IPC-7095C-2013.pdf - 第136页

higher melting point. Some component manufacturers have had unexpected yield losses due to low Ag alloys when they were not aware of their presence. The change to low Ag ball alloys may require a change to the PCA reﬂow …

8.2.2 Mixed Alloy Soldering During the transition to

lead-free reﬂow soldering, there will be occasions when it

may be necessary to solder tin/lead and lead-free BGAs on

the same PCBA with tin/lead or lead-free solder. Soldering

mixed alloy BGAs is an undesirable scenario, but it is

likely to happen. The most likely scenario is soldering

lead-free BGAs with tin/lead solder. This scenario occurs

when a component supplier provides end-of-life (EOL) tin/

lead BGAs in favor of lead-free BGAs. When confronted

with this situation there are several options:

• Refuse to build the PCBA

• Reball the BGA with tin/lead solder balls (may not be

acceptable to all companies)

• Convince the supplier to provide a Sn/Pb version

8.2.2.1 Reflow Solder Using a Lead-Free Profile Some

companies have successfully soldered mixed alloy BGAs

and the reliability results have been acceptable in many

cases. Studies have shown that the homogeneous contami-

nation of SAC solder with <20% wt. Lead will not degrade

fatigue life below that of tin/lead. However, be very cau-

tious. Perform adequate reliability testing (cross-sections,

thermal cycling, shock testing) to conﬁrm solder joint reli-

ability in the intended use environment.

All components on a mixed alloy (lead-free BGAs with

tin/lead solder) PCBA should be evaluated for lead-free

soldering process compatibility before soldering is

attempted. This evaluation will ensure that all components

will survive lead-free soldering temperatures. While most

tin/lead components are rated to a peak temperature of

230°C, evidence suggests that many tin/lead components

will survive lead-free soldering temperatures, though this is

not always the case. Once again, be very cautious.

Adequate reliability testing should be done to conﬁrm reli-

ability expectations. Use Table 8-1 as a guide for compo-

nent compatibility with lead-free soldering process tem-

peratures.

8.2.2.2 Reflow Solder Using a Tin/Lead Profile When

the assembly comprises both lead-free SAC BGAs and tin/

lead BGAs and/or when the PCBA has not converted to a

lead-free system or is not required to be a lead-free system,

such as the RoHS exempted industry sectors/applications, a

tin/lead eutectic paste continues to be used.

Under such scenarios, a tin/lead eutectic reﬂow proﬁle hav-

ing the adequate peak temperature, dwell time and proper

preheat must be used. More details about such scenarios

are contained in Section 7.

8.2.2.3 Implementation of BGAs with Low-Ag SAC, or

doped SAC, and Non-SAC BGA Ball Alloys

During the

initial lead-free transition, the BGA solder ball alloys used

typically matched the assembly process alloys of

Sn96.5Ag3.0Cu0.5 (SAC305), Sn95.5Ag3.8Cu0.7

(SAC387) or Sn95.5Ag4.0Cu0.5 (SAC405). Problems such

as balls dropping off and susceptibility to mechanical shock

and thermally-induced strains have led BGA suppliers to

evaluate and suggest the use of additional alloys for the

balls. For some packages, the change involves moving

from silver (Ag) content of 3-4% down to 1-3%. Depend-

ing on the speciﬁc Ag content, the reduced Ag content will

increase the melting point of the solder ball by as much as

10°C. This can have a signiﬁcantly adverse impact on the

PCBA assembly process. Great care must to be taken when

introducing new alloys into BGAs and new alloyed BGAs

into the assembly process.

The addition of doping elements to SAC system have also

been studied, offering improvements in some aspects of

mechanical properties. However, these additions largely do

not change the physical properties, particularly do not

lower the melting temperature of the alloy. Some doping

elements may affect solder under-cooling, the formation of

Intermetallic, matrix properties, Other non-SAC alloys, by

taking an entirely different metallurgical approach, offer

lower melting temperature, thus substantially favorable

assembly process and better mechanical properties from

reliability point view. But these alloys are more costly in

raw materials.

