IPC-7095C-2013.pdf - 第134页

For eutectic tin/lead BGA solder joints, during temperature cycling, grain coarsening occurs in areas of the solder joint where the strains due to thermal mismatch build up. These coarser grains are softer than the origi…

8.1.5 Mechanical reliability Mechanical reliability refers

to the response of the BGA interconnect to a mechanical

stimulus. These stimuli include shock events, which may

occur during shipping, installation, or ﬁeld use, transient

bend, which may occur during manufacturing (in-circuit

test, functional test, installation, etc.) or in the ﬁeld, cyclic

bend, such as repetitive keystrokes near a BGA, or vibra-

tion due to fans, or motors near or in the system. All of

these stimuli, and others, can have an effect on the

mechanical integrity of the BGA interconnect, and all of

them should be considered when using BGAs, which are

particularly susceptible to ﬂexure based stressors.

8.1.5.1 Shock A shock event is characterized by signiﬁ-

cantly high-speed, which does not allow time for intercon-

nect materials to respond to the applied force. Shock can be

induced via drop or impact, and can occur at any time dur-

ing the life of the product, although shipping and end-user

environment shock events are most typical. Shock events

can result in a separation, either partial or full, of a BGA

interconnect, at any of the interfaces making up the full

interconnect attachment. Even a partial interfacial fracture

can eventually, over the life of a product, cause an electri-

cal failure in a BGA. More brittle materials and interfaces

are particularly susceptible to shock induced failures.

8.1.5.2 Transient Bend Transient bend, or ﬂexure, is

characterized by a low number (typically 1-10 ﬂexure

events) of relatively slow strain. These events can be

induced during manufacturing, especially in test ﬁxtures

such as ICT or functional testers that use pogo-pin style

probes, during connector insertion, or screw-down of the

PCA into a chassis. Transient bend can also be induced

during unit repair, shipping, and in the end-user environ-

ment. As with shock, these failures typically occur at an

interconnect interface and can result in either a complete

open, or a latent ﬁeld risk as partial fractures can eventu-

ally open in the ﬁeld.

8.1.5.3 Cyclic Bend Cyclic bend is typiﬁed by multiple

relatively low-level strain events. These events can run into

the thousands or above, and are usually caused by repeti-

tive actions such as keypad actuation, transportation, and

connector insertion or extraction (such as in notebook

docking stations). Over time, fatigue failures may result.

8.1.5.4 Vibration Vibration is characterized by a con-

tinuous low-amplitude displacement of relatively high fre-

quency (as compared to the other mechanical stresses

described above). Vibration can impinge on a BGA due to

proximity of the component to a vibrating subsystem, such

as a motor, fan, or disk drive. It is possible, if the intercon-

nect system has already been stressed to the point of inter-

facial fracture, that vibration may exacerbate the issue,

leading to an electrical open; however, this is not yet well

understood.

8.2 Damage Mechanisms and Failure of Solder Attach-

ments

The reliability of an electronic assembly depends

on the reliability of the sum of the individual elements of

the mechanical thermal and electrical interfaces (or attach-

ments) between these elements. One interface type, the sur-

face mount solder attachment, is unique since the solder

joint not only provides the electrical interconnection, but is

also the sole mechanical attachment of the electronic com-

ponents to the printed board. It often provides the critical

heat transfer function as well. A solder joint in isolation is

neither reliable nor unreliable; it becomes so only in the

context of application.

The characteristics of these three elements—component,

substrate, and solder joint—together with the use condi-

tions, the design life, and the acceptable failure probability

determine the reliability of the surface mount solder attach-

ment.

The general characteristics of most lead-free solders as

compared to tin/lead solders, include (1) signiﬁcantly

increased alloy stiffness, (2) signiﬁcantly slower creep

rates, (3) greater difficulties of proper spread, and (4) sig-

niﬁcantly higher soldering temperatures.

The consequence of the greater stiffness or lower ductility

and the slower creep rates are higher stresses on the whole

solder attachment structure during either temperature

changes, causing thermal expansion mismatches or PCB/

component warping, or bending. These higher stresses,

combined with less strong solder to base material connec-

tions due to inadequate wetting or interfacial structural

weaknesses, may cause brittle interfacial failure.

8.2.1 Comparison of Thermal Fatigue Crack Growth

Mechanism in SAC vs. Tin/Lead BGA Solder Joints

Eutectic tin/lead solder joints develop different damage

mechanisms than SAC BGA solder joints during Thermal

Fatigue Tests, such as Temperature Cycling. Eutectic tin/

lead solder microstructure is multi-grained, with each BGA

solder joints having many individual grains of lead-rich

(dark phase) and tin-rich (light colored phases) precipitated

out, as seen in Figure 8-1. In contrast, each SAC BGA sol-

der joint has very few grains of tin. These grains can be

seen under cross-polarizing light. Figure 8-2 shows two

examples of SAC BGA solder joints. The one on the left

has 6 grains, whereas the entire solder joint on the right is

comprised of a single grain.

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For eutectic tin/lead BGA solder joints, during temperature

cycling, grain coarsening occurs in areas of the solder joint

where the strains due to thermal mismatch build up. These

coarser grains are softer than the original as solidiﬁed

grains, and promote crack propagation. This is shown in

Figure 8-3.

For lead-free SAC BGA solder joints, when subjected to

thermal cycling, re-crystallization of grains takes place in

regions of large thermo-mechanical stress. These

re-crystallized grains are smaller in size than the original

grains and are more prone to grain boundary sliding and

hence creep deformation. This causes the fatigue cracks to

propagate through this re-crystallized region, as shown in

Figure 8-4.

Figure 8-1 BGA Solder Joint of Eutectic Tin/Lead Solder

Composition Exhibiting Lead Rich (Dark) Phase and Tin

Rich (Light) Phase Grains

Figure 8-2 Socket BGA Solder Joints of SnAgCu Compo-

sition, Showing the Solder Joint Comprised of 6 Grains

(Top Photo) and a Single Grain (Bottom Photo).

Figure 8-3 Thermal-Fatigue Crack Propagation in Eutectic

Tin/Lead Solder Joints in a CBGA Module

Figure 8-4 Thermal-Fatigue Crack Propagation in

Sn-3.8Ag-0.7Cu Joints in a CBGA Module

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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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