IPC-D-279 EN.pdf - 第104页

joints are expected to withstand very high IC extrac- tion forces, both tensile and torsional, particularly if the extraction is performed with improper tools or technique. Similarly not-recommended components are TH soc…

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

joints are expected to withstand very high IC extrac-

tion forces, both tensile and torsional, particularly if

the extraction is performed with improper tools or

technique. Similarly not-recommended components

are TH sockets for SMT components.

Connector Mechanically restrain SM connectors to

resist expected tensile, torsional, shear forces in ser-

vice, repair and to resist lifting forces in assembly

operations. Long connectors may result in warp of

the PWA due to CTE mismatch effects during SM

reﬂow and cooldown.

• Surface mounted sockets for SMT components may

be considered a special purpose connector and should

not be used except under speciﬁc circumstances

which include the normal connector concerns AND:

• simulation of the effects of increased thermal resis-

tance from the component case and junction to air

(θ

) and (θ

) have yielded acceptable performance

and Tj results,

If your design can tolerate the increased θ

and θ

then-

• Simulate the effects of increased electronic parasit-

ics such as parallel lead capacitance and series lead

inductance and verify acceptable high frequency

performance. Watch for the effect of the added

parasitics at every lead on such parameters as

ground bounce, clock signal skew, and edge degra-

dation; higher power dissipation is often associated

with higher clock rates and faster switching times.

• No socket manufacturer warrants compatibility

between gold component termination ﬁnish and tin or

tin-lead plated socket contacts. Major socket manu-

facturers strongly discourage the use of gold socket

contacts with tin or tin-lead component lead ﬁnishes

because the socket systems for gold ﬁnishes are

designed for much lower per-contact normal force. In

either case, the concern is with the generation of high

resistance gold-tin IMCs rather than the generation of

corrosion products.

F-12.0 PRINTED BOARD

Caution

• SM printed boards are generally denser and may

require additional thickness or stiffeners for stiffness

during processing/testing/handling to avoid ﬂexure and

damage to solder joints and component bodies.

• Provide ‘‘balanced design’’ with similar areas of cop-

per on each side of the board (around the neutral axis)

particularly if, on one side of the printed board, MLCC

bridge from a large ground plane to a large power

plane.

• With ceramic components on FR-4 boards, accelerated

stress testing which exceeds the glass transition tem-

perature (Tg) of the board results in unreasonable,

decreasing failure rates of the solder joint because the

board material becomes more rubbery and less stress is

introduced into the joint; this was found in research in

the Mantech program at Martin Marietta.

F-12.1 Printed Board PTH/Vias

See Appendix B for details.

Identify risk sites threatening PTV reliability, such as:

• large environmental or power cycling temperature

excursions

• small diameter vias or PTHs

• thick board

• low T

board material

• high Z-axis CTE board material

• copper plating of low ductility and thin or non-uniform

thickness

• aspect ratio (board thickness to barrel diameter) > 3:1

Blind and buried vias, by their geometry, have a lower

AR; a series of these vias are more robust to thermal

stresses than a PTH with a high aspect ratio.

• Plated-through Vias (PTV) associated with through-

hole components (such as connectors or pin-grid

arrays) fail open at the knee after rework due to leach-

ing or dissolution of the copper during long term expo-

sure to molten solder (fountain or drag) during rework.

See publications from the Tin Information Center

(International Tin Research Institute) for the effect of

temperature on dissolution rate of copper and for the

protective nature of nickel plated barrier metal.

PTH thermal cycling failure risk reduction techniques

include use of minimum diameter vias and PTHs only

where needed, plating the barrel with nickel for reinforce-

ment, use of higher T

resin, additional innerlayer pads

where possible to spread barrel stress, use of 2 ounce cop-

per innerlayers to increase anchoring of barrel. See also

IPC-TR-579 and section 4.7 in this design guideline.

• Tent all PTVs, if using active water soluble ﬂux (paste

or liquid), on both ends; alternatively, do not terminate

open vias and PTHs under a component with tight

clearance. A third alternative is to ﬁll the vias and

PTHs with solder, epoxy or modiﬁed solder mask/

conformal coating material. These techniques mini-

mize barrel corrosion due to ﬂux entrapment and avoid

test ﬁxture corrosion and loss if SIR due to drips of

liquid ﬂux.

• Require supplier data to conﬁrm ability of PTV and

vias to meet your realistic environmental test require-

ment for temperature extremes; temperature transition

times; number of temperature cycles. Performance

July 1996 IPC-D-279

speciﬁcations (PTV T/S, Solder Resistance, IPC-SM-

840, etc.) should be deﬁned with agreed upon test

methods for evaluation.

