IPC-D-279 EN.pdf - 第101页

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 INDUCTO…

F-8.3 Solid Tantalum Capacitors Tantalum solid elec-

trolytics are preferred to aluminum electrolytics for stabil-

ity and reliability.

Caution

Excessive assembly soldering heat results in solder melting

and solder balls at internal connections. Some devices are

available in plastic encapsulation to limit internal heat

absorption during SM reﬂow.

Be aware of undamped reﬂections of switching transients

due to the parallel capacitance/series inductance geometry

on the power/ground distribution net of many PWAs. Visu-

alize the resemblance to the ideal conﬁguration for a delay

line with series inductors and high quality parallel capaci-

tors in the schematic and layout of SMT PWAs; the long

power and ground conductors contribute the series induc-

tance and high Q multilayer ceramic capacitors between

power and ground provide the parallel capacitance. At low

temperatures, silicon mobility increases and therefore

decreases the time rate of change of switching transients;

the edges get faster. Lossy electrolytic capacitors may have

to be applied between power and ground to dampen the

reﬂection of those switching transients.

Limitations include relatively high leakage current; voltage

range limited to 6 to 120 V. Only available polarized and

polarity must be observed; where handload or repair is

expected, mark the printed board to facilitate correct

insertion- this is also a DfM guideline.

Self healing effect of high leakage current on MnO

results

in lower incidence of shorts due to dielectric breakdown

but must use 3Ω/V current limiting resistor.

Opens due to poor solder or weld internal connections

which are damaged during vibration or shock.

F-8.4 Electrolytic Aluminum Capacitors Use where

large values are required and excess capacitance is allow-

able.

Caution

Low air pressure accelerates loss of electrolyte.

High body temperatures result in boiling of electrolyte, loss

of capacitance, increase in equivalent series resistance

(ESR), and spillage of possibly corrosive electrolyte. Some

styles are available in plastic encapsulation to prevent elec-

trolyte loss.

Storage at room or high temperature results in drying out

of electrolyte or corrosion of case. During storage, alumi-

num oxide dielectric electrochemically combines with elec-

trolyte and capacitance value decreases; connection

between electrode and aluminum dissolves in electrolyte

and opens. Operation at very severe derating conditions

also allows the aluminum oxides to ‘‘reform’’ and the

resulting capacitor is lower in voltage capability.

Low temperature can cause freezing of electrolyte.

Vacpk + DC < rated working voltage. Vacpk < DC Voltage.

No reverse voltage.

Rubber or elastomer seals cannot halt the ingress of com-

mon SMT cleaning halogenated solvents (and previously

after TH wavesoldering using the synthetic activated ﬂux/

Chloroﬂuoro-carbon cleaning combination). Rubber seal

degrades in halogenated solvent wash- the solvent is cata-

lytically degraded by the aluminum and the degradation

products include HCl. The HCl dissolves aluminium. This

halide corrosion is a major constraint on rework. Where

this kind of corrosion has occurred in OEM product, any

component cost savings were lost in the expense of the

recall, rework and repair of the product. Rubber-sealed alu-

minum electrolytics must be used in conjunction with non-

halogenated solvents, non-halogenated chemicals or water

wash to avoid the introduction of corrosives during manu-

facturing and factory rework or ﬁeld repair.

There are plastic encapsulated SM and TH versions of

rubber-sealed capacitors which seal out halides and seal in

dielectric ﬂuid.

Some aluminum electrolytics use the polar solvent

dimethyl-formamide (DMF)- a suspected carcinogen and a

solvent for and degrader of solder masks and epoxy con-

formal coatings. Better for environmental health and safety

purposes are aluminum electrolytic ﬂuids of gamma buty-

rolactone (GBL) or dimethyl acetamide which are also

polar solvents. Leaking polar electrolyte drastically reduces

the SIR of the PWA. Orient vent plugs of electrolytic

capacitors to minimize damage consequent to component

failure; face vent plugs away from substrate.

Reverse voltage results in burn out and opens.

F-8.5 Variable Capacitors Do not use variable capaci-

tors, if possible.

Caution

For greater stability, use air trimmer. Variable capacitors

require special precautions in manufacturing areas to avoid

contamination by the soldering and cleaning processes.

Sealed units require very tight process control by the sup-

plier to survive the rigors of wave soldering, SM solder

reﬂow processes and high pressure cleaning processes. If

you must use variable components, hand-install (backload)

the parts or process them sealed prior to and during

soldering/cleaning. Avoid variable SMT components which

have rubber or sliding seals around leads or shafts.

F-8.5.1 Variable Piston Capacitors Designs vary in

temperature stability.

July 1996 IPC-D-279

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