IPC-CM-770D-1996.pdf - 第142页

IPC-CM-770 Januaty 1996 Use conduclive or anti. bags and contamers n Usa wrist straps and I z Avord lead handimg // / Pick up static sensitive devices only by the body. Keep Discharge personal static before handling devi…

January

1996

IPC-CM-770

OETAIL

"A"

#14

gauge, black insulated

Gound

Wire

Terminal

Candull

808

Block

See

Oatall

"A"

Mat

firth

Ground

gauge, black insulat

nccrconnccling

Wire

rom

TOP

Mal

FLOOR

MAT

(or

conductive

floo

IPC-1-00299

Figure 26-4 Installation Guidelines for Grounded Work Benches and Floor Mats

provide automatic work-in- progress (WIP) status to a cen-

tral computer. Protection of ESD sensitive components

should be provided by:

Static free conveyer belts.

Grounding of machinery.

Personnel must be grounded at all times when they touch

devicedassemblies.

26.4.2 Automatic Assembly Machines

Handler capa-

bilities and features vary according to the type of unit and

device being accommodated. Automatic units provide

throughput rates on the order of

6,000,

and greater, compo-

nents per hour. Tube adapters are designed to take most

tubes currently in use. Automatic assembly machines range

from very specific machines which repeat the same task

over and over again, to the machines which can be pro-

grammed to do several similar processes.

Consideration must be given to complexity, volume of

units per run and the product mix to be assembled. Equip-

ment should be selected for high flexibility enabling it to

handle a large product mix and short production runs, or

choose highspeed dedicated equipment which does not

allow for much product mix.

There are three basic types of equipment available.

26.4.2.1 lnline Machines

Moderate volume machines

which utilize multiple placement heads. The machine can

have each head placing a single component or a program-

mable head which can place a variety of components up to

approximately

30,000

components per hour.

6-5

Licensed by Information Handling Services

IPC-CM-770

Januaty

1996

Use

conduclive or anti.

bags and contamers

Usa

wrist

straps

and

Avord lead handimg

Pick

static

sensitive

devices

only by the

body.

Keep

Discharge personal static before

handling devices.

í2<

handling

work

stauon cmap

Avoid all plastic, vinyl and Styrofoam in the

work

area, except

anti-static

conductwe plastics.

Assemate

only

at gmundecl

work

s~atron

AVOIO

Charge generalron

not

subject semconductors

shdlng movements over

any surface at any time.

IPC-I-O0301

Figure

26-5

Proper Handling Procedures

6-6

Licensed by Information Handling Services

January

1996

IPC-CM-770

26.4.2.2 Sequential Machines

Provides for flexibility of

programming placement of components-2,000 to 12,000

per hour. This level of automation allows for some compo-

nent testing prior to the placement the board.

26.4.2.3 Mass Placement Machines

Very high volume,

fairly dedicated machine used in the matrix of stocks or

multiplicity of tapes, which assembles a total circuit with

one or more codes of the machine, up to 200,000 compo-

nents per hour.

27.0 SOLDER-RELATED CONSIDERATIONS

This section details the printed wiring board, component,

surface finish, solderability, materials and various solder

process methodologies which can affect a manufacturer’s

soldering process yields.

27.1 General Considerations

The component mounting

technique used, the mounting sequence followed, and the

resultant assembly configuration will impact the soldering

techniques which can be applied for interconnection of

components and printed boards as well as the resultant

quality of the solder joints.

Excessively large clearances can result in draining of the

solder. Lead extension through the hole must be adequate

to ensure opportunity for a good solder joint and subse-

quent inspection but not

long as to cause interference

with tooling for subsequent processes or shorting to adja-

cent runs or assemblies.

Component and board considerations, independent of type,

that impact soldering should be considered.

27.1.1 Part Type

Mass soldering of assemblies is usually

done with either a solder wave or a reflow process with hot

air, radiation (infrared), condensation heat transfer or con-

ductive plate. Many leaded devices such as chip carriers

are currently not considered appropriate for wave soldering

and must be soldered with the reflow process. These

devices may appear on either the “solder source” or the

“solder destination” side of the board. All surface mounted

components mounted on the “solder destination” side must

be reflow soldered. Surface mounted devices with fewer

leads such as resistors, capacitors and small outline

(SO)

devices can be assembled with solder waves but the orien-

tation of the parts becomes important. Passive chips, SOTS,

SOICs and other components that can tolerate immersion

in the molten solder of a wave soldering machine may be

mounted on the “solder source” side of printed wiring

assemblies.

Through-hole mounted parts have the potential of resultant

lower quality solder joints if insulation material, potting

compound or other material is allowed to protrude into the

hole. Another characteristic of these devices is the ten-

dency to “raise” during the fluxinglwave soldering process

if not clinched or mechanically retained. For wave solder-

ing the through hole mounted devices should be mounted

on the “solder destination” side of the board. These

devices may not be compatible with reflow processes. The

component mass is another factor to be reviewed. Heavy

mass components require longer soldering times due to

their heatsinking characteristics.

27.1.2 LeadlLand Relationships

The leadless and

leaded termination provide different solder joint geometries

and the stress distribution is different in each case. If leads

are too short to protrude through the printed wiring board

or if the surface mount lands are too small, soldering may

become difficult. Printed board hole diameters must be con-

sidered for the type product to be used.

Costs and problems can be reduced if the designer selects

his devices prior to printed board design and then designs

the proper hole or land size, etc. “Non-standard’’ holes

and land patterns increase costs by making “non-

standard” devices mandatory.

Proper spacings can greatly increase automatic, semi-

automatic and manual speed of component placement.

Features should be, if possible, in straight line patterns

rather than random and they should be placed in such a

fashion that it will not be necessary to readjust the board

to the product.

Tooling holes should be placed as far apart as possible.

The designer must be certain no pre-mounted components

will interfere with the proper machine installation of

those devices that will subsequently be installed.

Printed board holes may be “non” plated or “through”

plated, drilled or punched, dependent upon the device

which will be eventually used and the quality required in

the final printed board. As a general rule, a drilled hole

will be more consistent in size and is advised where a

hole will eventually be plated through. Punched holes in

multilayer printed boards are not recommended since the

internal conductors may be damaged.

27.1.2.1 Leadless Component Terminations

This

geometry provides no compliancy, it results in a very rigid,

small lap solder joint depending on the reflowed solder

material system providing desired mechanical properties.

Visual inspection of the joint is limited to fillet appearance

on any castellation and pad extension. Cleaning is more

difficult with this geometry than with the leaded

termination.

27.1.2.2 Leaded Component Termination

This geom-

etry results in a narrow solder fillet. It provides compliancy

which can compensate for some degree of mismatch in

expansion between the component package and the sub-

strate. Visual inspection of solder joints is somewhat easier

6-7

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