F-12.2 Printed Board Conductor Design

• Only as needed, use minimum spacings from conduc-

tors to (conductor, mounting hole, component termi-

nals) and from barrels to inner plane conductors and

other barrels

• Minimize electromigration stresses by minimizing

temperature and current density/current crowding:

• Use conductor widths adequate to limit ∆T

conductor

less

than 5°C.

• Use rounded corners

• Use minimum number of sharp/acute angles

• Use smooth transition ﬁllets

• (With the above techniques, conductor cracking, foil

lifting, voltage breakdown are also improved)

• Use metal deposition techniques which result in large,

ductile grains

• Selectively plate pressure contact areas for reliable

electrical contact to avoid ‘‘fretting’’ corrosion; e.g.

gold to gold or tin to tin but not gold to tin. Gold

which is pore-free and ~0.6 µm is required for pressure

connections capable of ‘‘many’’ disconnects and for

SM boards, may require selective gold plating. A

nickel underplate of ~2 µm is required to prevent dif-

fusion of copper into the gold.

• Subdivide large copper areas on the surface to prevent

blistering/solder thieving

• Widen conductors which function as heat dissipators

F-12.3 Solder Joints

See section 4.6 and Appendix A for details.

Identify risk sites for solder joint non-reliability factors

such as:

• Large ∆α between component and substrate, particu-

larly for ceramic components on FR-4 substrate or

plastic components on ceramic substrate.

• Large components on any edge

• Non-compliant leads (short and stubby)

• Large ∆T in the use environment under power cycling

conditions

• Thin solder joints

• Solder joints containing gold > 3 weight percent

Gold thickness is more critical in SM than in TH

because the volume of solder is limited to that of the

paste ‘‘brick’’ and the gold content in the ﬁnal SM

joint is not as dilute as in the TH joint.

Use printed board gold plating ﬁnish 0.1 µm on printed

board surfaces to be SM soldered so that gold concen-

tration is < 3 weight percent in the ﬁnal joint to avoid

signiﬁcant solder joint embrittlement. See the note

above on gold thickness required to assure reliable

pressure connections.

Dipping of the component leads in molten solder to

dissolve and remove the thick gold is one historical

method of ‘‘converting’’ gold plate part ﬁnishes to a

safe termination ﬁnish; the resulting ‘‘solder dip’’ ﬁn-

ish thickness is not uniform and may interfere with the

formation of uniform solder joints.

• Solder joints to terminations ﬁnishes containing silver

Joints to components can fail immediately if the termi-

nations are manufactured with a ﬁnal terminal ﬁnish of

silver paste or palladium-silver paste; these materials

rapidly leach into molten lead-tin solder and the result-

ing joint is weak. Avoid leaching by requiring a barrier

metal layer of nickel over the silver for SM component

termination. Thin silver or palladium plating over the

nickel barrier is acceptable; the small amounts of Inter-

metallic Compound (IMC) formed are not detrimental.

Alternatives required to allow silver in the manufactur-

ing process and which result in reliable joints are

— expensive in terms of hand labor, special processing

or special materials

— use special silver-bearing solders in production,

rework and ﬁeld repair

— limit exposure of the solder joint to solder tempera-

ture

— limit volume of lead-tin solder in the joint; this

alternative may result in reducing solder attachment

cyclic fatigue life.

• Solder joints to ﬁnal terminations ﬁnish of nickel.

Nickel metallization as a ﬁnal termination ﬁnish

results in highly variable solderability or poor solder

joint yields if an aggressive ﬂux is not used in SM

reﬂow. Nickel is rapidly passivated (covered by a hard,

tenacious, non-solderable oxide). In their journal,

Siemens has reported the use of selective nickel plat-

ing on TAB leads to limit the spread of solder. The

consequences of using a nickel termination ﬁnish may

be extensive (uncontrolled) hand soldering to obtain

satisfactory metallurgical joints at the cost of degrad-

ing component reliability.

Strikes of porosity-free palladium or gold, on the order

of 0.1-0.2 µm, can preserve the solderability of the

nickel; silver plate on the order of 20-40 µm and tin or

tin-lead on the order of 400-100 µm are also used for

this purpose.

• Estimate solder joint reliability (see Appendix A).

• Solder joint risk reduction techniques include increas-

ing the solder joint height, adding compliant leads, or

IPC-D-279 July 1996