IPC-CM-770D-1996 - 第105页

IPC-CM-770

Januaty

1996

The manufacturing engineer usually establishes the flow

sequence for a particular line

so

that it can be used as a

planning aid, an equipment requirement guide, a manpower

requirement guide, and a list of the necessary process steps.

However, it is important that comparative process options

such as shown in Table

20-1

be available at the design

stage if the full benefits of each process sequency are to be

realized. For example, designers should be aware that the

assembly process for a double-sided or a mixed technology

PWB

will be more complex than for a single-sided

PWB.

Such a comparison also allows equipment cost and space

comparisons, and pinpoints necessary material changes.

Other decisions such as the choice of individual machine or

equipment processes may be based on component tempera-

ture sensitivity, cleanliness requirements, component mark-

ing stability, and component bond robustness. Ultimately,

all considerations will be combined in selecting an assem-

bly process.

20.4 Materials

20.4.1 Solders

Solders are generally metal alloys with

melting points in the range -150°C to -400°C. Below this

temperature range, alloys are commonly called fusible

alloys: above this they are called brazes. Tin-lead alloys are

most common, although more complex compositions have

been developed for special applications.

For electrical soldering, alloys near the eutectic composi-

tion (63% tin/37% lead) have the required combination of

properties. Although compositions either side of the eutec-

tic have higher liquidus (completely melted) temperatures

desirable for higher ambient temperature applications,

remember that the initial melting point (at which solder

softens) is 183"C, the same for all tin-lead alloys with

compositions between

20%

and

98%

tin. There is no ben-

efit of service temperature unless the tin content is less than

20%

or greater than

98%.

The range of properties of tin-lead alloys can be varied by

adding other metals such as bismuth or indium to lower the

melting point, or antimony, silver, etc. to increase hardness

and fatigue resistance. Alloys containing less than

10%

tin

are used for applications involving temperatures below

-40°C. As mentioned earlier, the choice of the soldering

flux depends primarily on the solderability of the base

material.

20.4.2 Part Type Description

Sections 4 through

19

pro-

vide guidelines for the specific component dealt with in

each section. The parts are described in detail and each

section outlines specifics affecting the part class. Descrip-

tions and classifications provided are those generally used

in the industry with reference to military and commercial

adaptations.

20.4.2.1 Lead Forming

When component leads require

forming, the leads should be formed with a bending tool.

The component leads should be formed to the final con-

figuration before assembly or installation (except for the

final crimp, where required). When being bent, welded

leads should be firmly held by a suitable tool on the side

of the weld away from the component body. Lead forming

should not damage the lead due to nicking. Energy from

the bending action detrimental to the part should not be

transmitted into the component body.

Lead forming tooling and forming tolerances have signifi-

cant impacts on maintaining functional quality of compo-

nents. Considerations must be given to lead material and

hardness when designing tooling. Component body materi-

als (glass, elastomers, metal case, plastic) react differently

to forming strains. Stresses from gripping and close bend-

ing may damage protective cases.

Nicks and cuts and section reduction to leads during grip-

ping and forming can provide failure sites and change elec-

trical characteristics. Care should be exercised in tool

design and materials. Following specific guidelines on

closeness of bend and minimum bend radii is necessary.

20.4.2.2 Component Securing

Forming of leads for

through-hole mounting components serves many purposes.

These include the desire to retain the component in the

substrate during subsequent handling prior to soldering or

provide a standoff. Such bends or loops are provided by

tooling with some forming machines and/or can be intro-

duced by hand forming. Care must be exercised such that

stresses are not introduced to the component, leads or sol-

der joint during solidification.

The shock and vibration to which printed board compo-

nents are subjected during normal handling, environmental

testing and use can damage the lead terminations and lead-

to-component body seals. For this reason, many compo-

nents should be mechanically secured to the mounting

base.

The more commonly used component securing methods

are: clips, clamps, and brackets; wire and elastic straps;

adhesives; and integral mounting provisions.

Most circuit malfunctions in a severe vibration environ-

ment are caused by cracked solder joints, cracked seals, or

broken electrical lead wires. These failures are usually due

to dynamic stresses that develop because of relative motion

between the electronic components and the board. This

relative motion is generally most severe during resonant

conditions.

Since shock and vibration vary widely with the specific

application, it is not possible to provide solutions to all

component mounting problems. The purpose is to suggest

some general guidelines which if observed will provide

reasonable assurance that the components and assemblies

will survive shock and vibration within their intended use.

