IPC-7095C-2013.pdf - 第54页

Laminates of dif ferent resins often have dif ferent glass transition temperatures resulting in dif ferent capabilities in high temperature applications. Lead free with its high tem- perature processing will require high…

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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
g
. 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
g
) 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
g
1
, CTE(z,<T
g
) (ppm/°C)
50 - 85 44 - 80 ~44 ~70 35 - 70 ~81
Coefficient of Thermal Expansion
z-axis above T
g
1, CTE(z,>T
g
) (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
2
,
T
g
(°C)
110 - 140 130 - 160 165 - 190 175 - 200 220 - 280 180 - 260
Decomposition Temperature
3
,
Td (5%) (°C)
310 - 330 320 - 350 330 - 400 ~334 ~376 ~376
Soldering Temperature Impact Index
4
,
STII
170 - 205 200 - 220 215 - 260 TBD TBD TBD
Flexural Modulus (GPa)
Fill
5
Warp
6
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
5
Warp
6
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
g
) is also known as Alpha 1, and CTE(z, >T
g
) 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
g
(TMA) ≈T
g
(DSC) -10°C
≈T
g
(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
g
/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
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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
g
(TMA) ≈
T
g
(DSC) -10°C ≈ T
g
(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
d
) 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
g
/2 + T
d
/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
g
Expansion
Temperature
T
g
-1
T
g
-2
Resin laminate
system 1
Resin laminate
system 2
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(especially OSP). As a general rule, PCBs should be only
be handled by their edges. To achieve maximum shelf life
with any surface finish proper packaging and storage is a
requirement.
5.3.1 Hot Air Solder Leveling (HASL) In this process, the
finished PCB is dipped either vertically or horizontally into
a molten solder bath at about 260°C and the excess solder
is blown away and leveled with hot air, giving the process
its name. HASL is the first heating stress that the PCB
experiences. Any evidence of nonwetting or dewetting is
immediately apparent as the board exits the process.
5.3.1.1 Tin/Lead HASL At one time tin/lead HASL was
the main surface finish solution for PCBs. However, coat-
ing thickness uniformity is a major concern for SMT and
BGA components - the solder thickness varies widely from
0.8 to 0.38 µm. It is generally held that the lower thickness
is not acceptable because the very thin layer of solder is
completely transformed into copper-tin intermetallic, which
Table 5-2 Key Attributes for Various Board Surface Finishes
HASL
SnPb/SnCu OSP
Electroless NI/
Immersion AU
Electrolytic Ni/
Electroplated Au
Immersion
Silver
Immersion
Tin
Shelf Life proper
Handling
1 Year 6-9 Months 1 Year 1 Year 6-9 Months 6 Months
Handling Normal
Avoid
physical
contact
Normal Normal
Avoid
physical
contact
Avoid
physical
contact
SMT land
Surface topology
Domed/
Flatter
Flat Flat Flat Flat Flat
Solderability after
multiple reflow
cycles (2x)
Good, Good Good Good Good Good
Hole fill after multiple
reflow cycles (2x)
Good
May have
problems
after 2x
reflow.
Good Good Good
May have
problems
after 2x
reflow.
Use on thick PCBs
No, holes
difficult
to fill
and clear
Yes
Nickel improves
hole reliability
Nickel improves
holel reliability
yes Yes
Use in thin PCBs
No, prone
to warping
Yes Yes Yes Yes Yes
Solder joint reliability Good Good
BGA
‘‘black pad’’
concerns
Gold
embrittlement
concerns
Planar
microvoid
concerns
Good
Sporadic brittle fracture failure
Card edge contacts
Additional
plating
operation
Additional
plating
operation
Additional
plating
operation
No additional
plating
Additional
plating
operation
Additional
plating
operation
Wire bonding No No No Yes No No
Test point probing Good
Poor,
unless solder
applied during
assembly
Good Good Good Good
Exposed Copper
after Assembly
No
Yes, along
land edges
No No No No
Switches/Contacts No No Yes Yes Yes No
Waste Treatment
and Safety in PCB
Fabrication
Poor/Fair Good Fair Fair Good Good
Process Control
Thickness
control
concerns
Good
Phosphorus
content
concerns
Gold thickness
control
concerns
Good
Thickness
control
concerns
Coating thickness/µm 0.38 - 0.8 0.2 - 0.5 0.05-0.10 0.8 - 2.5 0.07 - 0.10 1.0 - 1.3
General Cost
Comparison
1.0 0.4 - 0.6 2.0 - 3.0 2.0 - 3.0 1.1 - 1.6 1.0 - 1.5
January 2013 IPC-7095C
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Copyright IPC-Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
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