IPC-CM-770D-1996.pdf - 第115页

IPC-CM-770

Januaty

1996

Surface mount components mounted to the bottom side of

the board may also interfere with the lead cutting

operation.

21.5.2 Clinched Leads

Clinching of leads prior to sol-

dering is commonplace, either as part of machine insertion

or following hand insertion. The substrate land configura-

tion and spacing to adjacent lands must be considered.

Clinching in line with traces is good practice and trimming

of leads before clinching is recommended where clinch

direction may cause shorting to adjacent lands. It is gener-

ally not felt to be necessary to clinch all leads of a multi-

leaded device unless required by the customer and equip-

ment class. The lead is passed through-the-board and is

clinched to make contact with the land or conductor and is

then soldered. The lead or terminal should make contact

with the conductor pattern before soldering. Leads should

not extend beyond the edge of their lands, however,

if

overlap does occur, the lead should never violate electrical

spacing requirements. The lead termination hole may be

supported by eyelets or plated-through- holes or it may be

unsupported.

The lead should be cut and clinched prior to the soldering

operation. The maximum height of lead spring back should

not exceed the maximum lead diameter plus 0.8 mm above

the plated surface or eyelet. See Figure 21-7.

As its name implies, the component leads for this method

are clinched to the printed board land after they have

passed through the lead hole.

1

IPC-1-00329

Figure 21-7 Clinched Leads

In addition to the considerations common to all straight-

through mounting methods, this method has the following

advantages:

A reinforced mounting hole is not required; tear drop and

offset lands can be used.

This method does afford some resistance to movement

during soldering.

This method has some of the disadvantages mentioned for

unclinched straight-through mounting, in addition:

Care must be taken when cutting the lead to length and

forming the clinch to assure that minimum conductor

clearances are provided when the clinched lead overhangs

the land.

The lead clinching operation, if not controlled properly,

can unduly stress the component lead-to-can body seal.

21 5.2.1 Preformed Leads

A slight modification of the

mounting method just described is the offset multiple lead

can mounting method with clinched leads. As shown in

Figure 21-8 this method adds the feature of having the

leads clinched to the conductor land after they pass through

their mounting holes.

m

IPC-1-00330

Figure 21-8 Offset Clinched Lead

21.5.2.2 Semi-Clinched Leads

(The lead is passed

through a hole in the board, cut to length, and soldered.)

When straight-through leads are used inconjunction with

unsupported holes, the leads should extend from

0.5

mm

minimum to

1.5

mm maximum from the surface of the foil.

When straight-through leads are used in conjunction with

plated- through holes or eyelets, the lead should extend at

least to the surface of the plating or rim of the eyelet and

no more than 2.3 mm from the plating surface eyelet.

Semi-clinched leads should be considered as straight-

through leads providing the degree of clinch meets the

requirements shown in Figure 21-9.

n

a!

ID

IPC-1-00328

Figure 21-9 Straight Through Leads (Semi-clinched

Leads)

21.6 Assembly

Some basic principles for assembly,

especially automated are:

Design to the minimum number of different components,

and purchase these to industry or prescribed standards for

automatic insertion.

Arrange all components on

X

and, if necessary,

Y

axes.

Also arrange components in columns andor rows, if pos-

sible.

5-12

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

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

5-13

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

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

5-14

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services