IPC-CM-770D-1996 - 第116页

January

1996

IPC-CM-770

Sequence all types of axial lead components prior to

insertion.

Minimize the distance between the components and fol-

low a grid pattern for component layout.

Minimize the number of different center spacings.

Make provisions for tooling holes at or near the edge of

the board in an area not occupied by components.

Minimize the number of different hole sizes to minimize

manufacturing time if boards are drilled or die costs if

punched.

Provide clearance areas as large as the tooling footprint

between components for the insertion tools both above

and below the board.

22.0 SURFACE MOUNTING

22.1 General Considerations

Surface mounting is a

technique which is applicable to most component types,

and is used for a variety of reasons. The technique was

developed and proven for stripline and other high fre-

quency applications where lead placement and discontinui-

ties had to be strictly controlled.

The technique was found to be a reliable and viable solu-

tion to a number of problems, and is now in wide use.

Surface mounting consists of placing the component on the

printed board or other suitable substrate, and making the

necessary electrical connection to the component on the

same side of the board. The leads of surface mounted com-

ponents do not pass through the board to make electrical

connections. The specific connection technique to be used

will depend on several factors.

In general, leaded components are lap-soldered to the ter-

minal areas, while leadless components are attached by

solder fillets between a solderable area on the component

and the land area.

Orientation and placement of components on the “solder

side” of the assembly should be such that bridging or

depleted solder on the following component does not

occur. (Figure

22-

1).

f-

Direction

of

travel

F

Solder

wave

IPC-1-00369

Figure 22-1 Depleted Solder

22.1.2 Lead Forming

General requirements for lead

extension and forming for leaded components are illus-

trated in Figure

22-2.

When formed the leads must be held

close to the body to prevent damage to seal.

IPC-1-00326

Figure 22-2 Lead Forming for Surface Mounting

Leaded components should be mounted with the body

0.25

mm minimum above the surface of the printed board

except when the part body is sealed to the surface of the

board with adhesive such that no flux, moisture, or other

contaminants can become trapped. Round axial leaded

parts need not be elevated, since the body configuration

permits thorough cleaning.

A major consideration in using leaded components is to

assure that all leads are coplanar. A tolerance of

+

0.05

mm

is recommended to assure adequate solder filleting to each

lead.

Alternative definition: use the three longest non-colinear

leads to define a plane (sit the part on a flat surface) then

all leads should be within

+0.05

mm of the surface.

It should be noted that the JEDEC standards do not contain

such a stringent coplanarity requirement. Therefore, lead

forming by the user is usually required.

Lead forming is not applicable to leadless components.

Solderability, however, is of prime importance. All surfaces

to be soldered should be pretinned and verified as very sol-

derable through one of the recognized solderability test

methods, such as

J-STD-002.

The formed leads should be parallel and in contact with the

lands on the mounting base without unplanned overhang.

Maximum angle allowed between the lead pad and parallel

mounting base is

15.

22.1.2.1 Lead Support

Leads should be supported dur-

ing forming to protect lead to body seal.

22.1.2.2 Coined Leads

Round cross section leads usu-

ally are coined to enhance mounting stability.

Components with axial leads of round cross section should

be utilized for planar mounting only if the leads are coined

or flattened for positive seating.

22.1.2.3 Component Support

Depending upon weight,

components may be secured for soldering in the following

ways:

For light components, solder paste with relative high

solid content can be used to hold pretinned lead and land

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IPC-CM-770

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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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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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