IPC-D-279 EN.pdf - 第126页

T able L−1 Galvanic Compatibility of Metals MET AL GROUP I GROUP II GROUP III GROUP IV ANODIC - Corroded Magnesium/Magnesium Alloys X Zinc/Zinc Plating X X Aluminum ﬁlled (silver plated) Elastomer die Cut Edge (-740) Alu…

between conductors or barrels pre- and post-assembly

process stresses. To break possible CAF paths, use a

minimum of 2 plies for all layers and smooth and seal

board edges.

• printed board supplier’s inspection area is controlled

for humidity, temperature, cleanliness. Volume resis-

tance is a strong function of temperature changes, sur-

face resistance (SIR) is a strong function of humidity

and humidity changes, and delamination/measling/

vesication are functions of laminate cleaning and

cleanliness.

L-6.1 Galvanic Corrosion See also Sections 4 and 9,

ASM Electronic Materials Handbook, Volume 1, Packag-

ing, 1989; Contamination Effects on Electronic Products by

Carl Tautscher, Marcel Dekker, Inc., 1991, ISBN 0-8247-

8423-5. For sources of Ecorr vs. SCE, see Galvanic and

Pitting Corrosion-Field and Lab Studies, ASTM STP 576,

1976

Caution

Aluminum - 1% Silicon - 0.5% Copper alloy used for inte-

grated circuit metallization contains Al

Cu (Q phase)

which has a large oxidation (galvanic) potential with

respect to aluminum; in the presence of moisture, rapid

oxidation of aluminum occurs in the vicinity of copper

precipitates - and pits grow in the aluminum.

IPC-D-279 July 1996

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Table L−1 Galvanic Compatibility of Metals

METAL GROUP I GROUP II GROUP III GROUP IV

ANODIC - Corroded

Magnesium/Magnesium Alloys X

Zinc/Zinc Plating X X

Aluminum ﬁlled (silver plated)

Elastomer die Cut Edge (-740)

Aluminum/Aluminum alloys (-740 to -840 mV) X X

Beryllium X X

Chromium Plating X X

Tungsten X X

Molybdenum X X

Cadmium Plating (Restricted) X X

Carbon Steel, Cast Iron X X

Stainless Steel, Active X X

Lead, Tin-Lead Solder X X

Tin/Tin Plating (-440 V) X X

Tin/Indium X X

Nickel/Nickel Plating, Active (-250 mV) X X

Leaded Brass/Bronze X X

Copper-Zinc Alloys (Brasses)

Naval Brass X X

Brass, Commercial Yellow XX

Copper/Copper Alloys (-244 mV) X X

Beryllium Copper XX

Copper-Tin Alloys (Bronzes)

Copper-Nickel Alloys XX

Aluminum or copper ﬁlled (silver plated)

Elastomer (-190 to -200 mV)

Monel (-125 mV) XX

Silver Solder XX

Nickel, Nickel Plating, Passive X X

Cobalt, Cobalt/Nickel alloy XX

Stainless Steel, Passive XX

Silver-ﬁlled Elastomers (-50 mV) X

Silver/Silver alloys (-25 mV) X

Silver-ﬁlled Films X

Graphite/Carbon X

Rhodium X

Palladium X

Titanium X

Gold, Platinum, Gold/Platinum Alloys X

CATHODIC - Protected X

July 1996 IPC-D-279

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Appendix M

Solder Joint Variability

M-1.0 SOLDER JOINT VARIABILITY

Appendix A describes the design parameters which deter-

mine the mean cycles to failure. The Weibull parameter

describes the variability in the response of the solder

attachment and the individual solder joints due to manufac-

turing or processing conditions as well as the inherent vari-

ability in wear-out processes.

• Open surface mount solder joints have been found

after a period in service. These ‘‘cold’’ or ‘‘dry’’ solder

joints were characterized by a component termination

and substrate land which were mechanically touching

but not completed with a permanent solder connection

with intermetallic compound (IMC) formation can

open during service. Tin-lead to tin-lead mechanical

contacts (under light mechanical loading and not sol-

dered) oxidize and open in service under temperature

cycling or vibration/shock conditions. This failure

mode is similar to the ‘‘fretting corrosion’’ described

in these appendices.

Failed joints have been traced to

• planarity (or coplanarity) problems on the component

termination

• planarity (or coplanarity) problems on the substrate

land

• solderability problems on the component termination

and on the substrate land

• improper selection of solder paste/ﬂux.

• improper solder reﬂow processing.

Difficulty in inspection has resulted in ‘‘dry’’ solder joints

that are seen on J-lead solder joints. These ‘‘dry’’ joints are

insidious because they can be intermittent and be reported

as ‘‘NTF;’’ however, gull-wing terminations are not

exempt. Gull-wing leads are preferable to J-leads only

because they are not hidden under the component and sol-

der joints can be inspected and reworked more easily.

Some solder joints to gull-wing terminations have been

found to be open due to excessive IMC formation and

mechanical disturbance during solder solidiﬁcation. Some

companies use an electric or air driven plunger to ‘‘tap’’

PWAs during functional test to detect ‘‘cold’’ or ‘‘dry’’ sol-

der joints. Some repair technicians use a pencil ‘‘tapper’’ to

detect these solder joints. ‘‘3D’’ optical or X-Ray systems

may be fast enough and deﬁnitive in their detection of

defective joints.

• for the ideal SM solder joint of uniform thickness (for

minimum dispersion in the Nf or cycles to failure), the

surface of the solder paste on the lands of the printed

board must be coplanar (in the same plane), all the

leads of the SMT component must also be coplanar

and contact the solder paste simultaneously. If there is

a gap between any lead and the solder paste, a defec-

tive joint is likely.

Coplanarity and solder volume issues include:

• Maintenance of component lead coplanarity requires

appropriate shipping and storage containers, such as

trays rather than tubes for Ceramic Leaded Chip Car-

riers to prevent lead interlocking and damage and stor-

age of parts to be used for repair/rework or ‘‘kitting’’

in the original container rather than loosely ‘‘binning’’

them,

• Some of the requirement for perfect coplanarity is alle-

viated because the component leads sink into the sol-

der paste and into the molten solder; this requires that

cumulative coplanarity (between the substrate and the

component leads) apply over the dimension of the

component to ~100 µm. Solder paste thickness for

Fine Pitch and extra Fine Pitch (FP, XFP) is less than

that for the coarser pitch of 0.5 - 1.2 mm components

and therefore the cumulative coplanarity requirement

is tightened to ~75 µm, again over the dimension of

the component.

Large, uninterrupted stencil openings result in thin

paste in the middle of the opening; paste uniformity is

aided by assembling the large opening from a multi-

plicity of narrower openings with the long axis paral-

lel to squeegee blade travel.

• Planarity of the substrate lands is lost, particularly in

the case of Fine Pitch and extra Fine Pitch (FP, XFP),

when the substrate is processed through hot air solder

leveling or hot air leveling (HASL or HAL). These

processes leave uneven deposits of solder on the pads

with the distribution often a function of the location of

the pad on the panel.

• The volume of solder in the joints can be reduced by

via and feedthrough holes/pads, as well as large area

conductors or other large lands very close to compo-

nents lead pads; these features tend to draw solder

away from (steal or thieve) the intended solder joint

and should be covered with solder mask or separated

with narrow conductors, covered or not with solder

mask. The presence of a large number of solder balls

or a number of large solder balls also indicates a

reduction in the solder available for joints.

• The ideal SM solder joint is metallurgically joined to

the component lead and to the substrate land with no

voids in the joint nor sign of non-wetting or dewetting

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