5-2

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

Table 20-1 Through Hole and Surface Mount Assembly Process

Flow

Comparison

~~

Surface Mount

Single Sided

A

Surface

Mount

Single Sided B

Surface Mount

through Board

Mi

Surface Mount

Double Sided

PRECLEAN

Through

Board

n

PRECLEAN

W

PRECLEAN

u

PRECLEAN

APPLY SOLDER

I

CREAM

APPLY SOLDER

U

APPLY SOLDER

CREAM

APPLY COMP.

ADHESIVE

PICWPLACE

COMPONENTS

SIDE

1

ADHESIVE

SIDE

1

PlC"WUPTLOACE

I

COMPONENTS

I

PlC%$CE

I

COMPONENTS

AUTO INSERT

COMPONENTS

MANUAL INSERT

I

MANUAL

COMPONENTS

PICWPLACE

I

MANUAL

COMPONENTS

PICWPLACE

[Tl

CURE SOLDER

I

CRYM

I

CURE SOLDER

CREAM

SOLDERCREAM

CLEAN

U

FLOW MELT

SOLDER CREAM SOLDERCREAM

FLOW MELT

I

AUTO PICK

6

SIDE

2

AUTO INSERT

COMPONENTS

ANUAL INSERT

COMPONENTS

PUCL1

MANUAL PICK

6

I

PREHEAT THEN

MACHINE

SOLDER

I

PREHEAT THEN

MACHINE

SOLDER

I

FLOW MELT

SOLDER CREAM

CLEAN

MACHINE

SOLDER

I

CLEAN

I

REPAIR

.

RETEST

REPAIR

RETEST

REPAIR

RETEST

REPAIR

RETEST

I

RETEST

5-3

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

The extent to which the user wishes to implement these

guidelines may ultimately be validated by actual tests of

the assembled printed board in its intended shock and

vibration environment.

The ultimate ability of components to survive in shock and

vibration environments will depend upon the degree of

consideration given to the following factors:

The worst case levels of shock and vibration environment

for the entire structure in which the printed board assem-

bly resides and the ultimate level of this environment that

is actually transmitted to the components mounted on the

board. Particular attention should be given to equipment

which will be subjected to random vibration.

The method of mounting the board in the equipment to

reduce the effects of this environment, specifically the

number of board mounting supports and their interval and

complexity.

The attention given to the mechanical design of the

board; specifically its size, shape, type of material, mate-

rial thickness and degree of resistance to bowing and

flexing that the design provides.

The shape, mass, and location of the components

mounted on the board.

The component lead wire strain relief design as provided

by its package, lead spacing, lead bending, or a combina-

tion of these plus the addition of restraining devices.

The attention paid to workmanship during board assem-

bly

so

as to insure that component leads are properly

bent, not nicked and that components are installed in a

manner which minimizes component movement.

20.4.2.3 Clips, Clamps and Brackets

The following are

the basic requirements which should be adhered to when

components are mechanically secured by clips (Figure

20-l),

clamps or brackets:

All clips, clamps or brackets should be secured to prevent

their rotation, such as by using two fasteners or one fas-

tener and a non-turn device.

Clamps and brackets which require their removal in order

to replace the component should be secured with a

threaded fastener or other non-permanent fastener, unless

the subassembly in which they are used is considered to

be disposable or nonrepairable.

Spring type clips which need not be removed during com-

ponent replacement may be secured with permanent type

fasteners such as rivets or eyelets.

The use of twist type lugs, tabs, or ears, and clipping of

glass envelope components, should be avoided.

20.4.2.4 Strapping

When using wires and elastic straps

for mechanical securing, the strap is wrapped over the

component body and passed through holes in the mounting

Positive

re Clamp

to Board

IPC-I-O0032

Figure 20-1 Clip-mounted Component

base (see Figure

20-2).

When wire is used it is clinched

and soldered in the same manner as component leads to

lands. When wire is used with heat sensitive or fragile

components the part of the wire on the component should

be covered with a suitable sleeving.

Figure 20-2 Strap Securing

The elastic strap is secured by being stretched to reduce its

cross-section below that of the hole and then returned to its

larger than hole size by relieving the tension after it has

been passed through the hole. The resiliency of the strap

holds the component in place.

20.4.2.5 Adhesives

The advantages of using adhesives

are as follows:

A. Selective Deposition Not Required

Selective applica-

tion is not required and the entire circuit can be covered.

Conductivity only occurs where component leads make

contact during the thermal stage.

B. Extreme Simplicity

This is basically a one-step pro-

cess. The bonding film can be handled by the placement

equipment in a manner analogous to hot stamp printing.

This means that placement concurrent with heating is the

only step.

C. No Bridging

No

bridging or other shorting phenom-

enon occur.

D. No Defluxing.

5-4

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services

COPYRIGHT Association Connecting Electronics Industries

Licensed by Information Handling Services