8.2.2.4 PCA Manufacturing Impact of Pb-Free The new

lead-free BGA ball alloys may have an impact on the

printed circuit assembly (PCA) reﬂow process due to

Table 8-1 Tin/Lead Component Compatibility with Lead-Free Reﬂow Soldering

Tin/lead Component Type Compatibility

Ceramic resistors/capacitors Okay

Molded capacitors (tantalum) Use lead-free process temperature compatible components only

Aluminum electrolytic capacitors Use lead-free process temperature compatible components only

Molded ICs (SO, PLCC, QFP) Use lead-free process temperature compatible components only

Connectors, sockets, etc. Use lead-free process temperature compatible components only

Crystals, oscillators Use lead-free process temperature compatible components only

Transistors, diodes Okay

LEDs Use lead-free process temperature compatible components only

BGAs May or may not be Okay. Evaluate on a case-by-case basis

DIP switches Okay

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higher melting point. Some component manufacturers have

had unexpected yield losses due to low Ag alloys when

they were not aware of their presence. The change to low

Ag ball alloys may require a change to the PCA reﬂow tar-

get temperatures. For example, an increase of 10°C in

melting point for a speciﬁc BGA may cause other locations

on the PCA to exceed the temperature limits of either com-

ponents or PCB.

Reliability Impact – Improperly assembled low Ag parts,

assembled with too low of a reﬂow temperature, are at sig-

niﬁcant reliability risk since they may pass electrical test

but may still produce an unacceptable solder joint (see Fig-

ure 8-5). Some experimental data suggests that the drop

performance of well manufactured low Ag parts are better

than current SAC alloys, such as SAC305, SAC405 and

SAC387; however, the fatigue life of low Ag alloys is con-

sistently lower than their Ag counterparts. Additional fac-

tors (BGA pad and PCB land ﬁnish) are critical for reliabil-

ity and are being investigated.

8.3 Solder Joints and Attachment Types Solder joints

are not homogeneous structures. A solder joint consists of

a number of different materials, many of which are only

superﬁcially characterized. A solder joint consists of:

1) The base metal at the printed board.

2) One or more intermetallic compounds (IMC).

3) The bulk solder.

4) A layer from which the solder constituent forming the

component-side IMC(s) has been depleted.

5) One or more IMC layers of a solder constituent with

the component base metal.

6) The base metal at the component.

The grain structure of solder continues to change over time.

At room temperature, eutectic tin/lead solder is above its

recrystallization temperature; the grains will grow in size

over time. The grain structure growth reduces the internal

energy of a ﬁne-grained structure. This grain growth pro-

cess is enhanced by elevated temperatures as well as strain

energy input during cyclic loading. The grain growth pro-

cess is thus, to some degree, an indication of the accumu-

lating fatigue damage. This indication is signiﬁcantly more

pronounced for solder joints subjected to aging (and is less

pronounced in cyclic testing) than for solder joints in

operational use. Contaminants, like tin oxides and ﬂux resi-

dues, sometimes migrate to the outer surface of the solder,

but reside predominantly at the grain boundaries. As the

grains grow, the concentration of these contaminants is

increased at these grain boundaries, therefore weakening

the solder. After the consumption of about 25% of the

fatigue life of the solder, micro-voids can be found at the

grain boundary intersections; these micro-voids grow into

micro-cracks after 40% of the fatigue life. These micro-

cracks grow and coalesce into macro-cracks leading to total

fracture of the solder joint.

The solder joints frequently connect materials of differing

properties, one of which is global thermal expansion mis-

match. Solder often has properties signiﬁcantly different

than the bonding structure materials, causing local thermal

expansion mismatches. The severity of these thermal

expansion mismatches, and thus the severity of the reliabil-

ity threat, depends on the design parameters of the assem-

bly and the operational use environment. For lead-free sol-

der joints, there is a higher propensity for differential solid

diffusion causing Kirkendall voids, also known as IMC

Microvoids. The higher propensity for IMC microvoiding

is associated, at least in part, with the higher soldering

temperatures required for lead-free solders. Further, Planar

Microvoiding is seen on lead-free surface ﬁnishes such as

Immersion Silver, which entail typically more complex sol-

dering surface treatments than HASL.

