IPC-7095C-2013.pdf - 第57页

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/immersi…

has very poor solderability. The wide variation in tin/lead

HASL thickness can also cause component coplanarity and

solder paste printing problems. The uneven surface makes

the solder paste printing task more difficult because it is

difficult to achieve a good seal with the stencil. Lack of a

good seal will result in leakage of solder paste beneath the

stencil. The result is more frequent stencil cleaning or an

increased potential for bridging.

Tin/lead HASL is compatible with SMT, BGA and

through-hole components but it is not wire-bondable. It is

compatible with most solder masks. Tin/lead HASL coated

PCBs have a shelf life of 12 months. Typically, it can

withstand 4 to 5 heating cycles without affecting solder-

ability. Tin/lead HASL should only be used with tin/lead

solders.

5.3.1.2 Lead-free HASL Since tin/lead HASL is not

RoHS compliant, there has been a move to lead-free

HASL. The most likely candidates for lead-free HASL are

SnCu (227°C melting point) or SnAgCu (217°C melting

point). The SnAgCu alloy offers the advantage of a lower

melting point and the SnCu alloy offers the advantage of a

lower cost. Some lead-free alloys also add a small amount

of Ni. All high tin alloys are more expensive than the tin/

lead alloy they replace because a low cost material (lead)

is replaced by higher cost materials (tin and silver).

Lead-free HASL is compatible with SMT, BGA, and

through-hole components, but it is not wire-bondable. It is

compatible with most solder masks. Lead-free HASL

coated PCBs have a shelf life of 12 months. Typically, it

can withstand 4 to 5 heating cycles without affecting sol-

derability. Lead-free HASL should only be used with lead-

free solders.

Lead-free HASL is a reasonable alternative for those appli-

cations that need lead-free processing. The ﬁnish is smooth

and less domed than tin/lead HASL (see Figure 5-3); how-

ever, coating thickness uniformity is still a concern for

ﬁner pitch components. Lead-free compatible laminates

can tolerate the coating process without signiﬁcant degra-

dation and without unacceptable warpage (bow and twist).

Very thin boards are still problematic and may require ﬁx-

tures, which is true for any HASL process.

5.3.2 Organic Surface Protection (Organic Solderability

Preservative) OSP Coatings

OSP is an anti-tarnish coat-

ing of an organic compound (such as a benzimidazole-

based compound) which is applied over exposed copper

surfaces to prevent oxidation. An OSP is commonly a

water-based organic compound that selectively bonds with

copper to provide an organometallic layer that protects the

copper, preserving its solderability. Various chemistries of

OSPs are available. Some common ones are benzotriazol,

imidazol and benzimidazol. The coating is commonly

applied either by dipping or spraying. Either method will

work as long as the process is controlled to achieve a uni-

form coating. The coating thickness can range from thin

(0.2 µm) to relatively thick (0.5 µm). Thicker coatings are

preferred if there is a need for multiple reﬂow cycles

and/or a long wait (24 hour maximum) between soldering

of each side.

OSP coatings are compatible with SMT, BGA and through-

hole components but it is not wire-bondable. Solder mask

compatibility is usually not an issue. OSP coated PCBs

have a shelf life of 6 to 9 months if they are stored prop-

erly. OSP coatings are compatible with tin/lead and lead-

free solders; however, OSP coatings developed speciﬁcally

for lead-free soldering must be used.

OSP provides a ﬂat surface which reduces stencil printing

and component coplanarity issues. Since the OSP coated

surface maintains its copper appearance (OSP coating is

transparent), any solder paste misprint is more easily spot-

ted due to increased color contrast. Alcohol or other sol-

vents, if used for washing off the misprinted paste, will

also wash off coatings and, therefore, will increase the risk

of oxidation of the copper which impacts solderability.

However, such boards can be recoated if necessary. Wash-

ing and wiping the board is not recommended, but should

instead be processed in accordance with IPC-7526.

There are some potential process issues with OSP coatings.

Complete hole ﬁll at wave soldering may be difficult to

achieve due to OSP degradation after multiple reﬂow

cycles, especially when no-clean ﬂux is used. It is a good

idea to use a nitrogen atmosphere during reﬂow soldering

to limit the amount of degradation. This will reduce the

risk of hole ﬁll problems during wave soldering. It is also

a good idea to have a nitrogen atmosphere over the solder

pot during wave soldering. During reﬂow soldering, the

solder paste should cover the entire land surface to avoid a

dewetted appearance near the edges; this issue is cosmetic

but it often raises questions. In-circuit test probing may be

a problem because it is difficult to probe through the OSP

coating; test lands should be solder coated (reﬂow or wave

soldering) to provide a better contact surface.

5.3.3 Noble Platings/Coatings With the RoHS mandate

to remove lead from electronic solder, noble metal coatings

are seeing increased use as PCB surface ﬁnishes even

Figure 5-3 Hot Air Solder Level (HASL) Surface Topology

Comparison

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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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