IPC-CM-770D-1996 - 第67页

January 1996 IPC-CM-770 environment, since bare chips are being handled. General cleaning guidelines are discussed in Section 28. Attention should be placed on component and solvent compatibility. 14.1 O Coatings General…

IPC-CM-770

Januaty

1996

The width of wireAead bonding land should be at least

0.25 mm by 0.75 mm.

Spacing between adjacent wireAeads should be 0.25 mm

minimum.

The tip of the bonding wireAead should have a square

configuration.

The soldermask opening should be at least 1.25 mm from

the edge of the bonding land.

Thermocompression ball bonding requires relatively-large

bond- ing lands and wire bonding clear- ances for high-

yieldhigh-rate production. Figure

14-4

shows the recom-

mended layout features and limits for the determination of

COB land patterns.

14.3.3 LeadlLand Configuration After Assembly

The

three basic chip attachment and termination techniques are

wire bonding (chip-and-wire), tape automated bonding

(TAB), and controlled collapse (Flip Chip) bonding.

14.4 Mixed Technology

The use of unpackaged semi-

conductor components is feasible in mixed assembly

technology.

Subslrale

Conduclor

IPC-I-

Figure 14-4 Wire Bonding Layout Features (see Table

14-2)

14.5 Manual Assembly

Manual techniques are usually

used for assembling special parts that may have one or two

type parts on a particular assembly. This relates to trans-

formers, relays, crystals, etc. When handling semiconduc-

tor device components, manual placement techniques usu-

ally require the addition of magnification. The small size

and fragility of bare chips used in COB technology makes

manual assembly somewhat difficult and in some types of

COB, such as TAB, impossible. The manual wire/lead

bonding operator can often inspect and compensate for

deficiencies in board layout and/or bonding surfaces, while

automatic machines require conservative design features

and uniformity of bonding surfaces. However, machine

assembly is most desirable, in order to obtain the high vol-

ume benefits of COB technology.

14.6 Automated Assembly

The chip-on-board assembly

industry has now begun to closely follow similar manufac-

turing methods and procedures of those already well estab-

lished in the semiconductor and printed circuit industries.

Various unique approaches have been taken to solve COB

manufacturing problems with cost-effective solutions for

the particular applications involved.

Automated techniques are usually not employed for mount-

ing special components, unless a large variety of the com-

ponents constitute an entire assembly. This might espe-

cially be true for chip on board technology, where many

chips might be mounted on a single interconnecting

substrate.

Special requirements for wire bonding and automated

assembly for chip on board are defined in Section 23.

14.7 Handling and Storage

The handling and storage of

unpackaged semiconductor components should be in accor-

dance with the guidelines of Section 26.

14.8 Soldering

Most COB technologies do not involve

the use of solder, with the exception of flip-chip. Flip chips

are soldered by means of reflow soldering techniques simi-

lar to that used for other packaged devices such as leaded

or leadless chip carriers.

When metal sealing lids are used as a chip protection

method, soldering is generally used to attach the lid to the

board. The use of preforms is often the most convenient

method for applying the solder. The heat required to reflow

the solder can be applied using one of the soldering pro-

cesses that is associated with conventional surface mount-

ing technology.

General soldering guidelines are discussed in Section 27.

14.9 Cleaning

Cleaning requirements associated with

soldered through-the-board and surface mounted assem-

blies, in most cases, do not apply to COB technology.

However, very stringent cleanliness requirements are

needed for the boards to be used for COB. Also, strict

cleanliness standards should be maintained in the assembly

3-30

Licensed by Information Handling Services

January

1996

IPC-CM-770

environment, since bare chips are being handled. General

cleaning guidelines are discussed in Section 28. Attention

should be placed on component and solvent compatibility.

14.1

Coatings

General conformal coating characteris-

tics applying to all types of components and assemblies are

described in Section 29.

Coatings are used in COB technology to protect the bonded

chips on the board from handling damage as well as for

environmental protection. Typical coatings used for this

purpose are room- temperature vulcanizing @TV) disper-

sion coatings, silicon dielectric gel, and epoxy type coat-

ings, applied locally at the bonded chip site, as opposed to

entire board coating.

14.10.1

RTV

Dispersion Coating

Typical RTV disper-

sion coatings are one-component, room-temperature vulca-

nizing (RTV) silicone-rubber coatings that are supplied as

a xylene dispersion.

(No

mixing is required.) The curing

process used a crosslinking mechanism that generates

methanol during cure. Once applied and exposed, the mate-

rial vulcanizes by reaction with moisture from the air to

form a soft, resilient elastomeric coating that will withstand

long term exposure to temperature as high as 250°C

(482°F).

14.10.2 Silicone Dielectric Gel

Special controlled tech-

nology can yield a silicone gel that affords the nonflowable

permanence of a solid, but also gives the freedom from

large mechanical and thermal stresses of a fluid. Chemi-

cally, a typical silicone gel is very similar to silicone flu-

ids, but with just enough crosslinking to prevent separation

of the individual polymer chains and give nonflow thermal-

set properties. The fully-cured dielectric gel is a soft, jelly-

like material that exhibits tenacious pressure-sensitive

adhesion to virtually any substrate.

14.10.3 Epoxy Coatings

Epoxy coatings are also avail-

able for COB self-crowning or “glob-top’’ applications.

Typical materials are two-component, liquid epoxy/

anhydride systems that have been formulated for their

superior thermal shock performance, substrate adhesion,

moisture resistance, and glass-transition temperatures in the

range of from

165”

to 180°C.

3-3

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

3-32

Licensed by Information Handling Services