IPC-D-279 EN.pdf - 第102页

F-10.2 Digital Semiconductors Caution Where possible, obtain devices with BIT capability to ease testability requirements. F-10.3 Digital Silicon Semiconductors (MOS MSI/LSI) Not speciﬁed for most CMOS components (but sh…

Caution

Shorts due to contaminants within capacitor such as thread

wear debris or gold plating shaken loose by vibration.

Opens due to internal solder connections rupturing during

assembly solder operation.

F-9.0 INDUCTOR/TRANSFORMERS

Caution

Operation at less than rated lower frequency may result in

overheating and loss of inductance.

Multilayer SM ceramic and ferrite inductors (and ﬁlters)

are susceptible to soldering process thermal shock. Limit

the termination electrodes ∆T/∆t to less than 4°C/second

and ∆T to less than 100°C from preheat to peak body tem-

perature under SM reﬂow, wave solder and hand solder

process conditions to avoid cracking of the internal layers.

Components with the largest numbers of ferrite layers are

most susceptible to thermal shock and impact stresses.

Do not subject internal connections of encapsulated com-

ponents to assembly solder process temperatures > melting

point of speciﬁc solder alloys used; skip this caution if the

connections are welded.

Opens occur in the component body due to damage from

mechanical stresses such as overload due to vibration,

shock or ﬂexure of the PWA during service life.

Opens occur in the component body and in the body-to-

termination interface due to damage from mechanical

stresses under post-soldering stresses such as twisting, ﬂex-

ing or shock and vibration during assembly operations such

as TH component insertion, depaneling or testing. Identify

the high stress areas of the printed board and avoid placing

large, susceptible components in these areas. Minimize

PWA ﬂexure by providing adequate support and vibration

damping in the ﬁxturing.

Hard pick and place tooling can cause mechanical shock

during SM pick and place; test probes landing on compo-

nent body or component terminations may also result in

mechanical shock damage during testing. These tooling

shock cracks generally occur in the middle of the compo-

nent, although test probes can cause cracking and spalling

at the impact site.

The EIA 1210 size should be an upper bound for reﬂow

attachment of leadless ceramic and ferrite components to

polymer-glass substrates; these components include lead-

less chip carriers (LCCs), multilayer ceramic capacitors

(MLCCs), chip resistors, inductors and networks. At or

above this size, estimate solder joint reliability to evaluate

the impact of package size and lack of lead compliancy on

solder joint reliability on glass-epoxy substrates. For spe-

ciﬁc guidelines, see Appendix A.

Provide conductive heat transfer paths for these devices

and locate for favorable convection cooling.

Delay lines may be constructed of purely passive compo-

nents such as capacitors and inductors or may contain addi-

tional, active gain elements for impedance transformation

and independence from loading variations. Silicone rubber

is used as a conformal coating for protecting or relieving

the wires and core in transformers and delay lines from

molding stresses. The silicone with a high CTE of 200-300

ppm/°C will expand within its plastic enclosure during

reﬂow and stress the enclosure. If there is too much potting

compound or the enclosure wall is thin or weak, the wall

will fail and allow moisture to enter the package and cor-

rosion to start. The walls of the smaller SMT parts are

thinner than their TH counterparts and the higher body

temperatures during SM reﬂow make this phenomenon

more likely to occur than with TH processing. Subsequent

intrusion of moisture and ionizable contaminants into the

component has resulted in dendritic growth and functional

failures.

F-10.0 SEMICONDUCTORS

• Metallurgically (eutectic) chip bonded devices pre-

ferred but not feasible on very large die; hermetically

sealed devices preferred for humid or corrosive atmo-

spheres but generally costly.

• Devices with long, unsupported bond wires are more

susceptible to vibration and shock.

• Brazed terminations may employ combinations of met-

als which, if exposed at edges and cracks, can result in

galvanic corrosion under humid conditions.

• Packages such as SOT-23, with a metallic leadframe

comprising a high percentage of the plane area, dem-

onstrate an effective X-Y CTE closer to that of the

leadframe material than of the encapsulating plastic.

• SM metal electrode face bonded (MELFs) conﬁgura-

tion components may require special ‘‘U’’-shaped land

patterns to reduce rolling or may require adhesive to

retain position during reﬂow.

F-10.1 Light Emitting Semiconductor Diode (LED)

Caution

Packaging must be characterized to perform under the con-

ditions of your soldering and cleaning operations; LEDs

which exceed the 105-125°C glass transition temperature

) of the plastic body when immersed in wave solder and

experience delamination of the plastic from the leadframe.

