IPC-D-279 EN.pdf - 第106页

changing the materials of joint height, adding compli- ant leads, or changing the materials of substrate or component to decrease the ∆α . • Perform DfM review of printed board layout- include ease of inspection and rewo…

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

changing the materials of joint height, adding compli-

ant leads, or changing the materials of substrate or

component to decrease the ∆α.

• Perform DfM review of printed board layout- include

ease of inspection and rework of all solder joints, par-

ticularly those adjacent to ‘‘tall’’ components such as

connectors or in the PWA ‘‘valley’’ between such com-

ponents. Placing the non-termination portion of the

SMT component next to the TH component eases

inspection. Packages with heatsinks overhanging the

package edge lack inspectability and have a thermal

shadowing effect on neighboring parts during reﬂow; a

‘‘shadowed’’ part may require removal of the heat-

sinked part for rework.

July 1996 IPC-D-279

Appendix G

Coefficient of Thermal Expansion

G-1.0 COEFFICIENT OF THERMAL EXPANSION

Polymer systems expand with increasing temperature, dem-

onstrating a glassy phase response with a coefficient of

thermal expansion (CTE) or α

below T

, and a rubbery

phase response with a much higher α

, typically 3 times α

above T

. The transition from glassy phase to rubbery

phase is gradual, but for most polymer substrates may be

characterized by T

, the glass transition temperature. See

IPC-T-50.

Glass ﬁber reinforced substrates exhibit signiﬁcantly differ-

ent CTE in the z (CTE(z), out of plane) axis compared to

the CTE in the x-y axes; for example, below its T

, Quartz/

Bismaleimide material with 35% resin by weight exhibits a

CTE(x,y) of 6 ppm/°C and a CTE(z) of 41. Woven glass

ﬁber reinforcement exhibits an additional difference

between x and y axes on the order of 1-5 ppm/°C; this dif-

ference may be signiﬁcant where the CTE of a large SM

component package is to be matched to the CTE of the

substrate to enhance cyclic life of the solder attachments.

The CTE(z) is particularly signiﬁcant in determining the

cyclic life of PTH and vias in SM PWAs because the aspect

ratio (ratio of substrate thickness to ﬁnished hole diameter)

is generally much larger than the corresponding aspect

ratio achieved in printed boards manufactured for through

hole technologies. Higher CTE(z) values result in higher

cyclic tensile stress in the barrel of the PTH or via during

temperature excursion during SM reﬂow, or SM compo-

nent removal/rework/repair as well as during printed board

fabrication, solder dipping, hot air leveling, or wave solder.

See IPC-TR-579 and IPC-SM-782.

The thermal cycle reliability, vibration robustness, and the

thermal management of high performance Surface Mount

(SM) products are heavily dependent upon the constraining

core such as Copper-Molybdenum-Copper (CMC),

Copper-Invar-Copper (CIC) and Molybdenum-Graphite-

Molybdenum (MGM) composite material systems.

The ratios of the various materials in those composite sys-

tems can be adjusted to tailor the effective CTE to the opti-

mum value. The tradeoffs include weight and cost. See

IPC-MC-324.

G-2.0 CONSTRAINING CORES

A constraining core is an internal supporting plane in a

packaging and interconnecting structure, used to alter the

coefficient of thermal expansion of printed boards.

Table G−1 Typical Values for Coefficients of

Thermal Expansion (ppm/°C)

Insulator/Substrate

Material/System

CTE

lower value upper

E glass 5.5

S Glass 2.6

Glass-Ceramic > 3.0

Silicon 2.6

Diamond 0.9

Aluminum Nitride 4.5

Silicon Nitride 3.7

Quartz, fused silica 0.5 0.6

Kevlar 49 -5

Beryllia 6

Cubic Boron Nitride

x-y 3.7

z 7.2

E Glass/Epoxy

x-y 14.2 17.3

x8090

E Glass/Polyimide

x-y 12 16

E Glass/PTFE

x-y 24

z 260

Kevlar/Epoxy

x-y 5.1 7.1

z 5.1 7.1

Kevlar/Polyimide

x-y 3.4 6.7

z83

Quartz/Polyimide

x-y 5.0 8.0

z 68.4

Quartz/Bismaleimide

x-y, 35% Resin 6.2 6.3

z, 35% Resin 41

Alumina (90%) TF

Substrate

7.0

Alumina (Ceramic Chip

Carrier)

5.9 6.5 7.4

Epoxy (70% Silica) Plastic

Packaging

20 23

Mulite Co-Fired 4.2

IPC-D-279 July 1996