IPC-CM-770D-1996 - 第117页

IPC-CM-770

Januaty

1996

together.

For heavier components, adhesive or mechanical means

may be used.

22.1.3 Land Patterns

Land patterns must be designed

for maximum assembly yield. Land sizes must be large

enough to ensure an adequate fillet at the extremes of

allowable board and component tolerances while not wast-

ing space needed for routing and other component mount-

ing. Appropriate land and mounting patterns for the indi-

vidual components are contained in the respective sections

for the components considered. Surface mount land pat-

terns are addressed in IPC-SM-782.

22.2 Manual Assembly

Manual installation of surface

mounted components is accomplished in a manner similar

to through-hole installation, except for greater use

of

vacuum pick-up devices, which simplify the precise place-

ment of the components which is required for satisfactory

soldering. Boards designed for manual component installa-

tion need not follow the layout guidelines for automatic

assembly. However, space must be allowed for exposure to

the solder wave (when applicable), test access, and rework.

22.3 Automated Assembly

Most surface mounted com-

ponents can be assembled using automated equipment.

Automated assembly is simplified when the boards are

designed for this type of assembly. Figure 22-3 illustrates

several considerations which simplify automated assembly

and maximize productivity by restricting table motions.

mounted devices should be inspected for conformance to

the acceptability criteria of IPC-A-610.

23.0 CHIP-ON-BOARD (COB) TECHNOLOGY

23.1 General Considerations

The mounting of unpack-

aged semi-conductor dice for COB applications was once

restricted to the use of ceramic substrates in what is com-

monly referred to a "hybrid" circuits. However, interest in

COB technology has expanded the use of substrates to

include more conventional organic-base printed board type

structures.

COB components are always surface mounted. The follow-

ing paragraphs describe the mounting techniques and crite-

ria for COB technology. For more detailed information, see

IPC-SM-784.

23.2 Die Attachment

Die attachment may be accom-

plished by eutectic, adhesive or wire bonding, and is

dependent on the requirements of the application.

23.2.1 Adhesive Die Bonding

A die attach method,

eutectic or adhesive bonding, is selected for each applica-

tion.

23.2.1.1 Epoxy Bonding

Epoxy bonding can provide

both conductive and non-conductive attachments. Conduc-

tive epoxies have metal particles (usually silver or gold)

suspended in the basic epoxy resin. When the resin cures,

0.5

mm

[O

OZO"]

I

7

r0

9

mm

[O

035"l

mln.

Figure 22-3 Preferred Mounting Orientations

22.4 Quality Assurance

Assemblies containing surface the suspended metal particles remain in contact providing

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January

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IPC-CM-770

both electrical and thermal conduction. Non-conductive

epoxies contain a basic resin and can contain additives to

promote thermal conduction.

Epoxies designed for die attach usually limit content of

volatile solvents. This reduces the amount of out gassing

and formation of voids. Inspection for voids is simply done

by using a clear glass die and viewing through the die after

adhesive cure. Advantages of using epoxies include:

Low temperature cure, however:

Cure temperature should not reach Tg of the board

Cure temperatures may age solder joints

Ease of processing with silk-screening or dispensing

equipment:

Appropriate adhesive viscosity is necessary for each

process

High Yield: Dependent on die yield and handling

Ease of removal for repair: Before encapsulation

Mechanical strength for ruggedness:

Can be verified with a die shear test

Epoxy bonding also dictates certain precautions, including:

A.

Heat control:

When thermocompression bonding is

used, a heated probe should be used, as the heat associated

with some types of thermocompression bonding will soften

the epoxy. (Does not apply when thermosonic bonding is

used.)

B. Precision tools:

If a vacuum-hold chuck is used to

mount the chips, check that each component forms a posi-

tive seal against the probe tip. Otherwise, epoxy might be

drawn to the top of the chip.

C. Chip processing:

If the gold is evaporated from sin-

tered chips that have gold backing in order to alloy the gold

with the silicon, the epoxy can pull away from the chip and

form a high resistance contact.

23.2.2 Eutectic Die Bonding

Eutectic die bonding

requires the back of the silicon die to be gold plated. At

sufficiently high temperatures the gold plating on the back

of the silicon chip melts with the gold plating on the die

site and forms a eutectic bond with the silicon. The silicon

device can withstand these high temperatures without

endangering performance or life expectancy. The die-attach

station should have the die attach area flooded with an inert

(forming gas or nitrogen) atmosphere.

23.2.3 Polyimide

Polyimides and silver-filled glass

adhesives have the ability to withstand higher temperatures

than epoxies. The polyimides used are thixotropic pastes

containing approximately

70%

silver powder in a polyim-

ide resin that has been dissolved in a high boiling tempera-

ture solvent. The main objection to their use is that they are

organic polymers and it is difficult to remove all of the

organics, which may outgas and cause chip degradation.

