IPC-CM-770D-1996 - 第144页

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

so

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

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IPC-CM-770

Januaty

1996

with this geometry. Cleaning operations are aided by this

geometry since it provides clearance between the bottom

of

the component and the substrate. Excess solder fillets on

this geometry can stiffen the lead and reduce any compli-

ancy advantage.

27.1.2.3 LeadlTermination Finishes

Finishes on the

leads of leaded component packages and on the metallized

terminations of leadless packages, whether for through-

hole or surface mounting, preserve and assure the solder-

ability of the interconnection surfaces without decreasing

the assembly yield or the attachment reliability. The large

number of solder joints, which frequently cannot be visu-

ally inspected not only because of their sheer numbers but

also because they are physically inaccessible, makes it

imperative that the solderability of the component I/Os be

positively assured prior to assembly. The lead finish that

accomplishes this is fused or reflowed tin or solder. The

solder coating can be obtained either by solder dipping or

tidlead plating with a subsequent reflow process. It might

be necessary to remove excess solder, which could inter-

fere with the assembly or socketing process, with a hot air

knife or similar method.

The above conditions expressly exclude gold plating, since

gold forms brittle intermetallics with the tin in the solder

causing solder joint reliability problems, and silver plating

since silver forms a brittle silver intermetallics. Caution

should be used when leads are coated with tin or tidead

plating without reflowing prior to assembly because the

plating does not always assure solderability. Organics

which are sometimes Co-deposited during the plating pro-

cess can also cause poor solderability.

In addition, some user specifications require fusing of tin

plate to eliminate the possibility of tin whisker growth.

The foregoing considerations are more critical for surface

mounting than for through-hole mounting. Through-hole

mounting provides considerable margin because of the

mechanical anchoring of the leads and the larger solder

volumes.

For devices which are exclusively destined for non-

soldered interconnections (sockets, connectors), the surface

finish coatings depend on the design of the mating contact,

the number of insertion/withdrawal cycles and the service

environment.

It should be noted that the reliability of a tin/lead applica-

tion can be increased by lubricating the contact surfaces

with proper contact lubricants. Lubricants prevent fret and

corrosion that may occur on tidead due to changing envi-

ronmental conditions. A suggested source for additional

information of recommended finishes is IPC-GH-850

Handbook of Interconnection Contact Finishes.

27.1.3 Component Placement

The orientation of com-

ponents on the assembly can have adverse effects on the

final solder joint quality. See the placement descriptions for

each of the component types in Sections

1

through

8.

27.1.4 Assembly Sequence.

See the assembly sequence

information in Sections

19

through 25.

27.2 Solderability

A successful solder joint requires cer-

tain conditions. Primarily, the surface to be joined must be

solderable; this is known as solderability.

Good solderability can improve production rates, increase

reliability, lower costs and improve joint appearance. Pro-

duction rates increase significantly if board solder joints do

not have to be reworked or touched up. Furthermore

manual touch up can damage printed boards. Therefore,

production increases as a result of reduced printed circuits

board scrappage due to touchup operations. Reliability

increases and costs drop for this same reason. With regard

to cost, obtaining better solderability definitely increases

cost; however, in most cases this added cost is less than the

cost of bad joints and will therefore actually save money.

The solderability of both component leads and printed

boards must be high to obtain an easily and well made

joint. Degradation of either part will impede the formation

of good solder joints. The solderability of printed wiring

boards should meet the requirements of J-STD-O03 and the

components should meet the requirements of J-STD-002.

The emphasis in the control of solderability has been

placed on component leads rather than on printed boards,

because of the following characteristics of components:

Longer storage times

More rigorous processing during manufacture

Greater variety of base materials

Lower cost compared to cost of reworking the PWB

Considerable work on solderability control has been

applied to printed wiring boards. This includes both testing

for solderability and improving the surfaces to be soldered.

Specific information on solderability can be found in:

J-STD-001, “Requirements for Soldered Electrical and

Electronic Assemblies” J-STD-002, “Solderability Tests

for Component Leads, Terminations, Lugs, Terminals and

Wires” J-STD-003, “Soderability Tests for Printed

Boards”

27.2.1 TinninglSolder Dipping

Component leads not

meeting the designated solderability requirements may pos-

sibly be reworked by tinningholder dipping, prior to sol-

dering. All portions of the wires or leads which come in

contact with the area to be soldered should be tinned. The

solder should penetrate to the inner strands of stranded

conductors. To permit inspection for nicks or cuts at the

point of insulation termination, solder and wicking should

not conceal the individual outer wire strands.

