Kutz M. Handbook of materials selection
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5 PRODUCT PERFORMANCE 1219
3.4
3.6
3.2
3
0.3
-0.2
-0.1
-0.4
-1
0
1
2
3
4
184°C 204°C 232°C 246°C
Yellowness Index (YI)
S-1
S-2
Injection Molding Temperatures
X
X
Fig. 24 Clarity of PP flakes is essentially independent of injection-molding temperature.
9.8
5.5
10.5
5.3
7.2
9.1
3.6
3.7
6.5
6.8
2.5
2.7
5.6
6.1
2.2
0.8
218C/500Psi 218C/1000Psi 246C/500Psi 246C/1000Psi
Compression Molding Temperature and Pressure
Yellowness Index
3PHt/1Min
3PHt/3Min
4 Minutes
6 Minutes
0
2
4
6
8
10
12
Fig. 25 Compression-molding temperature, pressure, and time on PP clarity
(3 PHt / 1 min ⫽ preheating 3 min then molding for 1 min).
molded with the corresponding time and pressure continuously applied. The
degree of compression pressure, however—observed at either 500 or 1000 psi—
has much less influence on the YI of plaque than temperature and molding time.
Accordingly, for a given additive packaging, the YI of medical products made
from PP pellets can be as high as 10.5 or as low as 0.8, depending on how the
product is compression molded.
5 PRODUCT PERFORMANCE
Safety, efficacy, quality, integrity, functionality, and cost are the key factors that
determine a medical product’s success in a competitive market. A high-quality
product performance associated with a low product cost is the ultimate goal of
medical manufacturers throughout the world. To achieve this goal, medical man-
ufacturers must carefully select materials to fulfill product safety and efficacy,
ensure product integrity, prevent material–drug interactions, maintain strict qual-
ity control, and prohibit dose concentration change.
1220 SELECTING MATERIALS FOR MEDICAL PRODUCTS
806040200
-100
-50
0
50
100
% DEHP
Tg (C)
Fig. 26 PVC T
g
vs. plasticizer content.
5.1 Correlating Product Performance to Materials Characteristics
Numerous medical products, such as containers and tubing used in the device
and medication delivery system, are made from flexible plastics such as plasti-
cized PVC. This is due to its wide formulation latitude, ease of processing,
clarity for contamination inspections, and low cost. In many instances these
devices and delivery systems are subjected to the simultaneous conditions of
subambient temperatures and mechanical stress. For example, near freezing con-
ditions can be expected during winter shipping. Blood components are frequently
handled and centrifuge processed at near 4
⬚C. Certain drugs in premixed solu-
tions are often stored and shipped frozen at about
⫺20 to ⫺30⬚C to enhance the
chemical stability. Dry ice storage and transportation calls for temperatures of
about
⫺78⬚C, and in cryo-preservation liquid nitrogen at ⫺196⬚C is commonly
used. Since many products need to be stored in subambient temperatures, low-
temperature toughness is needed.
26,27
And yet since many medical products re-
quire terminal sterilization by steam, high-temperature survival is a must.
Early studies by Schmieder and Wolf
28
shown in Fig. 26 that glass transitions
of plasticized PVC followed a predictable functional relationship with the plas-
ticizer type and content. As expected, the higher content plasticizer samples had
lower glass transition temperatures (T
g
).
Figure 27 presents the product failure rate in a standardized test conducted
at
⫺20⬚C as a function of the glass transition T
g
. Failure rate percent is increased
as PVC T
g
is higher. This data indicated that the primary variable for the low-
temperature performance for flexible PVC is the location of the T
g
. This also
clearly indicates that the inherent properties of selected materials have a great
impact to the product performance.
5.2 Optimizing Product Performance
Product performance can be optimized through the combined synergetic effect
of the product design, materials selection, and manufacturing processes. One key
element is to define clear and complete product requirements at the very begin-
6 CONCLUSIONS 1221
20100-10-20
0.0
0.2
0.4
0.6
0.8
1.0
Tg (C)
% Failure
Fig. 27 ⫺20⬚C product failure rates vs. T
g
.
ning of product development. Based on the product requirements, product de-
signers, materials scientists, and manufacturing engineers can create synergy to
build a low-cost, high-performance medical product.