If you must wave solder these components, observe the

cautions for very moisture sensitive PSMCs. Some plastic

encapsulated LEDs are susceptible to thermo-mechanical

stress from the encapsulant at very low temperatures. LEDs

with shallow junctions or small area junctions may be sus-

ceptible to ESD. Proper current limiting or regulation must

be provided to observe average current guidelines. Peak

current ‘‘on’’ periods < time constants of the chip in pack-

age.

IPC-D-279 July 1996

F-10.2 Digital Semiconductors

Caution

Where possible, obtain devices with BIT capability to ease

testability requirements.

F-10.3 Digital Silicon Semiconductors (MOS MSI/LSI)

Not speciﬁed for most CMOS components (but should be

requested), is the allowed or preferred turn-on sequence of

power supplies, clocks, data and output loads. An improper

turn-on sequence may result in latchup and subsequent

damage to the chip.

F-10.4 Linear Semiconductors

Caution

Be aware of ESD vulnerability on high frequency/low cur-

rent devices- get evaluation data; with supply below .8X

nominal, device may be beyond recommended operation

range. Plastic cracking issues discussed in Appendix E,

particularly those associated with leakage currents, apply to

linear ICs.

F-11.0 OTHER COMPONENTS

F-11.1 Fuse

Caution

Fuse elements in SMT versions have less heat dissipating

capability (because there are no fuse clips) than otherwise

identical TH versions and may require additional tempera-

ture derating.

F-11.2 Separable Contacts (Relays, Switches, Connec-

tors, Sockets)

Caution

Condensation and contaminants on the printed board, ﬂex

circuit or wiring to the contacts can lower the insulation

resistance between the contacts; the same result is obtained

if the glass envelope of a reed capsule is contaminated. SM

relays, switches, connectors and sockets in high impedance

applications may require backloading during SM process-

ing.

Non-sealed components should be backloaded. ‘‘Sealed’’

components should be evaluated for the robustness of the

seal to SM cleaning factors such as solvents, water, high

temperature, high pressure/velocity.

Intermittent open circuits in slide switches have been found

due to washing out of contact lubricant during board clean-

ing with high pressure/velocity sprays intended to clean

under components with very small clearances.

For environments with anticipated vibration or thermo-

mechanical movements due to temperature cycling, con-

tacts must be prevented from micromotion (displacements

< 2.5 µm) during the manufacturing cycle as well as in the

service environment:

• Sealed or uncontaminated soft metal contact ﬁnishes,

such as gold, may weld under micromotion conditions

(as well as under severe shock and vibration condi-

tions such as SM panel routing or shearing).

• Volatile silicones are degraded in the contact area of

open contacts under micromotion conditions as well as

in the presence of electrical arcing; silicas and varnish

buildup result in intermittent as well as permanent

contact failure. This consideration may limit the choice

of silicones in such diverse areas as solder masking

tapes, adhesives, and resins to the non-outgassing

types.

• Open contacts with contact ﬁnishes from the active

catalyst platinum family, including gold may, in the

presence of micromotion and condensible organic

vapors, form insulating polymers; this consideration

may dictate the choice of printed board, solder mask,

ﬂux, ﬂuxing oils (under the conditions of SM process-

ing) and conformal coating (under service conditions).

With these catalytic metal contact ﬁnishes, non-out-

gassing plastics are required for switch and relay hous-

ings.

• Components of plastic materials susceptible to blister-

ing at temperatures > 200°C must be dry (baked out)

prior to SM reﬂow.

• After a soldering operation, assure that the component

housing, contacts and printed board are not in a

stressed condition; this stress-free condition is difficult

to achieve with in-line SM reﬂow processing.

• Service environment parameters such as humidity,

temperature, corrosivity must be known and accounted

for. These factors determine material choices, contact

conﬁguration and housing conﬁguration for robustness

and protection from the environment.

• Do not use the same connector or switch contact pair

for power and for ‘‘dry’’ or low power/low voltage cir-

cuit connections.

• ESD susceptibility evaluation of the assembly may be

required if a switch or connector is accessed by the

user; isolation of the circuit by plastic parts, air gaps or

grounded shields and shrouds may be required.

• Do not ‘‘repair’’ stuck (welded) reed contact capsules

by hitting them. Rhodium plating on reed contacts

serves to control long term contact resistance and con-

tact noise.

• Sheared edges of contact blades may be sites for gal-

vanic corrosion.

• Do not derate ‘‘dry’’ contacts (low voltage, low cur-

rent, low power); some electrical energy is required to

break down oxide ﬁlms. Gold or other noble metal

July 1996 IPC-D-279

may be required for ‘‘dry’’ circuits with 0.1-5.0 V and

1- 10mA.

