IPC-D-279 EN.pdf - 第33页

uniform in thickness. Plating may be followed by a reﬂow performed in hot oil. Dipped or reﬂowed solder ﬁnishes are thinner at the corners of the terminations. Dipped solder ﬁnish processes are sometimes followed by hot …

5.3.5 Plastic Surface Mount Components (PSMC)

PSMCs, including semiconductors and resistor/capacitor

networks, should have measured data on maximum body

temperature (Tmax) for the process ﬂow, including rework/

repair. PSMCs should be obtained from suppliers who have

control of moulding voids, internal and external package

cracks at the bonding ﬁngers, and qualiﬁcation or charac-

terization of the PSMCs to the peak process temperature

while meeting the requirements of IPC-TM-650, Method

2.6.20 (see Appendix F).

Surface mounted plastic ICs absorb moisture from atmo-

sphere humidity by diffusion. During assembly reﬂow pro-

cesses, rapid moisture expansion and material mismatches

can result in cracking and/or delamination of critical inter-

faces within the packages. The classiﬁcation levels deﬁned

in IPC-SM-786, Standard for Handling and Shipping of

Moisture/Reﬂow Sensitive ICs, are intended to be used by

IC producers and IC users (board assembly operations).

The levels of moisture sensitivity of product devised

should be used to avoid package cracking and delamina-

tion.

Susceptible components should be required to be dried

(baked) and bagged by the supplier and subject to appro-

priate documented internal or subcontractor treatment dur-

ing production and rework/repair; these treatments limit

additional internal delamination and cracking, but do not

reverse existing delamination and cracking.

See Appendix F and IPC-SM-786 for more details on

PSMC performance under SM solder reﬂow conditions.

See Appendix E for additional detail on rework and repair

effects on PSMCs.

5.3.6 Component Termination Coplanarity and Configu-

ration

A critical issue in SMDs is lead coplanarity which

is deﬁned as lying or acting in the same plane. Non-

coplanarity is the distance between the lowest and the high-

est leads when the package rests on a perfectly ﬂat surface.

The common problem caused by non-coplanarity is the

phenomenon known as solder wicking (the solder paste

wicks up the lead, causing open solder joints). It should be

kept in mind that poor land or lead solderability or uneven

or fast heat application during the reﬂow process also can

cause open solder joints. For all these reasons, the lead

ends should lie perfectly in the same plane to avoid manu-

facturing problems.

5.3.7 Component Lead Configuration Each lead con-

ﬁguration has its advantages and disadvantages. Butt-lead

or I-lead devices are not commonly used. The advantage

claimed for butt-leads is the possibility of converting

through-hole components into SM by clipping the leads

and accomplishing the soldering of all components in one

reﬂow operation. Butt-lead joints have normally 65% less

pull and shear strength than joints to J or gull-wing termi-

nations and are more sensitive to process-related handling,

placement and reﬂow soldering, and are less compliant

than J-leads or gull-wing joints.

5.4 Component Termination Finishes Ceramic and fer-

rite components such as multilayer ceramic capacitors, chip

resistors, and chip inductors are generally terminated with

a ﬁred-on silver or silver palladium paste. Because the sil-

ver content is easily and rapidly dissolved or leached into

molten solder, the soldering process window is tightly con-

strained as to temperature range and peak temperature

duration. A high silver content in the solder joint results in

loss of ductility. A termination with high loss of silver is

weakly adherent to the ceramic (see 5.4.1).

5.4.1 Nickel Barrier Layer An overplating of nickel

(which dissolves at 1/10th the rate of silver) is recom-

mended as a barrier layer between the silver bearing paste

and molten solder. Nickel oxide and nickel corrosion com-

pounds are resistive and difficult to solder; nickel oxidizes

or passivates rapidly. To prevent oxidation and corrosion of

the nickel and hence to preserve the solderability of the

termination, an easily soldered ﬁnal metallic termination

ﬁnish such as tin, tin-lead or gold is used; palladium is

used on some compliant leads.

5.4.2 Tin and Tin-Lead Solder Termination Finishes

The most common SM component termination ﬁnishes

include tin and tin-lead. Tin-lead ﬁnishes are also called

solder ﬁnishes. Because copper diffuses into the tin-lead

and forms copper-tin intermetallic compounds (Cu

Sn or

) at elevated temperatures, an ideal metallurgical

system for copper terminations includes a barrier layer of

nickel. The formation of the intermetallic compounds

results in a lead rich layer on the termination. The oxides

and chlorides of lead, copper-tin IMCs, Cu

Sn, and Cu

and the chlorides of tin often are difficult to solder.

5.4.2.1 Tin-Only Finishes Tin-only ﬁnishes may exhibit

tin ‘‘whiskers’’ as a result of stresses in the tin or nickel

under-layers; the whiskers may short closely spaced con-

ductors. Tin-only ﬁnishes may also exhibit stress induced

in tin ‘‘pest,’’ an allotropic transformation of the white tin

to a friable grey tin at low temperatures (~−40°C) alloys

with lead, bismuth, or antimony are less susceptible. Lead

content > 3% appears to suppress both whiskers and pest;

alloying of the tin with the lead in tin-lead solder pastes or

molten wave solder appears to be sufficient to prevent the

emergence of either problem according to extensive stress

tests conducted on tin-terminated multilayer ceramic

capacitors.