23.3 Wire Bonding Materials and Techniques

23.3.1 Bonding Wire

Both gold and aluminum wire are

used for chip on board (COB) applications (Figure 23-1).

Gold wires are typically alloyed with small amounts of

beryllium copper to control grain growth during bonding.

Aluminum wires are typically alloyed with

1%

silicon or

magnesium. For reference of standard wire see ASTM-F-

1.07.

Wire selection is dependent on the bonding techniques

(Table 23-1). The bonding technique determines the degree

of heat at the chip and board sites during bonding. The

amount of heat allowed and the wire termination selected

will limit the material choice and diameter of the wire.

Table 23-2 lists wire sizes and typical resistance values for

aluminum and gold wire. The current should be limited to

less than

0.5

amps for most applications.

A good gold wire bond should have a pull strength from

8

to

10

grams for 0.025 mm diameter wire. Aluminum wire

bonds usually achieve slightly less pull strength than gold.

Table 23-3, Minimum Bond Strength, lists the wire diam-

eter and minimum acceptable pull strengths. Failure occur-

ring in the wire and not at the bond sites means the bonds

are stronger than the wire. This failure is preferred because

wire strength is usually under more control than the bond

sites.

23.3.2 Metallization

Metallization of the die and board

bond pads are very important to successful bonding and

reliability of the bonds. Verification of metallization purity

and thickness are critical. Limiting intermetallics between

wire and bond pad is important. The purity, additives, and

age of the bond wire are also important. The vendor can

supply all of this information. Bond integrity can be veri-

fied by bond pull and shear tests. Aging the wire bonds and

repeating pull and shear tests will give additional informa-

tion on reliability of the bond. Typical board plating for

gold wire bonding should be a minimum of 0.001mm of

99.99%

pure soft gold over

0.004

to 0.005mm of conven-

tional nickel. Aluminum wire bonding can be done reliably

with board plating that is similar to that for gold wire

bonding.

23.3.3 Printed Boards

Printed boards produced with

polymer conductive materials on low-temperature sub-

strates are now being made that are wire bondable. The

application of a plated-metal surface over the printed thick-

film polymer conductor pattern makes this possible. The

printed polymers currently being used for this type of

board construction are thermal-set, epoxy-silver conductors

and non-noble nickel polymers.

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Licensed by Information Handling Services

IPC-CM-770 Januaty 1996

1

O0

70

30

20

::

10

I

O

J

v:

I

1980

1985

19

'NON-IBM YEAR

OTHER

GOLD

BALL

BONDING

73%

AI

BAL

L

3'1"

ALUMINUM

WEbGE

BONDING

22%

i%

t

2

2%

I

IPC-1-00338

Figure 23-1 Chip Termination Techniques

Table 23-1 Bonding Wire for COB Applications

Wire Diameter Substrate

(inch)

Comments

Temperature("C)

Attachment Method

Thermocompression (hot substrate)

I

>300

I

Limited repair ability

0.0007-0.001

(Gold)

0.0007-0.002

Thermosonic ball or wedge 125°C Quality of bonds improved if the

substrate is raised to 125°C. Can bond

chips without raising substrate

temperature.

(Aluminum)

I

Ultrasonic Wedge

0.0007-0.010

I

25 Low-temperature process, restricted due

to tool size

Table 23-2 Aluminum and Gold Wire Sizes and Ratings

Wire

Minimum

Standard Wires

Aluminum

1%

Si Aluminum

1%

Si Diameter

Resistance(Rm.)

(inch)

Gold

(Annealed) (Hard)

Gold Aluminum

I

0.0007

I I

X

I

36.06-39.86

I

34.62-38.273

I

27.71-29.43

I

0.001

I

X

I

X

I

17.67-19.53

I

16.0-18.75

I

13.58-14.42

I

0.002

I I

X

I

4.41-4.88

I

4.23-4.68

I

3.40-3.61

I

23.3.4 Wire Bonding Techniques

300"

to

400"

C. The majority of T/C bonding uses a ball

bond on the first bond site and a wedge bond on the sec-

23.3.4.1 Thermocompression (T/C) Bonding (Ball Bond-

ond bond site.

ing)

Thermocompression bonding utilizes heat and pres- When bonding an open flame (or spark discharge) melts

sure to bond wire to a metallized pad site.

A

typical system the end of the wire, molten metal wicks up the end of the

uses a capillary tool of refractory material, such as alumina wire forming the ball. Wire termination after the second

or tungsten carbide, gold wire, and a surface temperature of bond is achieved by flame

off

or as in wedge bonding cut

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