6 CONCLUSIONS
Polymeric materials have played a very critical role to enhance medical therapy
and product quality, particularly for the medical device and disposable product
sectors. Demonstration of product safety and efficacy are required prior to reg-
ulatory approval and product launch. Polymeric materials offer flexibility to
product design, easy manufacturing, and sterility integrity.
A good materials selection strategy for a specific medical product is to take
into account the material functional requirements with product design, manufac-
turing processes, and product performance simultaneously. The selected mate-
rials need to be compatible to the preferred sterilization method of steam
autoclaving, ethylene oxide, or ionizing radiation. High-performance low-cost
medical products are feasible as the selected materials are suitable for large-
scale production by automation processes.
PVC materials have been used for medical products for over 40 years because
of broad spectra of physical properties, sterilization compatibility, cost perform-
ance ratio, and meeting medical regulatory requirements. Recently, developments
in metallocene polyolefins have many unique features that have potential to
replace PVC for medical applications.
In addition to product safety and efficacy, the selected materials need to fulfill
product functionality, integrity, manufacturability, and cost constraints. Valida-
tion of the materials for specific product applications to the desired product shelf
is required. The validation includes functionality, stability, and extractives, with
focus on the toxicological and biological interactions of the selected polymeric
materials with the medication drugs and biological agents.
1222 SELECTING MATERIALS FOR MEDICAL PRODUCTS
REFERENCES
1. S. Shang, M. T. K. Ling, S. P. Westphal, and L. Woo, ‘‘Radiation Sterilization Compatibility of
Medical Packaging Materials,’’ J. Vinyl & Addit. Technol., 4(1), 60 (1998).
2. S. Shang, T. Yang, C. Sandford, M. T. K. Ling, and L. Woo, ‘‘Comparison Electron Beam and
Gamma Irradiation on Medical Packaging Materials,’’ Proceedings of the Society of Plastics
Engineers, ANTEC, 2740 (2000).
3. R. Carmen, ‘‘The Selection of Plastic Materials for Blood Bags,’’ Transfusion Med. Rev., 7(1),
1 (1993).
4. S. Shang and L. Woo ‘‘Selecting Materials for Medical Products,’’ Medical Device and Diag-
nostic Industry, 18(10), 132 (1996).
5. L. Woo, M. T. K. Ling, and E. Chan, ‘‘Application of the Oxidative Induction Test to Medical
Polymers,’’ Thermochimica Acta, 192, 209 (1991).
6. ASTM Standard: D3895-92.
7. G. N. Foster in Oxidation Inhibition in Organic Materials, Vol. 2, J. Pospisil and P. Klemchuk
(eds.), CRC Press, Boca Raton, FL, 1989.
8. H. E. Bair, Thermal Characterization of Polymeric Materials, E. Turi (ed.), Academic, New
York, 1981, p. 869.
9. Encyclopedia of Polymer Science and Engineering, Radiation-Resistant Polymer, 2nd ed., J. I.
Kroschwitz, (ed.), Wiley, New York, 1988, Vol. 13, p. 687.
10. R. J. Rolando, ‘‘Radiation Resistant Polypropylene: New Development,’’ J. Plast. Film & Sheet-
ing, 9(4), 326 (1993).
11. L. Woo, J. Palomo, M. T. K. Ling, E. Chan, and C. Sandford, ‘‘Shelf-Life Prediction Methods
and Applications,’’ Med. Plast. Biomat., 3(2), 36 (1996).
12. L. Matisova-Rychla and J. Rychly, in Polymer Durability, R. Clough, N. Billingham, and K.
Gillen (eds.), Am. Chem. Soc., Washington, DC, 1996.
13. K. Gillen and R. Clough, in Irradiation Effects on Polymers, D. W. Clegg and A. A. Collyer
(eds.), Elsevier Applied Science, New York, 1991.
14. D. Goodman, ‘‘Global Markets for Chlorine and PVC: Potential Impacts of Greenpeace Attacks,’’
J. Vinyl Techno., 16(3), 156 (1994).
15. D. C. Finney and R. M. David, ‘‘The Carcinogenic Potential of DEHP in Humans: A Review of
the Literature,’’ Med. Plast. Biomat., 2(1), 48 (1994).
