IPC-D-859.pdf - 第29页

surface and improves long-term solderability . T in-lead plating shall be in accordance with MIL-P-81728. 3.8.6 Organic Protective Coatings 3.8.6.1 Conformal Coatings Conformal coatings shall meet the requirements of IPC…

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

surface and improves long-term solderability.

Tin-lead plating shall be in accordance with MIL-P-81728.

3.8.6 Organic Protective Coatings

3.8.6.1 Conformal Coatings

Conformal coatings shall

meet the requirements of IPC-CC-830, and when required,

shall be speciﬁed on the master drawing.

3.8.7 Solder and Flux Table 3–11 lists important charac-

teristics of typical alloy combinations.

Solder compositions should be selected to be compatible

with subsequent processing steps such as epoxy curing and

case or cover sealing. Solder attachment is not generally

compatible with the eutectic attachment of semiconductor

die and requires careful consideration of its dispensing

method when used along with epoxy attachment.

Also, tin-bearing solder compounds are not compatible

with gold conductors. Therefore, indium-based solders are

generally considered for use with thin and thick ﬁlm gold

conductors.

Solders are generally encountered in four forms: preforms,

bulk, paste, and spheres (bumps). Bulk solder is typically

used in a solder pot. The substrate is dipped into the pot.

Then, with the metallization wetted with solder, the add-on

components are positioned and the solder reﬂowed.

Solder paste is solder in a screen-printable form that may

be printed onto the desired lands and subsequently

reﬂowed.

3.8.7.1 Flux The choice of base metal dictates both the

type of ﬂux required and its use. If the ﬂux is not suitable

for the assembly, the base metal must be changed or its

surface modiﬁed. This can be done by pretinning or plating

to allow the use of a more suitable ﬂux.

The effect of ﬂux on circuit components is a very impor-

tant consideration. Solder pastes generally contain the nec-

essary ﬂux. However, ﬂux must be applied with both pre-

form and bulk soldering. (Steps to remove the ﬂux and its

residue must be taken into account when processing the

circuit.)

3.8.8 Attachment Materials Thick-ﬁlm technologies use

a variety of adhesives to attach substrates to packages,

devices to substrates, lids to packages, and covers to sub-

strates. These adhesives, together with eutectic alloy com-

positions and conductive, low-melting glass compounds

may be used for die attachment. The materials selected for

this purpose must be compatible with temperature-sensitive

components.

When a series assembly approach is used (i.e., the sequen-

tial attachment of substrates, devices, and covers) a tem-

perature hierarchy must be established. Also, both the

mechanical and environmental characteristics of attach-

ment materials must be adequate to fulﬁll hybrid and sys-

tem requirements. Therefore, long-term outgassing prod-

ucts, ionic contaminants and electrical properties must all

be characterized with regard to the reliability and func-

tional requirements of the circuit.

The properties of attachment materials that are important in

determining their suitability for use in the assembly of

hybrid microcircuits include the following:

Table 3–9 Dielectric Comparisons

Parameter Units Glass/Ceramic Polymer Film

Insulation resistance range ohms >10

>10

Dielectric constant range @ 1 kHz — 6–14 4–6

Dissipation factor range % <3.5 >5

Voltage breakdown range Volts/mm 0.000079 mm

[0.002] ﬁlm >19700

low

Temperature coefficient of capacitance PPM/°C <250 NA

Hermeticity — excellent poor

Thermal coefficient of expansion range — low high

Conductor compatibility — PtAu, Au, Cu Cu, Ag

Resistor compatibility — good good

Capacitor value range (practical) pF ≤1000 small

Trimability — abrasive abrasive

Table 3–10 Gold Plating Uses

Contacts Wire

Bonding

Soldering

24 k soft C* S S

23* k hard S C** S

14-18 k hard C*** NR S

S=Suitable use NR=Not recommended C=Conditional use

*Thickness of deposit and number of expected insertions

over life cycle of product may limit use.

**May be used but will depend on type of wire bonding

being used. Run tests prior to specifying.