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COPYRIGHT Association Connecting Electronics Industries

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January

1996

IPC-CM-770

All gold plated leads and wires that are hand soldered or

surface mounted should be pretinned or solder dipped to

remove the gold prior to component soldering. Care should

be exercised to not adversely affect the component ele-

ments during this treatment.

27.2.1.1 Cleaning Prior to Tinning

Component leads

can be cleaned with a mechanical cleaning tool that does

not impart damage to the component or the component

lead. Other methods of cleaning can be used, provided that

these methods do not damage the component. Knives,

emery cloth, sandpaper, steel wool and other abrasive

should not be used.

27.2.2 Substrate Preparation

Printed wiring boards

should be tested for solderability before assembly.

If

sol-

derability is not acceptable, then pretreatment prior to

assembly may be required to enhance solderability and

subsequent quality of the soldered assembly. Solder mask-

ing may or may not be present dependent on specifications.

Typical pretreatments are degreasing (surface contamina-

tion, e.g., dirt, oils), brightening (chemical activation of

solder and/or copper), and baking (moisture). Baking of the

substrate may be necessary in order to prevent delamina-

tion of the substrate.

27.3 Materials

All soldering processes are only capable

of achieving optimum process yields if the materials used

in those processes are not substandard. Regardless of the

particular soldering process used (dip, wave, reflow), all

solder processes follow the same basic steps: flux, preheat,

and soldering. Some newer technologies use other materi-

als such as adhesives either in conjunction with or to

replace soldering. Adhesive attachment of components is

particularly attractive with temperature sensitive devices,

and for securing surface mount devices. The various types

of alternative solder materials will be discussed here.

27.3.1 Flux

Flux must have properties such that it

(1)

chemically removes the surface oxide or tarnish and

(2)

keeps the surface clean until the solder has melted and

flowed over the fluxed surface.

Soldering fluxes have been divided into three general cat-

egories. The traditional flux specifications classify fluxes

on the basis of their chemical make-up or flux base (rosin

base fluxes, for example, are classified as R, RMA or

RA).

J-STD-004 utilizes a unified approach to flux classification

based on fundamental, intrinsic corrosive and conductive

properties of flux and flux residues, rather than specifying

the flux base.

Flux are specified according to one of the following three

types per J-STD-004:

L

=

Low or no flux/flux residue activity

M= Moderate flux/flux residue activity

H= High flux/flux residue activity

Inorganic fluxes are not permitted for electronics soldering.

The flux and the cleaning process (or lack thereof) are

directly interdependent.

27.3.1.1

Many common fluxes use natural rosin as a

base. This natural product, derived from the gum of pine

trees, is a mix of abietic acid and numerous dehydrogena-

tion products.

Rosin is a glassy, non-crystalline mixture of organic acids

which are inert up to their softening point and only assume

an acidic nature when molten. After melting and resolidifi-

cation the hard glassy properties render the residue once

again inert. Residues from other added chemical activator

compounds usually become encapsulated in the rosin resi-

due, which renders them non- corrosive. The effectiveness

of this safeguard, however, depends on the quantity and

nature of the activator used.

The acidity of pure rosin alone is usually insufficient to

clean surfaces to be soldered,

so

rosin fluxes are usually

enhanced by a variety of chemicals called activators. Com-

mon activators are inorganic halides, organic halides, car-

boxylic acids, amines and halogenated amines. The degree

of activation achieved by the various chemicals depends

upon the compound used and the quantity. Since the over-

all activity developed is often a synergistic product of more

than one activator, activation is usually quantified not by

formulation alone but rather by some secondary property

such as the ability of the residue extract to dissolve a cop-

per from the “Copper Mirror Test”-or by its ionic con-

ductivity. The level of activator affects the rate of wetting.

27.3.1.2 Organic Acid (Water Washable) Fluxes

These

fluxes are significantly more active and aggressive than

rosin fluxes in removing oxides from the surfaces to be

soldered. They use strong organic acids and salts to achieve

these properties. As such they are more forgiving of poor

solderability characteristics of the surfaces to be soldered.

The increased activity yields flux residues which are more

corrosive than the residues of rosin fluxes. Because of their

characteristics it is necessary to completely remove the

residue with a post soldering cleaning operation to prevent

early failure of the soldered assembly.

27.3.1.3 No Clean Fluxes

(Low

ResiduelNo Residue).

This family of fluxes includes both rosin or modified resin

fluxes and non- rosin fluxes. The activators used are gener-

ally weak organic dicarboxcylic acids.

The low solids rosidresin based fluxes leave small

amounts of residue after soldering and except where these

may interfere with bed of nails testing or other post solder-

ing operations or operating characteristics generally have

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Licensed by Information Handling Services