IPC-7095C-2013.pdf - 第56页

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 dif f icul…

(especially OSP). As a general rule, PCBs should be only

be handled by their edges. To achieve maximum shelf life

with any surface ﬁnish proper packaging and storage is a

requirement.

5.3.1 Hot Air Solder Leveling (HASL) In this process, the

ﬁnished PCB is dipped either vertically or horizontally into

a molten solder bath at about 260°C and the excess solder

is blown away and leveled with hot air, giving the process

its name. HASL is the ﬁrst heating stress that the PCB

experiences. Any evidence of nonwetting or dewetting is

immediately apparent as the board exits the process.

5.3.1.1 Tin/Lead HASL At one time tin/lead HASL was

the main surface ﬁnish solution for PCBs. However, coat-

ing thickness uniformity is a major concern for SMT and

BGA components - the solder thickness varies widely from

0.8 to 0.38 µm. It is generally held that the lower thickness

is not acceptable because the very thin layer of solder is

completely transformed into copper-tin intermetallic, which

Table 5-2 Key Attributes for Various Board Surface Finishes

HASL

SnPb/SnCu OSP

Electroless NI/

Immersion AU

Electrolytic Ni/

Electroplated Au

Immersion

Silver

Immersion

Tin

Shelf Life proper

Handling

1 Year 6-9 Months 1 Year 1 Year 6-9 Months 6 Months

Handling Normal

Avoid

physical

contact

Normal Normal

Avoid

physical

contact

Avoid

physical

contact

SMT land

Surface topology

Domed/

Flatter

Flat Flat Flat Flat Flat

Solderability after

multiple reﬂow

cycles (2x)

Good, Good Good Good Good Good

Hole ﬁll after multiple

reﬂow cycles (2x)

Good

May have

problems

after 2x

reﬂow.

Good Good Good

May have

problems

after 2x

reﬂow.

Use on thick PCBs

No, holes

difficult

to ﬁll

and clear

Yes

Nickel improves

hole reliability

Nickel improves

holel reliability

yes Yes

Use in thin PCBs

No, prone

to warping

Yes Yes Yes Yes Yes

Solder joint reliability Good Good

BGA

‘‘black pad’’

concerns

Gold

embrittlement

concerns

Planar

microvoid

concerns

Good

Sporadic brittle fracture failure

Card edge contacts

Additional

plating

operation

Additional

plating

operation

Additional

plating

operation

No additional

plating

Additional

plating

operation

Additional

plating

operation

Wire bonding No No No Yes No No

Test point probing Good

Poor,

unless solder

applied during

assembly

Good Good Good Good

Exposed Copper

after Assembly

Yes, along

land edges

No No No No

Switches/Contacts No No Yes Yes Yes No

Waste Treatment

and Safety in PCB

Fabrication

Poor/Fair Good Fair Fair Good Good

Process Control

Thickness

control

concerns

Good

Phosphorus

content

concerns

Gold thickness

control

concerns

Good

Thickness

control

concerns

Coating thickness/µm 0.38 - 0.8 0.2 - 0.5 0.05-0.10 0.8 - 2.5 0.07 - 0.10 1.0 - 1.3

General Cost

Comparison

1.0 0.4 - 0.6 2.0 - 3.0 2.0 - 3.0 1.1 - 1.6 1.0 - 1.5

January 2013 IPC-7095C

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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

IPC-7095C January 2013

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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

January 2013 IPC-7095C

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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