IPC-D-859.pdf - 第27页

E. Platinum silver (PtAg) is used where soldering of com- ponent is employed. Its application is limited presently to nonmilitary applications. F . Palladium Silver (PdAg) is a low-cost solderable con- ductor designed fo…

3.8.1.1 Alumina Substrates Alumina substrates are

made of polycrystalline A1

with small amounts of

metal oxide glasses to achieve certain physical properties.

Alumina is the most popular substrate material since it is

readily available in sizes ranging from tiny chips to large

area substrates, in thicknesses ranging from 0.25 mm

[0.010 inch] to 1.25 mm [0.050 inch] or greater, and in a

variety of shapes and designs. It is refractory and the ﬁn-

ished substrate can be drilled or cut with diamond tools or

lasers.

3.8.1.2 Beryllia Substrates Beryllia (polycrystalline

BeO), with impurities for physical properties, is primarily

used in applications requiring rapid heat removal from the

circuit. Like alumina, Beryllia substrates are available in a

large variety of sizes, thicknesses, shapes and designs.

Beryllia is less-widely used than alumina due to its toxic

nature in the powder form and its higher cost.

3.8.1.3 Coated Metal-Core Relatively new as a sub-

strate, porcelainized steel substrates provide strength,

shock resistance, a built-in ground plane, and relatively low

cost for larger sized substrates. Prior to coating, the metal

can be inexpensively shaped and punched to allow it to be

used as a mounting bracket.

As a combination hybrid and discrete component mounting

surface, porcelainized steel substrates can withstand such

harsh environments as automobile engine compartments.

Processing of these substrates requires low temperature ﬁr-

ing and pastes compatible with that condition.

3.8.1.4 Quartz Substrates Quartz is available in two

forms: (1) fused quartz or silica, and (2) single-crystal

quartz.

A. Fused Quartz—Fused quartz is similar to glass and is

used in microwave applications because of its consis-

tent dielectric constant and low RMS surface ﬁnish. As

with glass, it is a poor thermal conductor and cannot be

used in high heat dissipation applications. It is very

fragile and extreme care must be exercised when it is

handled. Its cost is moderate and the material is readily

available from many sources.

B. Single-crystal quartz—Single-crystal quartz is used in

applications where an ordered crystal structure is nec-

essary, e.g., in acoustic wave devices. The tensile

strength of quartz varies according to the axis in which

the crystal has been cut. The material is brittle and must

be handled carefully. Its cost is generally considered to

be exorbitant and, as with most crystals, its thermal

conductivity is poor. However, single-crystal quartz

does offer a very low RMS surface ﬁnish and it exhib-

its little or no camber.

3.8.1.5 Sapphire Substrates Sapphire is used in micro-

wave applications as a replacement for alumina substrates

when a superior surface ﬁnish and low camber are

required. Sapphire is an expensive single-crystal material

(Al

) and is limited in size to the diameter of the boule

from which it is cut.

The material is relatively strong but is a poor conductor of

heat. The dielectric constant, as well as other mechanical

and physical properties, will vary depending upon the crys-

tal orientation. Therefore, care must be exercised to specify

the proper crystal orientation.

3.8.1.6 Ferrite Substrates Ferrites are typically used in

microwave applications where a magnetic material is

required. Circulators are one example of this application.

The material maintains a uniformity of substrate properties,

e.g., dielectric constant and dissipation factor. Ferrites can

be obtained with good surface ﬁnish qualities and low cam-

ber. However, ferrites are brittle and highly susceptible to

impact shock breakage.

3.8.2 Conductive Materials The conductor, analogous to

the wire in older technologies, is fabricated from materials

of very low resistivity that is used to advance the signal

(information to be processed or used) to the desired loca-

tion with minimal losses.

3.8.2.1 Thick-Film Metallizations Thick-ﬁlm conductors

are typically silver, gold and alloys of these materials with

either platinum or palladium. Glass frits and other additives

are used which will affect the electrical characteristic of

these materials. Copper-and nickel-based thick-ﬁlm pastes

are emerging as a new low-cost thick ﬁlm conductor.

(These materials are, however, in the development stage.)

3.8.2.1.1 Sheet Resistivity and TCR Table 3–6 lists

nominal sheet resistivities and temperature coefficient of

resistance (TCR’s) of various thick-ﬁlm conductor metalli-

zations.

3.8.2.1.2 Thick-Film Conductors

A. Gold (Au) conductors are used where high conductiv-

ity combined with long-term reliability are required.

B. Platinum gold (PtAu) conductors are used where sol-

dering is the only viable means of component attach-

ment.

