IPC-7095C-2013.pdf - 第55页

(especially OSP). As a general rule, PCBs should be only be handled by their edges. T o achieve maximum shelf life with any surface ﬁnish proper packaging and storage is a requirement. 5.3.1 Hot Air Solder Leveling (HASL…

Laminates of different resins often have different glass

transition temperatures resulting in different capabilities in

high temperature applications. Lead free with its high tem-

perature processing will require higher performance lami-

nates. These are normally a higher cost.

The glass transition temperature can be measured by three

different methods (TMA, DSC, DMA). Of these the values

obtained by TMA are the most pertinent for the purpose of

assessing reliability issues. A very rough relationship

between the results of these three methods is T

(TMA) ≈

(DSC) -10°C ≈ T

(DMA) -20°C.

5.2.3.3 Moisture Absorption Most organic materials are

hygroscopic to some degree and soak up moisture at differ-

ent rates; some do so relatively rapidly. This moisture

absorption changes the electrical properties of the material,

such as loss tangent and the processing characteristics of

the material, as outgassing can result in blisters. It can also

impact physical dimensions and the laminate’s weight.

Thus a simple way to determine that the material has

absorbed moisture is to note the increase in weight under

deﬁned moisture exposure conditions. Table 5-1 shows the

water absorption rate by weight for the various materials

highlighted in this section. IPC-1601 deﬁnes packaging

standards to reduce moisture absorption and test procedures

to determine the moisture content in a printed board.

5.2.3.4 Reliability Concerns with High Lead-free Solder-

ing Temperatures

The higher temperatures required for

soldering lead-free solders creates reliability concerns for

the survivability of the PCB resins as well as the integrity

of the PCB interconnect structures, such as plated-through

holes and vias. The properties that are most important in

this respect are the decomposition temperature, thermal

expansion, and the glass transition temperature. The

decomposition temperature (T

) measures the temperature

at which the resin decomposes irreversibly and thereby

loses weight; typically the temperature to a weight loss

of 2% or 5% is measured. The thermal expansion from

50-260°C, TE(50-260°C) is a composite of the thermal

expansions below and above the glass transition

temperature.

The impact of these three properties is captured with the

Soldering Temperature Impact Index, STII, which is

deﬁned as STII = T

/2 + T

/2 - (TE%(50-260°C) 10).

5.3 Surface Finishes The primary purpose of a surface

ﬁnish is to prevent oxidation of exposed copper on the

PCB; this ensures the surface is solderable when compo-

nents are mounted or inserted. Surface ﬁnishes also provide

several other functions; these include: reliable contact sur-

face for probes, contacts or switches, wire bonding and a

ﬂat surface for solder paste printing. Although BGAs are

the focus of this document, other components and assem-

bly operations must be taken into consideration when

choosing the most appropriate surface ﬁnish.

There is not an ideal surface ﬁnish that is best suited for all

applications; however, the search continues for improved-

surface ﬁnish solutions. Some of the application features

that must be considered in selection of a suitable surface

ﬁnish are given in Table 5-2. PCB handling is critical

because salts from hands can damage the surface ﬁnish

IPC-7095c-5-2

Figure 5-2 Expansion Rate Above T

Expansion

Temperature

-1

-2

Resin laminate

system 1

Resin laminate

system 2

IPC-7095C January 2013

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(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

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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

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