IPC-D-279 EN.pdf - 第125页
between conductors or barrels pre- and post-assembly process stresses. T o 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 i…
terminations of copper/nickel/gold which are sheared
after plating (e.g. exposed base metals on the edges of
contacts). These conditions can also lead to tarnish
creep, the extension of corrosion products of copper
over the gold. See a sample galvanic compatibility
table below.
• Identify mechanical stress levels in susceptible metal
parts (particularly formed terminations for possible
contribution to stress corrosion or plating discontinui-
ties); alternatively, metal parts are formed in the
annealed state and postplated.
• Identify susceptible ceramic package brazed and plated
terminations which may have brazed joints of metals
constituting galvanic couples, may have exposed
plated interfaces, and may have been or need to be
trimmed and/or formed after plating.
• Use components which do not release corrosive mate-
rials. Wet slug tantalum with sulfuric acid electrolyte is
not recommended for new designs. Avoid aluminum
electrolytics with dimethyl formamide electrolyte
which degrades solder mask and conformal coatings.
Orient vent plugs of unavoidable electrolytic capaci-
tors to minimize damage consequent to component
failure—face vent plugs away from substrate.
• Identify and if possible avoid galvanic corrosion
couples; these practices may mitigate consequences:
• Plating is to be complete with no exposed interfaces
in plating systems such as copper overplated with
solder, tin, or gold. Alternatively, interpose nickel
plating between the copper and the overplate.
• Interpose an intermediate compatible material. For
instance, for a steel screw fastened to aluminum,
interpose a washer plated with cadmium (an environ-
mental no-no) or use ‘‘active’’ stainless steel for the
screw where the S/S passivation tends to isolate the
screw.
• Selectively metallic plate as required for reliable
electrical contact between pressure contacts; e.g. gold
to gold or tin to tin but not gold to tin (to avoid
‘‘fretting’’ corrosion).
• Coat surfaces (with polymeric or conversion mate-
rial) for insulation and moisture exclusion.
• Anodically finish for insulation and moisture exclu-
sion but be very careful; the coating is thin and brittle
and entrapped residual anodic processing fluids are
corrosive.
• Design so that cathodic metal area is much smaller
than the anodic metal area.
• Assure that fretting ‘‘corrosion’’ has been minimized
or eliminated:
• The need for contact lubricants has been investigated
and satisfied.
• Contact finish and thickness/porosity/smoothness is
appropriate to the use environment, including fre-
quency of reconnections and current/voltage condi-
tions.
• Contacts are gold to gold or tin to tin but not gold to
tin.
• Card mounting stresses and flex circuit flexures
(static or dynamic vibration) are controlled by
clamps, screws, hold-downs; the stresses are not
transmitted to the connector or to the contacts.
• Identify conditions at possible electrochemical corro-
sion risk sites:
• bare metal
• tight spacing (very small diameter vias, complex
mechanical fitted parts, connectors, switches, vari-
able elements...)
• relative humidity 65% or condensed water films and
droplets
• ionizable contamination
• conductors with DC potential (particularly between
leads of fine pitch elements and where pin assign-
ment is optional, avoid large potential differences
between adjacent pins of connectors);
• Pressure contacts are sealed from condensing mois-
ture, high humidity, and corrosive gases; conformal
rubber seals under continuous high pressure appears to
be effective. Be cautious of compression set effect of
rubbers at low temperatures.
• Avoid exposed silver plating, silver pastes, and silver
adhesives; overplate silver conductor material with
nickel or conformally coat or locate the component so
that water will not condense and run onto the silver.
Includes MLCC, DIP, rotary and slide switch, variable
resistor and buzzer packages.
• Tent all vias and PTHs on both ends if using active
water soluble flux (paste or liquid); alternatively, open
vias and PTHs do not terminate 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
minimize barrel corrosion due to flux entrapment and
avoid test fixture corrosion and loss of SIR due to
drips of liquid flux.
• Hydrolyzable materials completely removed from the
PWA prior to application of any solder mask; similar
cleaning prior to any conformal coating application.
This is critical where water soluble flux systems are
used; otherwise, mealing and vesication can result
under high RH conditions.
• Avoid conductive anodic filament growth (CAF).
Evaluate printed board suppliers for delamination of
solder mask between conductors and laminate voids
July 1996 IPC-D-279
113
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
2
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
114
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
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