16. E. S. Shamshoum, L. Sun, B. R. Reddy, and D. Turner, ‘‘Properties and Applications of Low
Density Syndiotactic Polypropylene,’’ in Proceedings of the Worldwide Metallocene Conference,
Metcon ’94, Spring House, PA, Catalyst Consultants, 1, 30 (1994).
17. E. S. Shamshoum, ‘‘Syndiotactic Polypropylene Catalyst: Properties and Possible Applications,’’
in Proceedings of the Second International Business Forum of Specialty Polyolefins, SPO ’92,
Brookfield, CT, Society of Plastics Engineers, 1, 199 (1992).
18. S. Borman, ‘‘Elastomeric Polypropylene: Oscillating Catalyst Control Microstructure,’’ C&EN,
January 16, 6 (1995).
19. J. J. McAlpin, and G. A. Stahl, ‘‘Applications Potential of Exxpol Metallocene-Based Polypro-
pylene,’’ in Proceedings of the Worldwide Metallocene Conference, Metcon ’94, Spring House,
PA, Catalyst Consultants, 1994, p. 7.
20. G. W. Knight, and S. Lai, ‘‘Constrained Geometry Catalyst Technology: New Rules for Ethylene
Alpha-Olefin Interpolymers—Unique Structure and Property Relationships,’’ Polyolefins, Vo l
VIII, Society of Plastics Engineers, Brookfield, CT, 1993, p. 226.
21. S. Shang and R. Kamla, ‘‘Influence of Processing Conditions on the Physical Properties of
LLDPE Blown Film,’’ J. Plast. Film & Sheeting, 11(1), 21 (1995).
22. F. Stehling, C. Speed, and L. Westerman, ‘‘Causes of Haze of Low-Density Polyethylene Blown
Films,’’ Macromolecules, 14, 698 (1981).
23. M. Rokudai, ‘‘Influence of Shearing History on the Rheological Properties and Processability of
Branched Polymers. I,’’ J. Appl. Polym. Sci., 23, 463 (1997).
24. Yang, T., ‘‘Surface Roughness and Gloss of Latex Coated Paper,’’ Pittcon ’97, Atlanta, GA, 1997,
p. 172.
25. S. Shang, ‘‘What Makes Clear Polypropylene Discolor?’’ Med Plast Biomat, 2(4), 16 (1995).
26. S. Shang, M. T. K. Ling, S. P. Westpha, and L. Woo, ‘‘Correlating Resin Properties to the
Cryogenic Impact Performance of Medical Containers,’’ Proceedings of Soc. of Plastics Engi-
neers, ANTEC, 2862, (1996).
27. L. Woo, S. W. Shang, and S. P. Westphal, ‘‘Development High Heat, Metallocene Toughened
Polymers—A User’s Perspective,’’ in Proceedings of the Worldwide Metallocene Conference,
Metcon ’94, Spring House, PA, Catalyst Consultants, 1997.
28. K. Schmieder and K. Wolf, Kunststoffe, 41, 89 (1951).
1223
CHAPTER 39
MATERIALS IN ELECTRONIC
PACKAGING
Warren C. Fackler
Telesis Systems
Cedar Rapids, Iowa
1 GENERAL 1223
2 APPROACH 1224
3 DOMINANT CONSIDERATIONS 1224
3.1 Electrical Conductivity 1224
3.2 Thermal Conductivity 1225
3.3 Thermal Emissivity 1225
3.4 Thermal Expansion 1225
3.5 Chemical Inertness 1226
3.6 Corrosion 1226
3.7 Temperature Range 1227
3.8 Strength 1228
3.9 Density 1228
3.10 Electromagnetic and
Electrostatic Shielding 1228
3.11 Magnetic Shielding 1229
3.12 Fatigue Resistance 1229
3.13 Hardness 1229
3.14 Ductility 1230
3.15 Wear Resistance 1230
3.16 Sublimation 1230
3.17 Combustibility 1231
3.18 Creep 1231
3.19 Moisture Absorption 1232
4 OVERRIDING CONSIDERATIONS 1232
5 TYPICAL APPLICATIONS 1233
5.1 Equipment Attachment 1233
5.2 Equipment Racks, Frames,
and Mounting Structures 1234
5.3 Equipment and Module
Enclosures 1234
5.4 Temperature Control 1235
5.5 Mechanical Joints 1236
5.6 Finishes 1237
5.7 Position-Sensitive
Assemblies 1238
5.8 Electrical Contacts 1239
5.9 Encapsulation 1239
5.10 Harsh Environment
Endurance 1241
6 CANDIDATE MATERIALS 1241
6.1 General 1241
6.2 Metals 1241
6.3 Plastics and Elastomers 1245
6.4 Ceramics and Glasses 1249
6.5 Adhesives 1250
7 SUMMARY 1251
REFERENCES 1251
1 GENERAL
The process of materials selection for an electronic packaging application in-
volves a variety of considerations, and the end result is often a material chosen
for reasons not immediately evident at the beginning of the process. This seem-
ing paradox stems from the nature of the work faced by the electronic packaging
engineer. That is, the span of problems faced by the electronic packaging en-
gineer is multidisciplinary and involves finding a balance between function, per-
formance, manufacturability, reliability, and cost.
