IPC-D-279 EN.pdf - 第29页

T able 4−1 Advantages and Disadvantages of V arious T ypes of Substrates TYPE MAJOR ADV ANT AGES MAJOR DISADV ANT AGES COMMENTS ORGANIC BASE SUBSTRA TE Epoxy ﬁberglass Substrate size, weight; reworkable; dielectric prope…

For the assembly to function properly, the electrical,

mechanical, and thermal requirements of each material

used in the substrate must be considered for the operating

conditions and use environments.

Table 4-1 provides guidance in the material choices. See

IPC-D-275 for other material discussions.

4.3 Moisture and its Effects on Polymer Substrates

Polymers commonly used in SMT printed boards absorb

and adsorb water when exposed to moist atmospheres (high

relative humidity) for durations ranging from several days

to several weeks; the equilibration time depends upon the

thickness of the laminates and the geometry of the conduc-

tor pattern. The relative permittivity of water is 80 and that

of common substrates ranges from 3 to 5; the absorption of

1-3% by weight of water can signiﬁcantly (but reversibly)

increase the dielectric constant between conductors and

hence the capacitive coupling between conductors, over

time. The absorption and adsorption of water also

decreases the insulation resistance between conductors at

the surface (SIR). Together with ionizable contaminants

and DC bias, condensed moisture can lead to electro-

chemical corrosion and dendrites on the surface of the sub-

strates; conductive anodic ﬁlament (CAF) formation at the

glass ﬁber-resin interface; and electrochemical corrosion

and dendrites at delaminations and voids such as occur

between conductors on inner layers and between barrels of

PTVs and PTHs. Moisture effects are more signiﬁcant in

SM printed boards because the spaces between conductors

and the interbarrel distances are much less than the corre-

sponding dimensions in through hole printed boards; in

addition, solder masks may not be easily applied between

land patterns in the ﬁne and extra ﬁne pitch SM patterns.

See Appendix C for DfR information on SIR. See also

IPC-TR-476.

Some materials with higher glass transition temperatures

) such as bismaleimides and polyimides, appear to

absorb more water than the lower T

materials, such as the

epoxies. Drying of the higher T

materials (as well as

thicker buildups of the epoxy systems) prior to SM reﬂow

exposure or rework/repair is recommended to minimize

delamination or separation, for instance, of the conductor

from the resin or the glass ﬁber from the resin.

The laminate surface is porous when treated by etching to

enhance adhesion of conductors; this surface porosity can

retain hydrolyzable and ionizable contaminants and water,

as well as hydrophilic materials such as polyglycols which

are used in the formulation of some water soluble SM sol-

der pastes. Solder mask and conformal coating materials

cover and seal the porous surface and help to retain SIR

values and reduce the risk of corrosion.

Common solder masks (and conformal coatings) are per-

meable to water vapor; the presence of water soluble con-

taminants between solder mask or conformal coating and

the underlying surface can result in vesication or mealing

and in electrochemical corrosion/migration.

Chemisorption of water into polymers appears to reduce

the T

slightly, reduces the adhesion of the polymer to

other materials and reduces the strength of the polymer.

See also the bibliography of IPC-SM-786.

4.4 Coefficient of Thermal Expansion (CTE) of Polymer

Systems

Polymer systems expand with increasing tem-

perature, demonstrating a glassy phase response below T

with a CTE or α

and a rubbery phase response above T

with a much higher α

, typically 3 times α

. The transition

from glassy phase to rubbery phase is gradual, but for most

polymer substrates may be characterized by T

, the glass

transition temperature.

Glass ﬁber reinforced substrates exhibit signiﬁcantly differ-

ent CTE in the z (out of plane) axis compared to the CTE

in the x and y axes; for example, below its T

, Quartz/

Bismaleimide material with 35% resin by weight exhibits a

CTE(x-y) of 6 ppm/°C and a CTE(z) of 41. Woven glass

ﬁber reinforcement exhibits an additional difference

between x and y axes on the order of 1-5 ppm/°C; this dif-

ference may be signiﬁcant where the CTE of a large SM

component package is to be matched to the CTE of the

substrate to enhance cyclic life of the solder attachments.

The CTE(z) is particularly signiﬁcant in determining the

cyclic life of PTH and PTVs in SM PWAs because the

aspect ratio (ratio of substrate thickness to ﬁnished hole

diameter) is generally much larger than the corresponding

aspect ratio achieved in printed boards manufactured for

through hole technologies. Higher CTE(z) values result in

higher cyclic tensile stress in the barrel of the PTH or PTV

during temperature excursion during SM reﬂow, or SM

component removal/rework/repair as well as during printed

board fabrication, solder dipping, hot air leveling, or wave

solder. See IPC-TR-579 and IPC-SM-782.

The thermal cycle reliability, vibration robustness, and the

thermal management of high performance Surface Mount

(SM) products are heavily dependent upon the constraining

core such as copper-molybdenum-copper (CMC), copper-

Invar-copper (CIC) and molybdenum-graphite-

molybdenum (MGM) composite material systems.

