IPC-7095C-2013.pdf - 第58页

appearance varying from grey to black in color , which is where the term ‘ ‘black pad’ ’ comes from (see Figures 5-6 and 5-7). SEM analysis shows a distinctive nickel nodular structure similar to ‘ ‘mud cracks.’ ’ EDX in…

though the price of some metals have surpassed historically

high levels. As a result noble coatings typically cost more

than other coatings.

5.3.3.1 Electroless Nickel/Immersion Gold (ENIG) The

electroless nickel/immersion gold surface ﬁnish is applied

through the deposition of an initial layer of nickel followed

by a thin, protective layer of gold onto the exposed copper

surfaces of the PCB. The thin layer of immersion gold pre-

serves solderability by preventing oxidation of the highly

active nickel surface. The presence of nickel plating pro-

vides extra strength for the through-hole barrels during

multiple reﬂow, wave and hand soldering cycles. IPC-

4552, Speciﬁcation for ENIG Plating, is a valuable refer-

ence document.

ENIG is compatible with SMT, BGA and through-hole

components. It is not considered to be a wire-bondable sur-

face. ENIG coated PCBs have a shelf life of 12 months.

Typically, it can withstand 4 to 5 heating cycles without

affecting solderability. ENIG is compatible with tin/lead

and lead-free solders. It provides a ﬂat surface which

reduces stencil printing and component coplanarity issues.

ENIG application can be performed using a variety of

chemistries, which may lead to different results depending

on the chemistry used. Also, the chemistry and process

may be incompatible with some solder masks. The reduc-

ing agents used in the electroless nickel process contain

either phosphorous or boron. In the reduction of the nickel

in the electroless nickel deposition, either phosphorous or

boron is incorporated into the nickel deposit. The level of

these co-deposited elements should be controlled within the

speciﬁed limits. Too much phosphorous or boron variation,

outside the speciﬁed limits, may have adverse effects on

solderability and possibly solder joint reliability.

Many companies have used ENIG successfully. However,

when BGAs are used with the electroless nickel/immersion

gold ﬁnish the results, at times, can be unpredictable. Two

failure modes have occurred in recent years. The ﬁrst fail-

ure mode is a nonwetting or dewetting condition referred to

as ‘‘black pad.’’ Figure 5-4 shows the location of crack

constituting a black pad related failure. The failure is

between nickel and Ni-Sn intermetallic (not between the

ball and Ni-Sn intermetallic).

The second failure mode is an interfacial fracture that is

associated with mechanical stress and the failure will occur

between the BGA ball and Ni-Sn intermetallic. Figure 5-5

shows an illustration to highlight the differences between

the two failure modes and the location of their occurrence.

Results from industry consortia and studies by individual

companies suggest that ‘‘black pad’’ is caused by an

aggressive attack (hyperactive corrosion) of the electroless

Ni plating during the immersion Au plating process. The

gold ions from the plating solution attract electrons from

the metallic nickel surface as they plate out as the gold

metal; in return, a nickel ion is released to the bath. Due to

certain microstructure features, such as grain boundaries

and the electrochemistry involved, the exchange does not

always occur locally, i.e., the gold can be deposited to one

feature or area and the nickel ion released from a different

feature or area. The possible consequence of this process is

that selected nickel features become attacked leaving

behind a rough and phosphorous rich layer that forms a

weak bond with solder. The affected solder joints do not

form a robust mechanical bond with the PWB, and as a

result, the solder joints fail with a relatively small applied

force, revealing lands with little or no solder left on them.

The exposed nickel surface on the land is smooth with an

Crack

Ni-Sn IMC

BGA Solder Ball

IPC-7095c-5-4

Figure 5-4 Black Pad Related Fracture Showing Crack

Between Nickel & Ni-Sn Intermetallic Layer

Crack Interface

Solder

Ni-Sn IMC

Ni-P

(a)

(b)

Solder

Ni-Sn IMC

Ni-P

Crack Interface

IPC-7095c-5-5

Figure 5-5 Crack Location for a) Black Pad Related Failure

and (b) Interfacial Fracture When Using ENIG Surface

Finish

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appearance varying from grey to black in color, which is

where the term ‘‘black pad’’ comes from (see Figures 5-6

and 5-7). SEM analysis shows a distinctive nickel nodular

structure similar to ‘‘mud cracks.’’ EDX indicates high

amounts of phosphorous and nickel and low amounts of

tin. Occurrence of the ‘‘black pad’’ condition does not

appear to be sufficiently common to advise against use of

electroless nickel/immersion gold as a surface ﬁnish.

Assemblers using PCBs with this ﬁnish should be aware of

the potential problem, learn to recognize it, and take cor-

rective action.

Recent analysis indicates that the interfacial fracture of the

solder joint happens between the nickel-tin intermetallic

layer and the BGA ball, or under a high level of both

applied strain and strain rate even if hyperactive corrosion

does not take place. Failures have occurred under a variety

of laboratory testing conditions including bending,

mechanical shock, and thermal cycling. Data indicates that

increasing the strain rate shifts the failure mode to an inter-

facial fracture of the solder joint. Therefore, interfacial fail-

ure may occur under a reduced strain if the strain rate is

high enough. Currently there is no industry speciﬁcation

that quantitatively assesses the mechanical strength of

assembled BGA components on any surface ﬁnish.