Pure tin plating is not recommended.

5.4.2.2 Tin-Lead Finishes Tin-lead ﬁnishes can be

applied by electroplating or by dipping of the termination

into molten solder. Plated (but non-reﬂowed) ﬁnishes are

July 1996 IPC-D-279

uniform in thickness. Plating may be followed by a reﬂow

performed in hot oil. Dipped or reﬂowed solder ﬁnishes are

thinner at the corners of the terminations. Dipped solder

ﬁnish processes are sometimes followed by hot air ‘‘kniv-

ing’’ to obtain a uniform thickness of metal on the ﬂat por-

tion of the termination; the result is the thinner metal at the

corners. Copper terminations are commonly less solderable

at the corners due to exposed intermetallics and oxidation

of the intermetallics more quickly than areas with thicker

tin-lead coating.

5.4.3 Termination Recommendations When Using Elec-

trically Conductive Adhesives

Where electrically con-

ductive adhesive is used, component termination ﬁnishes of

silver, gold or palladium are recommended, since silver

oxide is conductive and gold and palladium do not oxidize.

Tin, lead, or tin-lead terminations are not recommended,

particularly when the service environment exposes the

joints to humidity. The moisture permeates the adhesive

and oxidizes the adhesive/metal interface, increasing the

series resistance.

5.4.4 Gold, Palladium, Silver Termination Finishes

Where gold or palladium plating is used, the gold or palla-

dium content in the solder joint should be less than 3 wt%.

This level can generally be met if the gold or palladium

ﬁnish is ~0.1 µm thick on both the component termination

and the printed board. Alternatively, the component termi-

nations can be dipped in solder to remove the gold; the

solder bath must be monitored to prevent excessive gold

buildup. The presence of gold plating on the printed board

might be driven by the use of pressure contacts such as

solder-less or edge connectors.

Copper leads should be separated from gold overplating by

a pore-free barrier of nickel plating to prevent diffusion of

the copper into the gold; oxidation of diffused copper

degrades the solderability of the gold ﬁnish.

Where thin, uniform, solderable platings are needed which

can be exposed to high temperatures without loss of solder-

ability, precious metal platings are chosen. Silver, palla-

dium and gold are some of the precious metals available.

Gold is the best of these materials having excellent solder-

ability and the best shelf life (indeﬁnite). However, it is

expensive and prone to intermetallic failure. Gold requires

a barrier layer of nickel plating, ~1.5 µm thick, to prevent

diffusion of copper to the surface of the gold which

degrades solderability.

Gold plating requires a careful soldering process to avoid

formation of intermetallic phases which may embrittle the

solder joint and lead to premature failure. Surface mounted

solder joints are more susceptible to embrittlement failure

because of the limited amount of solder in the joint and the

reliance on the solder joint for mechanical strength. The

concentration of the gold in the tinning bath needs to be

monitored to prevent the gold concentration from exceed-

ing 3 wt%.

Silver requires care in handling and storage to avoid tar-

nishing which can interfere with solderability.

5.5 Solderability of Termination Finishes Solderability

describes an observed condition which results from the use

of a ﬂux together with a component termination ﬁnish or

printed board land pattern ﬁnish under the inﬂuence of a

heat source.

The period of time over which tin-lead ﬁnishes demon-

strate acceptable solderability depends upon the plating

thickness, plating conditions and storage conditions; sol-

derability decreases with increases in such factors as tem-

perature, humidity and oxygen content. Benign atmo-

spheres and proper storage conditions may result in a shelf-

life on the order of years. Some plated systems are

unsolderable after several months under factory ﬂoor con-

ditions. Techniques for evaluating and quantifying soldera-

bility are found in J-STD-002, Solderability Tests for Com-

ponent Leads, Terminations, Lugs, Terminals and Wires.

Organic contamination such as oil, grease, and ﬁnger prints

or particulate contamination such as dust can also degrade

solderability. The mild no-clean or ‘‘leave-on’’ ﬂuxes may

not be effective in the presence of these contaminants.

Co-deposited organic materials (electroplating brighteners

and levelers) will shorten the solderable shelf life of the tin

or tin-lead plating.

Tarnished ﬁnishes may be difficult to solder and result from

the reaction of a silver termination ﬁnish with sulfur com-

pounds which may be emitted by chemical processes using

sulfur or storage containers made from paper containing

residual sulfur compounds. Silver-plated terminations

should be stored with an oxidation inhibitor.

Assure solderability through testing by the producer. The

wetting balance method permits measurement of the time-

for-wetting as well as the degree of wetting. This testing

provides a reproducible method of evaluating process vari-

ables with a standard sample geometry. At this time, inter-

pretation of the results varies. A cheaper, non-quantitative

test is visual examination of the solder tinned surface (‘‘dip

and look’’).