HandbookofMaterialsSelection.EditedbyMyerKutz
Copyright Ó 2002 John Wiley & Sons, Inc., NewYork.
1224 MATERIALS IN ELECTRONIC PACKAGING
The focus of this chapter is on narrowing the range of choices regarding
selection of the most appropriate solution to a materials selection problem, and
this chapter does not set forth a detailed summary of material properties. As a
result, the reader is directed to other chapters in this handbook and references
cited for specific details regarding the properties of materials, specific manufac-
turing and assembly processes, and applicable analytical procedures that may
apply to the candidate materials.
2 APPROACH
The selection of a material from a set of materials includes the following steps:
1. Define the dominant considerations for the design application under con-
sideration.
2. Define any overriding considerations that narrow the field of choice.
3. Review typical applications regarding materials selection.
4. Compile a list of the most likely set of candidate materials.
5. Compare the properties of the selected material(s) and select one or more
materials that appear to offer the most likely solution.
6. Test each selected material physically and against the original design
criteria to verify suitability.
3 DOMINANT CONSIDERATIONS
A dominant consideration refers to the primary technical performance require-
ment that must be offered by the selected material to achieve the technical
objectives of the design. The design may also have a set of secondary require-
ments; however, if the dominant characteristic is not achieved, the material is
unsatisfactory for service regardless of whether desirable secondary requirements
are present.
It is not unusual for a material requirement to include more than one dominant
characteristic: for example, a heat sink material with high thermal conductivity
and electrical insulation, or an item to be used in outer space requiring high
thermal emissivity and low sublimation, or a component bonded to substrate
where the substrate material must have a coefficient of thermal expansion match-
ing that of the component and chemical inertness over a given temperature range.
Dominant technical performance criteria include:
3.1 Electrical Conductivity
A material may be required to conduct electrical currents. This includes metals
and some nonmetallic elements such as adhesives, greases, and other compounds
loaded with graphite or metal powder. If electrical conductivity is important,
then the resistivity of the material must be considered because electrical resis-
tance creates voltage drop and heat generation, either of which may be a desir-
able or an undesirable consequence. In screening for electrical conductors, a
maximum electrical resistance requirement must be defined, thus materials with
equal or lower electrical resistivity become candidates for selection. Electrical
resistivity is temperature sensitive; thus it is necessary to make sure that
the material electrical resistivity is satisfactory over the temperature range of
interest.
3 DOMINANT CONSIDERATIONS 1225
Conversely, it may be necessary to electrically isolate a component or current-
carrying member to protect from undesired electrical shorts. Materials used as
electrical insulators, such as mica or glass or ceramics and many plastic mate-
rials, have very high electrical resistivity, are poor conductors of electricity, and
are employed to provide electrical isolation.
1
Some high-voltage dielectrics may
be liquid, which often act as heat transfer media in addition to providing elec-
trical insulation. Examples include mineral oils, silicon oils, polybutanes, flour-
ocarbons, and organic esters including castor oil. Problems with liquid dielectrics
include limited temperature range, chemical decomposition over time, combus-
tibility, and loss of dielectric strength.
3.2 Thermal Conductivity
All materials conduct heat; however, among materials there is a wide range of
thermal conductivity. Good conductors of heat include metals, and poor con-
ductors of heat such as ceramic and plastic foam materials are considered to be
thermal insulators. The cooling of electronic components and the protection of
electronic components from excessive temperatures is managed by employing
mounting devices, heat sinks, and thermal insulators using materials of the ap-
propriate thermal conductivity.