5.3.3.2 Electrolytic Nickel/Electroplated Gold Another

version of a nickel/gold combination is the electrolytic

nickel/electroplated gold surface ﬁnish. This plating is

similar; however, it results in a different grain structure

from electroless nickel/immersion gold, and does not

exhibit the ’black pad’ joint cracking phenomenon. Electro-

lytic nickel/electroplated gold is applied after pattern plat-

ing and most often before solder mask, and therefore car-

ries some risk of surface contamination. Solder mask

applied over electrolytic nickel/electroplated gold exhibits

lower solder mask adhesion than other surface ﬁnishes.

This can create problems during assembly of BGAs, espe-

cially during rework. If the solder mask dams covering the

traces between BGA lands and vias peel off, solder will

ﬂow from the BGA lands into the vias and cause insuffi-

cient or open solder joints.

Electrolytic nickel/electroplated gold PCBs have a shelf

life of 12 months. It is compatible with SMT, BGA and

through-hole components and it is wire-bondable. Typi-

cally, it can withstand 4 to 5 heating cycles without affect-

ing solderability. Electrolytic nickel/electroplated gold is

compatible with tin/lead and lead-free solders. It provides

a ﬂat surface which reduces stencil printing and component

coplanarity issues.

Another concern is that it can be difficult to control the

gold thickness across the board. The gold may be too thin

(i.e., in areas with dense circuitry) or the gold may be too

thick (i.e., in isolated circuits). This latter situation may

lead to gold embrittlement due to excessive gold (>3%) in

Figure 5-6 Typical Mud Crack Appearance of Black Pad

Surface

Figure 5-7 A Large Region of Severe Black Pad with

Corrosion Spikes Protruding into Nickel Rich Layer

through Phosphorus Rich Layer Underneath Immersion

Gold Surface

IPC-7095C January 2013

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the solder joints. Gold embrittlement creates weak solder

joints that will eventually fail (see Figure 5-8).

5.3.3.3 Electroless Nickel/Electroless Palladium/

Immersion Gold (ENEPIG)

The electroless nickel/

electroless palladium/immersion gold process is similar to

ENIG except it uses a palladium metal layer that is depos-

ited after the nickel layer prior to the ﬁnal gold layer. The

palladium layer is much harder than gold, providing added

strength to the surface ﬁnish for wire-bonding while pro-

tecting the underlying nickel from oxidation. The ENEPIG

structure is illustrated in Figure 5-9. ENEPIG is not widely

available from PCB suppliers and it is not widely used by

PCBA assemblers.

This ﬁnish is compatible with SMT, BGA and through-hole

components and it is wire-bondable. Nickel/gold ﬁnishes

typically can withstand 4 to 5 heating cycles without

affecting solderability. ENEPIG-coated PCBs have a shelf

life of 12 months. It provides a ﬂat surface which reduces

stencil printing and component coplanarity issues. The high

plating temperatures and low pH of the nickel/palladium/

gold plating process can be incompatible with some solder

masks.

ENEPIG is best suited for lead-free soldering. Studies have

shown that ENEPIG does not produce very reliable tin/lead

solder joints. Pd cannot form an alloy with lead. The non-

alloyed lead disrupts the IMC formation. As Pd is intro-

duced into the Ni3Sn4 layer, it aggregates into distinctive

clusters (away from lead) creating a nonuniform IMC

layer. The excessive growth and lack of uniformity of the

IMC leads to poor reliability when ENEPIG is used with

tin/lead solder.

5.3.3.4 Direct Immersion Gold (DIG) DIG is a ﬁnishing

process that can directly deposit gold onto the copper sur-

face by utilizing an electroless plating process. A graphic

representation is shown in Figure 5-10. It is possible to

directly deposit gold on the copper surface with excellent

coverage. The main gold depositing reaction is an auto-

catalytic rather than displacement. Solderability is good

when the gold ﬁlm thickness is within the range of 30 to

80 nm; however, underlying copper roughness can affect

solder wetting. Good wire-bonding characteristics can also

be derived from deposits plated by a neutral pH, auto-

catalytic type heavy electroless gold plating bath on top of

the ﬂash gold. DIG is not widely available from PCB sup-

pliers, and it is not widely used by PCBA assemblers.

DIG typically can withstand 3 to 4 heating cycles without

affecting solderability. Solder mask compatibility is usually

not an issue. DIG coated PCBs have a shelf life of 9 to 12

months. It provides a ﬂat surface which reduces stencil

printing and component coplanarity issues. DIG may be

best suited for tin/lead soldering applications (one reason it

is not widely used). Lead-free solder results are usually

inferior (in terms of solderability and wetting) compared to

tin/lead solder.

5.3.3.5 Immersion Silver The immersion silver ﬁnish is

produced by the selective displacement of copper atoms

with silver atoms on the exposed metal (copper) surface of

Figure 5-8 Gold Embrittlement

Copper Surface Cleaned and Microetched

Electroless Ni 5 µm (200 microinches)

Electroless Pd 0.06 µm (2.4 microinches)

Immersion Au 0.03 µm (1.2 microinches)

IPC-7095c-5-9

Figure 5-9 Graphic Depiction of Electroless Nickel/

Electroless Palladium/Immersion Gold

Copper Surface Cleaned and Microetched

Immersion Gold 0.06 µm (2 microinches)

IPC-7095c-5-10

Figure 5-10 Graphic Depiction of Directed Immersion

Gold

January 2013 IPC-7095C

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No reproduction or networking permitted without license from IHS

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