IPC-7095c-8-5

Figure 8-5 Incomplete Solder Joint Formation for 1% Ag

Ball Alloy Assembled at Low End of Typical Process Window

Unmelted solder ball

CSP Package

PCB

CSP Package

PCB

Unacceptable solder joints

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8.3.1 Global Expansion Mismatch The global expansion

mismatches result from differential thermal expansions of

an electronic component or connector and the printed board

to which it is attached via the surface mount solder joints.

These thermal expansion differences result from differences

in the CTEs and thermal gradients as the result of thermal

energy being dissipated within active components. Global

CTE mismatches typically range from about 2 ppm/°C for

CTE-tailored high reliability assemblies to ≥14 ppm/°C for

ceramic components on FR-4 printed boards. Figure 8-6 is

showing a solder joint failure due to CTE mismatch on a

wafer level CSP. The silicon die on the top expands much

less than the FR-4 substrate on the bottom. This causes

shear stress on the solder joint, which eventually fails at the

die to solder interface when subjected to temperature

cycling stresses.

8.3.2 Local Expansion Mismatch The local expansion

mismatch results from differential thermal expansions of

the solder and the base material of the component or PWB

to which it is soldered. These thermal expansion differ-

ences result from differences in the CTE of the solder and

those of the base materials from thermal excursions. Local

CTE mismatches typically range from 7 ppm/°C with cop-

per to 18 ppm/°C with ceramic and 20 ppm/°C with Alloy

42 and Kovar. Local thermal expansion mismatches typi-

cally are smaller than the global expansion mismatches,

since the acting distance, the maximum wetted area dimen-

sion, is much smaller: in the order of hundreds of microm-

eters instead of thousands.

8.3.3 Internal Expansion Mismatch In tin/lead solders,

an internal CTE mismatch results from the different CTEs

of the tin-rich and lead-rich phases of the solder. Internal

thermal expansion mismatches typically are the smallest,

since the acting distance, the size of the grain structure, is

much smaller than either the wetted length or the compo-

nent dimension - in the order of less than 25 µm.

For lead-free solders, the metallurgy is more complicated

due to the ternary or quarternary alloy composition concen-

trated at very high tin contents.

8.4 Solder Attachment Failure Failure of the solder

attachment of a component to the substrate to which it is

surface mounted is commonly deﬁned as the ﬁrst complete

fracture of any of the solder joints of which the component

solder attachment consists. Given that the loading of the

solder joints is typically in shear, rather than in tension, the

mechanical failure of a solder joint is not necessarily the

same as the electrical failure. Electrically, the mechanical

failure of a solder joint results, at least initially, in the

occasional occurrence of a short-duration (<1 µs) high

impedance event during either a mechanical or thermal dis-

turbance.

From a practical point of view, the solder joint failure is

deﬁned as the ﬁrst observation of such an event. For some

applications this failure deﬁnition might be inadequate. For

high speed signals with sharp rise times, signal deteriora-

tion prior to the complete mechanical failure of a solder

joint might require a more stringent failure deﬁnition.

Similarly, for applications which subject the electronic

assemblies to signiﬁcant mechanical vibration and/or shock

loading, a failure deﬁnition that considers the mechanical

weakening of the solder joints as the result of the accumu-

lating fatigue damage might be necessary.

8.4.1 Solder Attachment Failure Classification There

are some common BGA failure signatures. These defects

can be induced during the assembly process or they could

be latent solder joint defects or failures. Such defects

and/or failures are the result of an inadequate assembly

process, defective material or excessive mechanical stress

during assembly. The defects could be a partial open or

very weak interfaces, the latent failures could be hairline

cracks, open joint with a full contact and partially lifted

land. They are difficult to detect using conventional process

veriﬁcation tools such as X-ray and ICT test. They are a

major reliability concern since they can be intermittent.

Subsequently, they are tracked down following high levels

of ﬁelds returns.

8.4.2 Failure Signature-1: Cold Solder Cold solder joint

is a result of a low peak temperature during the reﬂow pro-

cess (<Liquidus+20°C). The Liquidus temperature of a sol-

der alloy is the temperature above which the whole solder

volume is liquid. The solder paste partially melts and will

form a cold solder joint. The joint may have a rough sur-

face and sometimes necking at the interface to the board.

The solder will appear grainy as shown in Figure 8-7 and

there will be a difference between the grain structure of the

Figure 8-6 Solder Joint Failure Due to Silicon and Board

CTE Mismatch

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