IPC-CM-770D-1996.pdf - 第35页

IPC-CM-770

Januaty

1996

Figure 7-4

A

Chip Inductor

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Figure 7-5 Typical Surface Mount Inductor

7.1.3 Other Devices

Other passive devices are being

adapted to surface mounting applications. One noteworthy

example is a surface mount sealed single-turn cermet trim-

mer in a vertical and horizontal style. This particular prod-

uct is available in a 1K to 200K ohms resistance range.

Figure 7-6 shows an outline of this trimmer.

B

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Figure 7-6 Surface Mount Cermet Trimmer

7.2 Through-Hole Mounting

Chip components are nor-

mally leadless or have ribbon leads that are not appropriate

for through-hole assembly.

7.3 Surface Mounting

7.3.1 Component Preparation

When surface mounting,

good solderability of chip component terminations is

essential for high assembly yield. The solderability of the

end terminations should be tested on all lots of new com-

ponents upon receipt and after any prolonged (more than

one month) storage.

The preferred termination may be reflowed or dipped only

if excess solder is removed from the end terminations. Bare

silver-palladium terminations should be avoided because

such terminations tend to lose solderability when exposed

to a solder bath or any other large volume of solder.

Component terminations should have a diffusion barrier

layer (typically nickel or copper) under the solder to pre-

vent the leaching of silver from the underlying silver-

palladium termination.

No

silver should be detectable on

the surface of the component.

Conductors may be connected to a land at any portion of

the land perimeter but vias should not be located on or in

contact with the land. However, when reflow soldering is

used, conductors should be covered with soldermask to

minimize scavenging of solder away from the component

termination. This is especially important when conductors

connect to plated-through-holes near the component termi-

nation land.

7.3.2 Land Patterns

Land patterns for mounting chip

components should be designed for high assembly yield in

order to maintain product performance and high relia- bil-

ity of the finished product. Various land shapes are used to

solder attach chip components, the most common being

rectangular, circular or diamond shaped lands. Some vari-

ants such as ovals, semi- circles or rectangles with rounded

comers are also used. Figure 7-7 shows some typical land

configura- tions. (See PC-SM-782 for more details.)

Recommended

Alternative

IPC-1-00191

Figure 7-7 Chip Component and Lands

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

January

1996

IPC-CM-770

Printed Wiring Board

(Pm)

lands for the solder assembly

of rectangular chip components should be approximately

the same width as the component terminations, Lands that

are significantly narrower than the component do not pro-

vide adequate space for the formation of good fillets and

lands that are much wider than the part waste space. Lands

that are either too narrow or too wide can allow too much

lateral motion of the part.

If the components are attached to the substrate with adhe-

sive before soldering the size and shape of the land is less

critical than when the components are held in place with

solder paste and reflowed. Very wide or very narrow lands

are undesirable for solder paste reflow assembly because

they allow excessive motion of the component during the

reflow process. Lands for cylindrical (MELF) components

should have a width approximately equal to the diameter of

the component. Adhesive bonding of cylindrical parts to

the substrate is recommended for both wave and reflow

soldering.

The length of the bonding land for rectangular chips must

be adequate to provide a satisfactory fillet at the allowable

extremes of the tolerances on board, placement and com-

ponent dimensions. For components that are bonded to the

substrate for wave soldering, the primary limitations on the

length of the land are solder process restrictions. Either the

length of the land must be restricted or other measures

must be taken to prevent excessive solder build-up on the

end terminations of the components. Excess solder on the

end terminations can damage the components.

When parts are held to the lands with solder paste and then

reflow soldered, the lands must also be restricted to limit

the motion of the part during reflow soldering to prevent

“tombstoning” (a phenomenon where by chip components

fail to make a joint on one land and stand on end on the

other).

The spacing between the inner edges of the lands should be

approximately the distance between the inner edges of the

terminations on the components. If the distance between

lands is greater than the distance between the component

termination, the solder contact can be inadequate and if the

spacing between is significantly less the parts can move

excessively during soldering.

The outside comers of lands on wave soldered assemblies

are commonly rounded or cut

off

to reduce the incidence of

solder bridges to adjacent board features. Cutting or round-

ing the comers on the inside of the land pattern does not

reduce bridging and can impair the solder fillet formation

for parts that are displaced toward the land edge and should

not be done.

7.3.3 Mounted Component Configurations

When

design criteria permits, chip components to be wave sol-

dered should be oriented such that the longitudinal axis of

the chip components (a line passing from the center of one

termination to the other) is perpendicular to the direction of

motion through the wave

so

that the two terminations meet

the wave at the same time. Chip components that pass lon-

gitudinally through the wave can generate “no-solders’’ or

insufficient solder joints on the following termination.

Adequate space must be left between adjacent components

and between components and other board features to avoid

shorting or bridging during soldering.

7.4 Mixed Technology

Chip components are commonly

assembled to boards where both surface mount and through

board mount components are used. Frequently the chip

components are attached to the “solder side” of the board

with adhesives and soldered to the lands at the same time

as the through board components by passing through a sol-

der wave. Chip components that are to be wave soldered

must be qualified for immersion in molten solder.

Alternatively, the chip components can be attached on the

“solder destination side” of the board with solder paste

(and sometimes with adhesive to guarantee mechanical

attachment) and reflow soldered to the mounting lands.

After the reflow soldering of the surface mount compo-

nents the through board mounted components are wave

soldered. The wave soldering process should not melt the

surface mount joints on the top of the board. (See Section

25

for more information on mixed assembly technology.)

7.5 Manual Assembly

Because of the small size of most

chip components, manual assembly is limited to low vol-

ume production or to designs where only a small number

of chip components are to be mounted on each board. Chip

components can be handled with tweezers or vacuum pick-

ups and can be manually soldered with a temperature con-

trolled soldering iron. Manually placed components can

also be reflow soldered if the manual placement is properly

located in the assembly sequence. Generally, however,

manual assembly of chip components to boards is done

after all other assembly sequences but before cleaning,

inspection and test. Manual assembly techniques are also

used in touchup and repair operations.

7.6 Automated Assembly

Chip components are well

adapted to automated assembly lines. Fully automated

assembly lines for all surface mount products depend

heavily on chip component placement for their high

throughput. Most chip components are supplied in tape-on

reel packaging to interface with automatic placement

equipment. The automated placement machines frequently

place adhesive dots and then place the chip component on

the adhesive for wave soldering processes.

When reflow soldering is used, solder paste is screen

printed before the parts are placed and the paste is used to

hold the chip components in place until the solder is

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

reflowed. Automated board handling facilitates precise and

rapid movement of assemblies through the process.

7.7

Handling and Storage

The handling and storage

of

chip components should be in accordance with the guide-

lines of Section

26.

7.8

Soldering

General soldering techniques applying to

all types of components are described in Section

27.

7.9

Cleaning

General techniques for cleaning applying

to all types of components are described in Section

28.

7.10

Conformal Coating

General techniques for confor-

mal coating for all types of assemblies are described in

Section

29.

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COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services