IPC-7095C-2013.pdf - 第142页

coef f icient of thermal expansion (CTE) for materials can generate undue stress to the solder interface. The solder attachment integrity for area array packages will vary , depending on the loading conditions to which t…

• Stiffening Mechanism

• Solder Volume

• Solder Mechanical Properties

• Solder Creep Properties

• Quality of solder Joint Interfaces

As a remedy, some designers especially in the cell phone

industry had adopted larger corner lands, elongated lands,

and underﬁll to enhance robustness. From the assembly

perspective, enforcing proper ﬁxturing and handling are

keys to avoid solder joint damage.

8.4.8 Failure Signature-7: Insufficient Reflow This fail-

ure signature occurs when the BGA ball does not receive

sufficient heat for the solder to reach a temperature above

liquidus temperature of the solder. Insufficiently reﬂowed

solder joints, when they occur, are typically found under-

neath the center portion of the BGAs, because those areas

heat the slowest during soldering process, and would be the

most prone to not reaching adequate soldering tempera-

tures.

Sometimes, insufficient reﬂowed solder joints occur due to

the existence of a feature on a component, such as a cam

on a socket, depriving the heat from that area during the

reﬂow process. Figure 8-18 below illustrates an example of

insufficiently melted solder joints due to this reason.

8.5 Critical Factors to Impact Reliability

8.5.1 Package Technology

Area array components

come in a variety of styles and materials. The majority of

the commercial array devices utilize plastic encapsulation

and a reinforced rigid organic substrate interposer material

for packaging. For package-to-board interconnect, a metal-

lized land or alloy sphere (ball) is employed. Land grid

array (LGA) packaged ICs are often speciﬁed when pack-

age height is an issue, while the ball grid array (BGA) uses

small alloy spheres for the interconnect system. The con-

tact alloys furnished on the majority of the plastic based

BGA is a Sn/Pb (eutectic) or a Sn/Ag/Cu (lead-free) com-

position. Area array packages using a ceramic based sub-

strate interposer may be supplied with high-lead solder ball

or solder column, e.g., Pb90/Sn10. A growing number of

area array package variations adapt nonreinforced ﬁlm

dielectric for the substrate and a diverse combination of

encapsulation materials. Miniature ﬁne-pitch (FBGA) and

die-size package (DSP) are also widely used (especially in

portable or hand-held electronic products) and many higher

power applications incorporate an in-package heat spreader

or heat-spreading layer (see Section 4).

Long-term reliability of the solder attachment of the area

array package, when soldered to a conventional printed

board, is of primary concern. The difference in the

Figure 8-16 Two Examples of Pad Cratering (Located at

Corner of BGA)

Figure 8-17 Pad Crater Under 1.0 mm Pitch Lead-Free Sol-

der Ball. Crack in Metal Trace Connected to the Land is

Clear; However, the Pad Crater is Difficult to See in Bright

Field Microscopy.

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coefficient of thermal expansion (CTE) for materials can

generate undue stress to the solder interface. The solder

attachment integrity for area array packages will vary,

depending on the loading conditions to which the solder

joints are subjected and the reliability requirements for the

product. CTE mismatch is further aggravated when large

silicon die are attached to an organic substrate with a non-

compliant epoxy compound. Silicon CTE is near 3 ppm/°C

while the organic substrate is closer to 16 ppm/°C. Package

warp during assembly processing and even the power dis-

sipation within the package can subject the solder joints to

signiﬁcant tensile stresses. Excessive stress and strain at

the solder interface will cause solder joint failure and even

separation of the metallized lands.

When the die is attached to the package substrate with rigid

epoxy, the substrate material directly beneath the die may

be restricted to a CTE nearer that of the die. When solder

balls are retained in the same zone and exposed to a wide

variable of operating temperature, the solder interface will

be subjected to excessive strain. For ‘‘cavity-up’’ compo-

nents (die attached facing away from the package sub-

strate), only a thin dielectric layer separates the solder

joints from the die. The larger the die is, the more acute the

concerns for solder attachment reliability. Furthermore,

when the BGA solder joints fracture, they are typically near

the ball-to-package interface. This is a consequence of the

local expansion mismatch between the solder and the die-

constrained BGA substrate.

The current trend for larger BGA package outlines is to

move the contacts toward the package perimeter, with the

possible exception of some thermal solder balls and vias

retained at the central area of the package. Several manu-

facturers that cannot move the ball contact outside the die

attach area have adapted a more compliant die attach mate-

rial. The compliant die-to-package interface is slightly

thicker and exhibits a dramatic reduction in stress at the

solder-to-board interface, furnishing a substantial increase

in fatigue life.

