IPC-D-859.pdf - 第30页

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- ti…

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

Q. Reliability—Assembled hybrid microcircuits should be

able to withstand the temperature-cycling, thermal-

shock and constant-acceleration tests deﬁned in MIL-

STD-883.

A list of some of the better know general types of attach-

ment materials, and their associated advantages and limita-

tions, is given in Table 3–12.

3.8.9 Chip to Substrate Interconnect Gold or alumi-

num wire bonds are used to interconnect the semiconduc-

tors and IC’s to the hybrid network and to the I/O pins. The

wires are bonded from the chip to a metallized bonding

land that is connected to the circuitry or the I/O pins.

The gold wires are typically alloyed with small amounts of

beryllium or copper to control grain growth during bond-

ing. The aluminum wires are typically alloyed with

approximately 1% silicon.

The following factors will inﬂuence the choice of wire

bonding materials and equipment:

A. Maximum processing temperature, including rework

(see Table 3–13)

B. Size of the bond sites on the semiconductor or IC 0.025

mm [0.001 inch) diameter wire thermo-sonic ball bond-

ing requires land sizes greater than 0.06 mm [0.0025

inch] square

C. Hybrid substrate conductor materials

D. Final operating and non-operating environment

E. The geometry of the hybrid package which may restrict

bonding tool access.

3.8.9.1 Bonding Process The choice of wire type and

speciﬁc properties, such as hardness, tensile strength and

percent elongation, are a function of the bonding process

used as well as speciﬁc bonding parameters. The standard

wire bonding process used for hybrids is thermosonic gold

ball-bonding. The principal advantages of this process are

the low substrate temperatures (<150°C) required and the

ability to bond in any direction.

Wedge bonders, used most frequently with aluminum wire,

require that the second bond be positioned directly behind

the ﬁrst. This is time consuming because the entire hybrid

package must be rotated to position each bond.

3.8.9.2 Gold and Aluminum Wire Selection The fusing

current of wire is dependent on the material, its diameter,

its length and application of the wire. The application is

important because it determines the operating temperature

and the degree of heat sinking required to the I/O leads or

substrates.

Table 3–14 lists sizes and typical current rating for alumi-

num and gold wires. Note: Most hybrid package leads are

made of Kovar [Fe, Co. Ni alloy] and are subject to I

heating effects.) Most leads should be limited to less than

0.5 amps.

Wire leads used to interconnect semiconductors to the

hybrid network, and/or package I/O pins that are not in

Table 3–12 The Advantages and Limitations of Various Adhesive Types

Type Advantages Limitations

Phenolics Very high bond strength Used mostly for structural applications, possibly corro-

sive, difficult to process at low temperatures

Polyurethanes Easy to rework Not suitable for temperatures above 120°C, relatively

high outgassing, some decomposition

Polyamides Easy to rework High moisture absorption, high outgassing, variations

in electrical insulation properties, especially when

exposed to high humidity

Polyimides Very-high temperature stability High cure temperatures, require solvents as vehicles

Silicones High’temperature stability, easy to rework, high purity,

low outgassing

Moderate-to-poor bond strength, high coefficient of

thermal expansion

Epoxies Some are easy to rework by thermomechanical

means, some are low outgassers, easy to process,

can be ﬁlled to 6-–70 percent with a variety of conduc-

tive or nonconductive ﬁllers

Depending on the type of curing agent used and

degrees of cure: outgassing, catalyst leaching, corro-

sivity

Cyanoacrylates Very rapid setting (10 sec), give very high initial bond

strengths

Bond strengths often degrade under moist or elevated

temperature (≤150°C) conditions

Glass–Silver Die

Attach Materials

High bond strength, with bond stable at high tempera-

tures. Non-corrosive with low moisture absorption and

little, if any outgassing.

Requires high cure temperature and is not easily

reworkable. Contaminant level fair to good.

Eutectic Alloy No outgassing. Good electrical and thermal properties.

Rapid setting.

Restricted in die size. Not repairable.

Solder No outgassing. Good electrical properties. Rapid set-

ting. Repairable.

Temperature sensitive for subsequent assembly pro-

cesses. High CTE.

December 1989 IPC-D-859