3.3 Thermal Emissivity
The thermal emissivity of a material is a measure of the efficiency by which a
material will radiate or receive infrared energy. The thermal emissivity is a
surface characteristic of a material and to some extent a function of material
color. Thus a highly polished light-colored material will have a low thermal
emissivity and dark-colored, high surface roughness materials will exhibit a
higher thermal emissivity.
When an electronic component is mounted near a high-temperature emitter,
the heat absorbed by the component may be reduced by using material and finish
with a low thermal emissivity to protect the component.
This is of key importance for electronic equipment used in spacecraft and
that may face the sun or be in shadow. Low-emissivity materials, such as plating
an equipment case with gold, are used to limit the heating from the sun and also
to reduce heat loss when the equipment is facing dark space.
If thermal emissivity is a design requirement, often the selected material may
be chosen for some other dominant criteria, and thermal emissivity is controlled
by surface finish selection, to include application of an electrodeposited highly
emissive metal on the exposed surfaces of the part.
Painting a surface with a light-colored paint may slightly lower the emissivity
in comparison to darker shades of paint; however, the surface roughness is often
essentially the same regardless of paint color, thus the improvement is minor.
Also, to coat a dark polished surface with a light-colored paint often leads to a
higher emissivity and not a lower emissivity.
3.4 Thermal Expansion
Measured by a property known as the coefficient of thermal expansion, this
material property allows the electronic packaging engineer to predict the linear
or volumetric change (expansion or contraction) of a material when the material
is exposed to a change in temperature.
1226 MATERIALS IN ELECTRONIC PACKAGING
It is often very important to determine the coefficient of thermal expansion
for materials that are bonded or mechanically secured together. For example, for
a ceramic component (very low coefficient of thermal expansion) soldered to a
printed circuit board (a higher coefficient of thermal expansion) exposed to re-
peated temperature cycles, perhaps caused by turning the equipment on and off,
can lead to solder joint failure. Another example is when two materials of dif-
ferent thermal expansion characteristics are mechanically joined and exposed to
a temperature change; one material will be placed in compression and the other
material will be placed in tension. This will result in the composite assembly
bending in the direction of the material placed in tension. Also, depending on
the relative strength of the materials, one or the other material may fracture, or
the joining method (adhesive joint, solder joint, weld, rivet, screw, etc.) may
fracture or otherwise be degraded. Another effect caused by thermal expansion
in a circuit where mechanical position defines a circuit operating parameter
would be a change in position of parts during a change in temperature, which
causes the circuit operating parameters to deviate.
3.5 Chemical Inertness
Materials may be exposed to chemicals during the life of a product. This may
include fuels and lubricants, cleaning fluids, fluxes, and other chemicals used in
industrial processes. Most tables that list material properties also provide limited
information regarding chemical resistance. If the material is exposed to condi-
tions not covered by information in the tables, then the designer must contact
the provider of the material for additional information. It also may be necessary
to experimentally determine and to verify the suitability of a material in the
presence of a chemical. It is important to note that chemical reactions are af-
fected by temperature and the presence of other chemicals.
Chemical deterioration may be induced by ionizing radiation (which alters
the molecular structure and thus the physical properties of materials) and ultra-
violet radiation (such as from sunshine), which may cause depletion of plasti-
cizers from elastomers, like polyvinyl chloride (PVC) plastic, which is used as
a wire insulation material, thus rendering the material brittle and weak.
3.6 Corrosion
Most tables of material properties provide information of corrosion resistance;
however, this information may not be sufficiently specific to give confidence
regarding the intended usage of a material. This may (for metals) include sus-
ceptibility to intergranular or stress corrosion cracking in the presence of certain
acids or bases. For all materials, and especially nonmetals, corrosion deteriora-
tion includes processes similar to those identified regarding the chemical inert-
ness of a material.
The process of galvanic corrosion
2
is a primary concern to the electronic
packaging engineer and applies mostly to metals, and under certain conditions
galvanic corrosion may be a leading cause for loss of electrical conductivity or
loss of strength in a material or at a riveted, bolted, or welded joint as well as
failure of electroplated finishes.
Table 1 lists the galvanic series relative position of several materials used in
electronic packaging.