The ratios of the various materials in those composite sys-

tems can be adjusted to tailor the effective CTE to the opti-

mum value. The tradeoffs include weight and cost. See

IPC-MC-324.

4.5 Constraining Cores in Substrates A constraining

core is an internal supporting plane in a packaging and

interconnecting structure, used to alter the coefficient of

thermal expansion of printed boards.

July 1996 IPC-D-279

Table 4−1 Advantages and Disadvantages of Various Types of Substrates

TYPE MAJOR ADVANTAGES MAJOR DISADVANTAGES COMMENTS

ORGANIC BASE SUBSTRATE

Epoxy ﬁberglass Substrate size, weight; reworkable;

dielectric properties; conventional

board processing, availability,

cost/performance value.

Thermal conductivity x, y, and z axis CTE a concern for

high density applications.

Polyimide ﬁberglass Same as epoxy ﬁberglass plus

high temperature z axis CTE;

substrate size; weight; reworkable,

dielectric properties.

Thermal conductivity; moisture

absorption.

Same as epoxy ﬁberglass; x, y,

and z axis CTE a concern for high

density applications.

Epoxy aramid ﬁber Same as epoxy ﬁberglass; x-y axis

CTE; substrate size; lightest

weight; reworkable; dieletric

properties.

Thermal conductivity; resin

microcracking; z axis CTE; water

absorption; cost; resin adherence.

Volume fraction of ﬁber can be

controlled to tailor x-y CTE. Resin

selection critical to reducing resin

microcracks.

Polyimide aramid ﬁber Same as epoxy aramid ﬁber; z

axis CTE; substrate size; weight;

reworkable; dielecric properties.

Thermal conductivity; resin

microcracking; water absorption;

cost; resin adherence.

Same as epoxy aramid ﬁber.

Polyimide quartz (fused silica) Same as polyimide aramid ﬁber; x,

y, z axis CTE; substrate size;

weight; reworkable; dielectric

properties.

Thermal conductivity; drilling;

availability; cost; low resin content

required.

Volume fraction of ﬁber can be

controlled to tailor x-y CTE. Drill

wearout higher than with

ﬁberglass.

Fiberglass/Teﬂon laminates Dielectric constant; high

temperature stability; thermal

conductivity; x and y axis CTE.

Same as epoxy ﬁberglass; low

temperature stability; thermal

conductivity; x and y axis CTE;

difficult processing.

Suitable for high speed logic and

high frequency applications. Same

as epoxy ﬁberglass.

Flexible dielectric Lightweight; minimal concert to

CTE; conﬁguration ﬂexibility.

Size Rigid-ﬂexible boards offer tradeoff

compromises.

Thermoplastic 3-D conﬁgurations; low

high-volume cost.

High injection molding setup costs;

additive processing.

Very limited applications.

Bismaleimide/triazine glass Improved dielectric properties;

multiple thermal shock; minimum

cost penalty for upgrade.

Thermal conductivity; x, y, and z

axis CTE.

Applicable to MCM-L technology.

Composite CEM-1 and CEM-3 Punchable at room temperature;

cost; stiff enough for SMD.

x and y axis CTE; thermal

conductivity.

Substrate of choice for consumer

products with SMDs.

Paper-based phenolic Punchable with heat; lowest cost. Single-sided only; stiffness;

availability; x and y axis CTE.

Majority of world market is

paper-based.

NONORGANIC BASE

Alumina (ceramic) CTE; thermal conductivity;

conventional thick ﬁlm or thin ﬁlm

processing; integrated resistors.

Substrate size; rework limitations;

weight; constant; brittle; dielectric

constant.

Most widely used for hybrid circuit

technology.

SUPPORTING PLANE

Printed board bonded to plane

support (metal or nonmetal)

Substrate size; reworkability;

dielectric properties; conventional

board processing x-y axis CTE;

stiffness; shielding; cooling.

Weight The thickness/CTE of the metal

core can be varied along with the

board thickness, to tailor the

overall CTE of the composite.

Sequential processed board with

supporting plane core

Same as board bonded to

supporting plane.

Weight Same as board bonded to

supporting plane.

Discrete wire High speed interconnections; good

thermal and electrical features.

Licensed process; requires special

equipment; cost; availability.

Same as board bonded to low

expansion metal support plane.

CONSTRAINING CORE

Printed board bonded with

constraining metal core

x and y axis CTE; uses FR-4 or

polyimide/glass materials.

Weight; internal layer registration;

delamination; via hole cracking, z

axis CTE.

Can be used as power/ground

planes.

Printed board bonded to low

expansion graphite ﬁber core

Same as board bonded to low

expansion metal cores; stiffness,

thermal conductivity; low weight.

Cost; microcracking; z axis CTE. The thickness of the graphite and

board can be varied to tailor the

overall CTE of the composite.

Compliant layer structures Substrate size; dielectric

properties; x-y axis CTE.

z axis CTE; thermal conductivity. Compliant layer absorbs difference

in CTE between ceramic package

and substrate.