C. Palladium gold (PdAu) is limited to use in those areas

discussed under PtAu and where cost is to be kept as

low as possible. PdAu is a relatively low cost alterna-

tive to PtAu.

D. Silver (Ag) is used primarily in commercial applica-

tions where low cost is essential. Typical applications

are in microcircuit displays and polymeric composites,

adhesives, and sealants.

IPC-D-859 December 1989

E. Platinum silver (PtAg) is used where soldering of com-

ponent is employed. Its application is limited presently

to nonmilitary applications.

F. Palladium Silver (PdAg) is a low-cost solderable con-

ductor designed for good solder-leach resistance.

G. Palladium platinum silver (PdPtAg) is a ternary metal

composition which attempts to reduce the silver migra-

tion which has limited the utilization of silver-bearing

inks in military applications. It provides a solderable

conductor with good solder-leach resistance.

H. Copper (Cu) was developed primarily for high conduc-

tivity and to reduce costs associated with noble conduc-

tors. The use of this material must be carefully evalu-

ated, as the process employs tighter controls than those

associated with noble metals.

I. Nickel (Ni) is another low-cost substitute for noble

metals. It ﬁnds its greatest use in display applications.

J. Tungsten/molybdenum manganese (W/MoMn) materi-

als require reducing atmospheres. The ﬁring conditions

requireconsiderably greater controls which are gener-

ally not available in small laboratories. The inks are not

readily available.

3.8.2.2 Attributes Table 3–7 lists typical attributes for

thick- ﬁlm conductors.

3.8.3 Resistive Materials A variety of characteristics are

provided by materials that impede signal ﬂow in a circuit

and these entities are considered separately as passive ele-

ments in the circuitry. Table 3–8 lists the properties of

thick-ﬁlm resistors.

3.8.3.1 Ceramic Thick-Film Resistors Ceramic thick-

ﬁlm resistors have sheet resistivities in a large range of

values and are available in a range from a very few ohms/

square to high megohms/square. The resistivity is con-

trolled by varying the metal-to-glass content which causes

large variations in all mechanical and physical properties.

After screen printing, the pastes are dried and ﬁred. Firing

temperatures range from 750° to 900°C.

Most thick-ﬁlm resistor systems are based on the use of

ruthenium-oxide or lead/bismuth ruthenate metallurgies.

These are relatively inexpensive systems which can be for-

mulated in a range of sheet resistivities from 10 ohms/

square to 1 Mohm/square.

Temperature coefficients as low as +50 PPM/°C are readily

achievable in the mid-range of from 100 ohms/square to

100 Kohms/square. In the extended ranges, TCR’s of +100

PPM/°C are available in resistors as low as 10 ohms/square

and as high as 1 Mohm/square.

Table 3–6 Typical Film Conductor Resistivities and TCR’s

Line Resistance

Conductor compositions Sheet Resistivity (Ο/square) TCR PPM/°C

25.4 x 0.5 mm

[1 x 0.020]

25.4 x 0.13 mm

[1 x 0.005]

Platinum gold 0.100 +800 5 ohms 20 ohms

Gold 0.003 +1250 0.15 ohms 0.6 ohms

Palladium silver 0.035 +1400 1.75 ohms 7 ohms

Silver 0.002 +1800 0.1 ohm 4 ohms

Table 3-7 Conductor System Attributes

Die Bondability Methods Wire Bondability

Thick ﬁlms Eutectic Solder Organic Gold Aluminum

Solder

Wetting

Solder

Leach

Resistance

Corrosion

Resistance

Au Good Poor Excellent Excellent Good Excellent Poor Excellent

PtAu NG Good Excellent Fair to poor Fair Excellent Good Excellent

PdAu NG Fair Excellent Fair to poor Fair Good Good Excellent

Ag NG Poor Excellent Good NG Excellent Poor Poor

PtAg — Good Excellent Good NG Excellent Fair to Good Good

PdAg — Good Excellent Good Good Good to Exc. Fair to Good Good

PdPtAg — Good Excellent Good Good Good Good Good

Cu NG Good Excellent NG Fair Good Excellent Poor

Ni NG NG Excellent NG NG NG Excellent Excellent

W/MoMn NG NG Excellent NG NG NG NG Good

December 1989 IPC-D-859

In the ‘‘as-ﬁred’’ state, thick-ﬁlm resistors normally exhibit

a tolerance of ±20%. To achieve precise values, the resis-

tors are adjusted or trimmed to value by air abrasion or

laser. With care, ±0.5% tolerance can be achieved.