5.6 Soldering Considerations SMDs must withstand the

higher solder process temperatures and must be selected,

placed and soldered more carefully to achieve acceptable

manufacturing yield. See also Appendix D, Components

and PSMCs.

5.7 CTE Mismatch Considerations Difference in the

coefficient of thermal expansion (CTE) of the materials of

an SMD is very important to its reliability. This is because

IPC-D-279 July 1996

at one temperature, there may be no stress where two mate-

rials join but at a different temperature, if there is a differ-

ence in CTEs, the same joint may well be under such con-

siderable strain that the part fractures.

Matching the CTEs at a joint is no guarantee of freedom

from the problem because electronic components are their

own heat sources and because there is a temperature differ-

ence between component and substrate. The problem is

directly related to the size of the component, the thickness

of a solder joint and the compliance of the lead. See Sec-

tion 3.4 and Appendix A. The major problem arises from

thermally induced cyclic stress in the solder joint of the

larger leadless ceramic chip carrier (LLCCC) components.

5.8 ESD Packaging Requirements All SM pick-and-

place feeder parts, sensitive or not, if they are dispensed

adjacent to ElectroStatic Discharge Susceptible (ESDS)

components, should be packaged in antistatic materials.

5.9 Specials or Custom Devices Use Precaution

Devices with unusual or exotic characteristics, extremely

tight speciﬁcations, or low volume ‘‘custom’’ processing

are not as reliable as ‘‘standards.’’ The tightened speciﬁca-

tions result in a smaller or unknown C

. The unique pro-

cess of evaluation and selection used to meet special

requirements is more vulnerable to errors. It is very diffi-

cult to demonstrate product quality and reliability (or to

improve the process) with low volume of product.

5.10 Components to Avoid or to Use with Caution

• Printed board with T

< 125°C

• Printed board with PTH and PTVs aspect ratio > 3:1

• Components not on the preferred parts list

• Components using obsolete technologies

• Components containing liquid and sealed only with

rubber

• Components with rotating seals

• Components with thick silver or gold plating or paste

as the solderable termination

• Components with corrosive or polar liquids

• Aluminum electrolytics with silver anode (obsolete

technology)

• Components with exposed moving electrical contacts

• Electro-Mechanical connections between contact ﬁn-

ishes of tin and gold (dissimilar metals)

• Film resistors trimmed more than 50%

• Multilayer ceramic components such as capacitors,

inductors, ﬁlter networks assembled into PWAs using

assembly processes with ∆T/∆t > 4°C/second.

• Solder immersion or wave soldering of surface mount

components other than simple chip resistors and chip

capacitors.

• Components with ESD susceptibility

• Variable resistors, particularly wire-wound

• Variable capacitors

• Multilayer capacitors trimmed to value

5.11 Component Selection Considerations for Military

and Space Applications

Military and aerospace applica-

tions exposed to severe environments may require pack-

ages that are more robust. Hermetic packages may be

required to be robust to a life cycle environment which

includes high relative humidity. However, under extreme

levels of shock and vibration, hermetic packages (with ﬂy-

ing internal leads) are less robust than plastic encapsulated

packages. See Appendix P.

6.0 SOLDER MASK AND CONFORMAL COATING

CONSIDERATION

Because of the ﬁne lines and close conductor spacing and

the assembly processes utilized in SMT, solder mask and

conformal coatings may be a new design requirement. Sol-

der mask performs two functions, one to limit the ﬂow of

solder paste, the other to protect adjacent traces from cor-

rosion. Conformal coating protects the solder joints and

component conductors from corrosion. Detailed descrip-

tions of solder masks and conformal coatings may be found

in Appendix N.

6.1 Solder Mask Considerations for SM The decision to

use solder mask for surface mount technology applications

is usually based upon the need to prevent migration of sol-

der away from the device pads during solder reﬂow. An

alternative to the use of solder mask is a pads-only

approach.

Typically, solder mask openings are designed 125 µm

larger than the component land or through-hole pad. This

allows for alignment tolerances during solder mask pro-

cessing. Since solder mask or its residue on surface mount

pads will reduce the size of the solder ﬁllet, it is highly

recommended that printed boards be procured which

exclude solder mask from extending onto these areas.

The ‘‘do’s’’ and ‘‘don’ts’’ of solder mask are important ele-

ments in the construction of a reliable surface mount

assembly. The type, thickness, coverage areas and proper

application are essential parameters in this complex equa-

tion. Since surface mount design begins to encroach upon

the limits of the solder mask process, it is important for the

designer to understand solder mask process capabilities.

The solder mask must be compatible with the surface

mount processes being used, especially the heat and sol-

vent resistance characteristics. The choice of a particular

solder mask should be validated with the proposed assem-

bly processes (reﬂow, wave and rework soldering; clean-

ing, conformal coating) to ensure that there are no adverse

July 1996 IPC-D-279