***Check requirements; generally may be suitable for Class

1, possibly for Class 2, but not allowed for Class 3.

December 1989 IPC-D-859

A. Electrical—Stable electrical properties must be main-

tained over wide ranges of temperature and humidity

(e.g., insulating materials should maintain a volume

resistivity greater than 1014 ohm-cm in the dry condi-

tion).

B. Handling convenience—From the standpoint of both

economy and user convenience, factors such as storage

conditions, pot life, whether the attachment material is

a single- component or a two-component system, and

whether or not the material is available in a ready-to-

use tube will inﬂuence selection.

C. Ease of application—Attachment materials must be

capable of being applied in controlled amounts and

thicknesses, and also must give void-free bonds. Insuf-

ﬁcient thickness can result in electrical breakdown,

while excessive amounts can result in stresses during

temperature cycling.

D. Flow during cure—Excessive ﬂow must be prevented

during cure to avoid the coating of adjacent areas that

must subsequently be soldered, the bridging of conduc-

tor lines, and, possible, electrolytic corrosion.

E. Shrinkage during cure—Excessive shrinkage during

cure must be prevented to avoid mechanically stressing

components, possibly cracking them or inducing

parameter changes.

F. Component creep—The tendency of an adhesive com-

ponent to separate, as a result of capillary action or

creep during cure, is undesirable due to its possible

contribution to electrolytic corrosion or to the degrada-

tion of wire bonding.

G. Outgassing—Both the release of condensable volatiles

during cure and the continued outgassing after cure are

undesirable because electronic components can absorb

outgassed constituents which may degrade properties.

H. Ionic content—Attachment materials must not contain

water- extractable ionic constituents, such as Cl

–

, that will promote corrosion or electrical leakage

between conductors.

I. Tackiness—The exposed edges of electrically-

insulative adhesives must be ‘‘tack free’’ in order to

avoid the capture of conductive particle contaminants

that can cause electrical failure.

J. Solvent resistance—The degradation of bond strength,

or the leaching of adhesive components, must not be

caused by solvents used in cleaning electronic compo-

nents, modules, or subsystems.

K. Corrosivity—Attachment materials must not be

innately chemically corrosive or electrolytically corro-

sive to the metallization system(s) with which they are

used.

L. Flexibility—The adhesives must be sufficiently pliable

to relieve mechanical stresses between thermally mis-

matched materials in order to avoid warping or crack-

ing of the substrates and components.

M. Repairability—It is desirable that the adhesive bond be

fracturable at some reasonable temperature and with

minimal mechanical force to avoid, during rework,

damaging any metallization(s), or breaking the sub-

strate.

N. Hydrolytic stability—The attachment material must not

chemically degrade (i.e., reverse back to a liquid) upon

its exposure to high temperature and humidity for long

periods of time.

O. Thermal stability—The attachment material must not

decompose at high temperatures (+225°C) or crack at

low temperatures (–65°C).

P. Bond strength—The attachment material must have

adequate bond strength at maximum use temperature,

after exposure to commonly used solvents, high humid-

ity, and extended aging.

Table 3-11 Typical Solder Systems

Parameter 80Au20Sn 88Au12Ge 63Sn37Pb 62Sn36Pb2Ag 95Sn5Ag 50Pb50In

% Elongation 28-30 30.0 55

Electrical Conductivity

% IACS*

11.5 14.0 12.6 5.1

Thermal Coefficient of

Expansion (mm/mm°Fx10

-6

)

13.7

Solidus Temperature (°C) 280 356 183 180 221 180

Liquidus Temperature (°C) 280 356 183 185 245 210

Reﬂow Temperature (°C) 300 375 200 200 275 230

Tensile Strength (MPa) 46.2 58.6 55.1 32.2

Shear Strength (MPa) 41.8 48.2 18.5

Brinell hardness 17.0 13.7 9.6

*IACS = International Annealed Copper Standard, i.e. copper having a resistance of 0.15328 abs. ohm (meter, gram) at 20°C, a

density of 8.89 per cm

at 20°C of 0.00393.

IPC-D-859 December 1989