3 DOMINANT CONSIDERATIONS 1227
Table 1 Galvanic Series Position of Common
Electronic Materials
Magnesium and alloys Most Anodic
Zinc
↑
Aluminum 1100 alloys
Cadmium
Aluminum 2024 alloys
Steel, iron, cast irons
Type 304 and 316 CRES (active)
Lead–tin solders
Lead
Tin
Nickel (active)
Inconel (active)
Brasses, copper, bronze
Monel and copper nickel alloys
Monel
Silver solder
Nickel (passive)
Inconel (passive)
Type 304 and 316 corrosion resistant
steel CRES (passive)
Silver, gold, platinum
Titanium
↓
Graphite Most Cathodic
Galvanic corrosion is the result of an electrochemical reaction, which occurs
when a design that employs metals ranking at different levels on the galvanic
chart is in the presence of an electrolyte (i.e., saltwater, contaminated water, or
other conducting chemicals). One material acts as an anode and the other as a
cathode as in a simple storage cell. The result is ionic migration during which
the anodic element suffers sacrificial deterioration. This is a mechanism for
leaching elements from grain boundaries and a corresponding weakening of the
affected material and other undesirable effects. This process may be hastened or
hindered by passing a small electrical current through the affected area.
3.7 Temperature Range
The operating and storage temperature ranges applicable to the application of a
material must be known and the properties of the material reviewed for stability
over the range of temperatures. This is important for any material including
metals especially when the temperatures are very high or very low. Metals and
nonmetals will become brittle and stiff at lower temperatures, and may soften,
lose strength, and creep or flow at elevated temperatures. This is especially true
with nonmetal materials, where the temperature ranges may be severely limited.
It is necessary to review the temperature range suitablity of a candidate material
because temperature influences each material’s physical property differently,
even for different alloys or formulations within a given class of materials.
1228 MATERIALS IN ELECTRONIC PACKAGING
3.8 Strength
A guide to the load-bearing capacity or strength of a material is its tensile
strength, which for many metals is a function of process conditioning or heat
treating (i.e., is the material annealed, half-hard, or hard?). Strength of nonmetals
(plastics) varies significantly with temperature and the rate at which a load is
applied (impact strength). Weight reduction is achieved in electronics by using
smaller quantities of high-strength materials for forming brackets, structures, and
fasteners.
3.9 Density
Density is a measure usually taken as the weight of a material for a given unit
of volume. This property may be used as a guide to reducing weight in a design.
Material density establishes the inertia and resonant frequency of mechanical
elements and thus the degree to which they are affected by mechanical vibration
and mechanical shock loading.
Another indicator of the density of a material is its specific gravity, which is
a comparative ratio of the weight of a material compared with the weight of an
equivalent volume of water.
3.10 Electromagnetic and Electrostatic Shielding
Electromagnetic shielding is employed to prevent the imposition of electrical
waves from the environment (such as radio waves) on circuit elements and thus
inducing currents and voltages leading to circuit malperformance and possible
component damage.
Electrostatic shielding is employed to prevent penetration of high-voltage
charges on the surface of an electronic enclosure (such as caused by static dis-
charge) from entering the enclosure and therein causing damage to components
or causing circuit performance malfunction.
Protection of an equipment from both of these sources of potential equipment
performance deterioration involve shielding the equipment such as to prevent
radio waves or other electromagnetic noise from entering the enclosure, to shield
the equipment to promote the collection of static charges on the surface of the
equipment, and to dissipate the unwanted energy by providing a conductive path
to drain the energy to an appropriate sink (ground).
A circuit may be shielded by enclosing it in a conductive enclosure, thus
creating a Faraday cage where the imposing electromagnetic waves or static
charges cause currents in the skin of the enclosure and do not penetrate the
enclosure. An example would be to protect a high-gain amplifier circuit from
malfunction when exposed to radio waves.
Conversely, essentially identical techniques are used to contain radio wave
emissions from a circuit from escaping the enclosure and causing malfunction
in other equipment that may be susceptible to such emissions. High-speed digital
circuits at speeds of 10 kHz and above are known to cause such emissions and
often require electromagnetic shielding to meet the emission requirements set
forth by Part 15 of the Federal Communications Commission Rules and Regu-
lations.
3
Nonmetallic enclosures are sometimes used to control electromagnetic
field by coating the material with a paint that contains metal powder.