IPC-D-279 July 1996

As with printed boards with supporting planes, one or more

supporting metallic or non-metallic planes can serve as a

stiffener, heatsink, and/or CTE constraint in constraining

core printed boards.

The results of ‘‘accelerated’’ life tests which incorporate

temperatures which approach or exceed the T

of the sub-

strate should not be extrapolated to predict service life;

these tests may be used to discriminate between alterna-

tives.

4.5.1 Printed Board Stiffness and Damping Con-

strained core systems with skins of high modulus material

form boards which in comparison with standard base mate-

rials are stiffer and have higher damping frequencies. These

characteristics may be beneﬁcial, depending upon the envi-

ronmental vibration and noise spectrum.

4.6 Flexible Printed Board with Metal Support Plane

Another arrangement for a printed board with leadless

components involves conventional ﬁne-line polyimide ﬂex-

ible printed wiring. These assemblies can be constructed in

multilayer form while retaining the low-modulus feature

that reduces residual strain at the solder joints. Further-

more, lasers can drill very ﬁne holes in the thin, printed

wiring laminate. These holes can be plated-through or

ﬁlled with solid copper as required.

To retain inherent ﬂexibility while dissipating heat from the

solder joint, cutouts in the ﬂexible circuit accommodate

pillars from the metal heatsink support plane. Although this

appears to be heavy and cumbersome, if the heatsink base-

plates are made from thin sheets of aluminum, the result-

ing density of the combined circuit/heatsink assembly

might actually be less than other constructions.

4.7 Discrete Wire Structures with Metal Support

Plane

Discrete wire printed boards have been developed

speciﬁcally for use with surface mounted components.

These structures are usually built with a low-expansion

metal support plane that also offers good heat dissipation.

The interconnections are made by discrete 0.06 mm diam-

eter insulated copper wires precisely placed on a 0.03 mm

grid by numerically-controlled machines. This geometry

results in a low-proﬁle interconnection pattern with excel-

lent high-speed electrical characteristics and a density nor-

mally associated with thick ﬁlm technology.

The wiring is encapsulated in a compliant resin to absorb

local stresses and dampen vibration. Electrical access to the

conductors is by 0.25 mm diameter copper vias. The small

via size can be accommodated in the component attach-

ment land, thus eliminating the need for fanout patterns

when using components with terminals on centers as close

as 0.6 mm, and allowing very-high packaging densities.

The high level of water absorbed into polyimide tape auto-

mated bonding (TAB) substrate materials during exposure

to high ambient moisture levels has been observed to result

in conductor corrosion and delamination.

4.8 Outgassing of Polymer Substrates See also the dis-

cussion of solder mask and coatings in Appendix N, and

section 6. The printed board may contribute emissions of

cleaning solvent, polyglycols, and lighter fractions of ﬂux

vehicles in addition to the emission from the solder mask

and conformal coating of the printed board and of the com-

ponent encapsulation materials.

4.9 Assembly Process Effects on Polymer Substrates

See also the discussion of rework and repair in section 7.6

and Appendix E.

In addition, ﬂuxes for wave soldering and for water soluble

solder pastes can contain high boiling point hydrophilic or

hygroscopic solvents such as the polyglycols. These sol-

vents can penetrate the resin-glass ﬁber interface and con-

tribute to conductive anodic ﬁlament (CAF) formation. See

Appendix C for DfR information. See also IPC-TR-476.

4.10 Printed Board Solderability The land patterns in

IPC-SM-782, particularly for those intended for ﬁne pitch

and extra ﬁne pitch components, clearly demonstrate the

very small areas available for affecting the solder joint in

SM technology. A solderability defect with an area of 125

µm by 250 µm which might be discounted on a through

hole board may constitute the single land which is non-

solderable on a SM board and render that SM PWA non-

functional; worse, the component lead may contact the land

and mechanically affect an electrical contact which

becomes intermittent in service and the product is a NTF or

No Trouble Found at the repair center.

Although solder dipping and hot air solder leveling

(HASL) are said to constitute ‘‘proof’’ that the land is sol-

derable, these processes do not characterize solderability of

the land at the critical time which is just before the solder

paste is applied and the components are placed. Printed

board land solderability is degraded by oxides or chlorides

of tin and lead oxides or chlorides of tin-lead phases;

oxides of tin-copper intermetallic compounds; and organic

ﬁlms such as residues from ﬂuxing oils, ﬁnger prints or

solder mask. These oxides, chlorides and organic ﬁlms can

form after the HASL process. See also IPC-PE-740 and

IPC-S-816. Quantiﬁcation of the solderability of the SM

printed board is difficult but is addressed in ANSI/J-STD-

003; an earlier speciﬁcation is IPC-S-805.

4.11 Design for Reliability of Plated-Through-Hole Vias

(PTVs)

The material in Appendix B gives a detailed treat-

ment of DfR for PTVs.

5.0 GENERAL COMPONENT SELECTION CONSIDER-

ATIONS

1. During circuit design and veriﬁcation, primary

impact on manufacturing and reliability lies in the

July 1996 IPC-D-279