3.8.4 Dielectric Materials Dielectric compositions are

playing an increasingly important role in the hybrid tech-

nology and will probably continue to do so. The ﬁrst

dielectric compositions used were sealing glasses that

served to cover other circuit elements and protect them

from attack by atmosphere and other processes. Since the

start, the use of dielectrics has spread until the sealing

functions are secondary to the more directly functional

capacitor, crossover, and multilayer uses.

Dielectric compositions are now ﬁrmly established in three

important classes of ﬁlm compositions: resistors, conduc-

tors and dielectrics. The three types of dielectric composi-

tions are crossover and multilayer compositions, capacitor

compositions, and sealing or encapsulating compositions.

Table 3–9 lists typical properties of ﬁlm dielectrics.

3.8.4.1 Ceramic/Glass Dielectrics The most-widely

used thick-ﬁlm dielectrics are overglazes, multilayer

(crossover) dielectrics, and capacitor dielectrics. These

materials have a dielectric constant of 6 to 14 and a break-

down voltage greater than 500 volts per 0.025 mm [0.001

inch].

The overglaze is a low-melting temperature vitreous glass

material. It is generally used as a resistor overcoat to pas-

sivate thick-ﬁlm resistors and to protect them from

mechanical damage. It can also be used as a conductor

insulator and solder barrier.

A multilayer (crossover) dielectric is a mixture of ceramic

and devitrifying glasses. It is used as an insulating barrier

between conductor crossover layers and between multi-

layer conductors.

3.8.5 Metallic Platings and Coatings The primary func-

tion of all metallic coatings used in hybrid circuit manufac-

turing is to allow for the passage of the electronic signal

from point to point. Beyond this primary function, speciﬁc

platings offer such additional beneﬁts as corrosion preven-

tion, solderability enhancement, wear resistance and others.

3.8.5.1 Gold Plating While not commonly used, gold

plating can serve several functions on a multilayer hybrid

circuit.

A variety of gold platings are available for deposition on

hybrid circuits: 24K soft golds, 23 + K hard golds (which

are hardened by cobalt and nickel or iron) and a number of

low karat golds (14K–18K).

Different types of gold plating serve different uses (i.e., a

corrosion resistant contact surface, a wire bondable surface,

or a corrosion or aging resistant solderable surface). Table

3–10 will help to clarify some uses for the various alloys.

All electrolytically deposited gold shall be in accordance

with MIL-G-45204. A low stress nickel or tin nickel alloy

(see 3.8.5.2) shall be used between the gold overplating

and copper.

Care should be taken when soldering to gold to prevent the

formation of brittle intermetallics that can fail prematurely.

Special indium bearing solders are available to lessen this

effect. Gold plated component leads should be double

dipped and agitated in solder to prevent this brittle interme-

tallic formation.

3.8.5.2 Nickel Plating Nickel plating serves a dual func-

tion in contact plating; (1) it provides an anvil effect under

the gold adding an essential extra hardness to the gold, 2)

it is an effective barrier layer (when its thickness exceeds

2.5 (µm [0.0001] preventing the diffusion of copper into

gold. This diffusion process can result in a room tempera-

ture alloying of the gold, degrading the electrical and cor-

rosion resistance characteristics of the contact.

All electrolytically deposited nickel plating shall be low-

stress and conform to QQ-N-290.

3.8.5.3 Tin-lead Plating Tin-lead plating is applied for

purposes of enhancing long term solderability of the mul-

tilayer hybrid circuit. The electrodeposit is generally fused

by one of several techniques (hot oil immersion, infra-red

exposure, exposure to hot vapors of inert liquids). The fus-

ing operation results in the formation of a true alloy on the

Table 3-8 Film Resistor Characteristics

Parameter Units Thick Film

Resistivity Range Ohms/Sq 1–1000 M

Temperature Coefficient

of resistance

PPM/°C ±50–300

TCR Tracking PPM 10

Power Handling Capacity Watts/mm

≤0.155

Thermal Stability

(150°C/1000 hrs.)

%∆R ≤±0.25

Voltage Coefficient

of resistance

PPM/V 0.5–5

High Voltage stability %∆R<±0.5

Noise dB –35 to +20

Trim stability %∆R ≤±0.25

Load life 25°C–1000 hrs.

(rated power)

%∆R ≤±0.25

Aging Stability 25°C/50%

RH/1000 hrs.

%∆R ≤±0.25

Conductor Compatibility — Au, Ag,

Cu, PdAg,

PtAg

IPC-D-859 December 1989