IPC-1601 deﬁnes packaging

standards to reduce moisture absorption and test procedures

to determine the moisture content in a printed board.

5.2.3.4 Reliability Concerns with High Lead-free Solder-

ing Temperatures

The higher temperatures required for

soldering lead-free solders creates reliability concerns for

the survivability of the PCB resins as well as the integrity

of the PCB interconnect structures, such as plated-through

holes and vias. The properties that are most important in

this respect are the decomposition temperature, thermal

expansion, and the glass transition temperature. The

decomposition temperature (T

) measures the temperature

at which the resin decomposes irreversibly and thereby

loses weight; typically the temperature to a weight loss

of 2% or 5% is measured. The thermal expansion from

50-260°C, TE(50-260°C) is a composite of the thermal

expansions below and above the glass transition

temperature.

The impact of these three properties is captured with the

Soldering Temperature Impact Index, STII, which is

deﬁned as STII = T

/2 + T

/2 - (TE%(50-260°C) 10).

5.3 Surface Finishes The primary purpose of a surface

ﬁnish is to prevent oxidation of exposed copper on the

PCB; this ensures the surface is solderable when compo-

nents are mounted or inserted. Surface ﬁnishes also provide

several other functions; these include: reliable contact sur-

face for probes, contacts or switches, wire bonding and a

ﬂat surface for solder paste printing. Although BGAs are

the focus of this document, other components and assem-

bly operations must be taken into consideration when

choosing the most appropriate surface ﬁnish.

There is not an ideal surface ﬁnish that is best suited for all

applications; however, the search continues for improved-

surface ﬁnish solutions. Some of the application features

that must be considered in selection of a suitable surface

ﬁnish are given in Table 5-2. PCB handling is critical

because salts from hands can damage the surface ﬁnish

IPC-7095c-5-2

Figure 5-2 Expansion Rate Above T

Expansion

Temperature

-1

-2

Resin laminate

system 1

Resin laminate

system 2

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