• Contact ﬁnal ﬁnish material/underplate/thickness/

porosity/smoothness is appropriate to the use environ-

ment, including frequency of reconnections and

current-voltage conditions.

• Identify and avoid exposed galvanic couples such as

terminations of copper-nickel-gold which are sheared

after plating; in addition, exposed base metals on the

edges of contacts can lead to tarnish creep, the exten-

sion of corrosion products of copper over the gold. See

a sample galvanic compatibility table in the appendi-

ces.

• Suppliers’ data usually arises from margin-testing

which may not be long enough in duration under stress

to make meaningful comparisons or judgments of per-

formance in service; no failures under accelerated con-

ditions means no data with respect to mean time to

failure or with respect to scatter have been obtained.

Minimize fretting corrosion:

• Assure that only compatible contact ﬁnish combina-

tions are used, such as gold-gold, with per-contact nor-

mal forces of 30-50 grams-force.

Non-noble metal ﬁnishes require the availability at the

contact interface of high current, high voltage or high

energy to break down any developed oxides and other

corrosion products; these conditions are not generally

available with ICs which operate at required higher

per-contact normal forces, e.g. contact mating ﬁnishes

of tin to tin or tin-lead to tin-lead: < 200 grams-force

initially and < 100 grams-force at end of life.

• Do not use incompatible contact ﬁnish combinations

such as gold-tin.

Under micromotion conditions arising from mechani-

cal or thermo-mechanical causes, gold-tin intermetallic

compounds are generated. These IMCs are high in

resistance and result in intermittent or permanent resis-

tive or opens connections.

• Investigate and satisfy any need for contact lubricants.

With non-noble contact ﬁnishes such as tin or tin-lead,

the environmental circumstances may indicate a pos-

sible need for oxygen/corrodant exclusion techniques

such as the use of ‘‘lubricants;’’ lubricants are not pre-

ferred because they hold dust particles, may slowly

evaporate or oxidize, and require special attention dur-

ing service/replacement.

• Card mounting stresses and ﬂex circuit ﬂexures (static

or dynamic vibration) must be controlled by clamps,

screws, hold-downs; the stresses must not be transmit-

ted to the connector or to the contacts.

F-11.2.1 Batteries Keep in a suitable, insulated or origi-

nal container, not loose, in inventory, on the line and at

repair stations. Otherwise, shorts may result in extremely

high temperatures in the storage container. Use a battery

holder designed with very high pressure contacts. To avoid

the effects of fretting corrosion, do not interface dissimilar

metals such as nickel and tin at the contacts. Welded con-

nections are preferred for high vibration environments.

Liquid or gel electrolyte may boil or expand at SM reﬂow

temperatures.

F-11.2.2 Separable Electrical Interconnections Sepa-

rable SM interconnections include randomly laid plated

wire bundles in holes in a hard insulative substrate (fuzz

buttons); stamped and plated preformed springy material in

an elastomeric matrix; stamped and plated preformed

spring material in an injection molded connector housing;

strips of metal ﬁlm over an elastomeric core; plated etched

or stamped metal ﬁlms on an insulative ﬂexible substrate,

mated under pressure from an additional mechanical part;

metal particles dispersed in an insulating polymeric matrix;

or carbon or silver particles dispersed in a polymeric

matrix and separated from each other by insulative poly-

meric material. The stamped/plated preformed spring

mechanisms appear to afford a ‘‘wiping’’ action which

scrubs tarnish from the mating surfaces. The silver plated

contact materials may lead to dendriting with moist corro-

sive environments in combination with low powered cir-

cuits, if the elastomer does not form a gas-tight seal.

For reliability, the contact materials should be of noble

metals and, in particular, no gold-tin contacts should be

employed due to the formation of resistive gold-tin inter-

metallics under fretting corrosion conditions. Use caution

with tin-tin or tin-lead contacts and specify contact normal

force > 100 grams (force) per contact at end of life.

Mechanical restraint of the mating parts to reduce micro-

motion to < 2.5 µm is recommended.

• Identify mechanical stress levels in metals (particu-

larly formed terminations) which might contribute to

stress corrosion or plating discontinuities; alterna-

tively, form metals in the annealed state and post-

plate.

• Verify that the operating temperature rating applies to

the mated connector under the required combination

of current/voltage/impedance. Where the required

normal contact force depends in part on the plastic

housing, that normal contact force may decrease with

exposure at high temperature during assembly or ser-

vice.

• Current rating per connector pin applies to the

as-stuffed condition; the heat rise per pin must be

accounted for in high current (paralleled power sup-

ply) situations.

• Evaluate each SM connector and socket style for

inspectability of the solder joints as well as repair-

ability. In many cases, the invisible socket solder

IPC-D-279 July 1996