8.5.2 Stand-Off Height Stand-off height signiﬁcantly

affects reliability of solder joints. The higher the stand-off,

the better is the reliability of solder joints. BGAs attached

with Sn63Pb37 solder balls result in solder joint heights

that are less controlled and lower (height400 to 640 µm),

while the Sn10Pb90 solder balls (diameters of 760-890 µm)

result in uniform solder joint heights of the same dimen-

sion since the Sn10Pb90 solder has a Liquidus temperature

signiﬁcantly above the near-eutectic tin/lead solders and

does not melt during a typical reﬂow process. Table 8-2

provides information on typical stand-off heights for tin/

lead ball and solder paste metallurgy packages.

Weight of the package also affects reliability of solder joint

since it impacts solder joint or stand-off height. The key

Figure 8-18 Cross-Section Photographs Illustrating Insufficient Melting of Solder Joints During Reﬂow Soldering. These

Solder Joints are Located Below the Cam of a Socket.

Table 8-2 Typical Stand-Off

Heights for Tin/Lead Balls (in mm)

Ball

Pitches

Stand-off

Heights

Ball Diameter

Prior to Reﬂow

PCB

Land Size

1.27 0.40-0.60 0.75 0.65

1.00

0.45-0.55 0.60 0.45

0.35-0.45 0.50 0.40

0.30-0.40 0.45 0.35

0.80

0.35-0.45 0.50 0.40

0.30-0.40 0.45 0.35

0.28-0.35 0.40 0.35

0.18-0.25 0.30 0.25

0.50

0.18-0.26 0.25 0.25

0.08-0.15 0.17 0.25

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factors that control stand-off are land size, available solder

volume, and the weight of the component. The lower the

weight, the smaller the land size; the larger the solder vol-

ume, the higher the stand-off.

8.5.3 PCB Design Considerations Another inﬂuence on

reliability is the geometry of the solder joints as well as the

solder land metallization. Solder masks can have a negative

inﬂuence if they are used for solder mask-deﬁned (SMD)

lands with the solder mask on the metallization lands

affecting the solder joint geometries. Stress concentrations

created by the SMD solder joint geometries can be the ori-

gin of solder joint failures and reduced reliability. More

than that, the solder mask shape and thickness could inﬂu-

ence the reliability of the solder joint. Figure 8-19 shows

crack due to stress concentration at the solder mask.

For equal solder joint height, increases in fatigue life by

factors of about 1.25 to 3 can be anticipated with the use

of nonsoldermask-deﬁned (NSMD) vs. SMD lands with the

larger improvements for solder joints with the more severe

loading conditions. Surface ﬁnish also plays a critical role

in BGA solder joint reliability. HASL, a commonly used

surface ﬁnish may be too thick or too thin. Insufficient sol-

der thickness may be consumed as intermetallic, which is

unsolderable. Immersion gold over electroless nickel is

prone to the black pad defect which leads to brittle interfa-

cial solder joint failures under mechanical and/or thermal

stress. The ‘‘black pad’’ defect is thought to be caused by

excessive corrosion of the nickel during the gold plating

process.

Laminate cracking is also a possible failure mechanism

under BGA solder joint lands. Such failure is thought to be

caused by thermal mechanical stress during reﬂow and or

subsequent mechanical stresses on the joints. Via-in-pad or

via next to land may cause drainage of solder. This is gen-

erally not recommended. Via-in-pad is being tried by some

companies with successful results. However, such an

approach should be considered only by companies with

extensive internal resources to validate reliability of solder

joints with via in pad technology.

Microvias are becoming more common in BGA lands.

Most of the BGAs will have voids whenever microvias are

used. Studies showed that most voids are not a reliability

risk to initiate a crack, however, they reduce the joint area

and will shorten the time to failure when a crack is propa-

gated. Figure 8-20 shows a failure after reliability testing

where the void was so large the ball collapsed.

8.5.4 Reliability of Solder Attachments of Ceramic Grid

Array

Ceramic CTE is about 6 ppm/°C; the CTE of

organic-based PCBs is in the range of 16-20 ppm/°C. Thus,

a global CTE-mismatch of about 10-14 ppm/°C exists

between ceramic components and organic printed boards.

To compensate for the large global CTE-mismatch, ceramic

components typically require solder columns to function

reliably in most applications. Since the corner joints are

loaded more than other solder joints (they are farthest from

the neutral point or DNP), they fail ﬁrst.

The solder columns, which currently are only used for

ceramic Grid Array Components (GACs), are 10Sn/90Pb

columns with lengths of 1.27 mm to 2.29 mm that are

either cast onto the CGA or are wire soldered to both the

CGA and the substrate with near-eutectic tin/lead solder.

With all other conditions being equal, the ratio of CBGA

solder joint fatigue lives of three column heights, 0.41 mm

[16 mils], 0.76 mm [30 mils], 2.29 mm [90 mils] is 1:4:45.

The height of the solder columns is limited by the require-

ment that the column aspect ratio (height-to-diameter) does

not produce slender columns thus changing the loading

conditions; cast columns can accommodate larger aspect

ratios.

8.5.5 Lead-Free Soldering of BGAs This section covers

various aspects of board assembly of BGAs using lead-free

solders. A description of various available lead-free alloys

and their selection is described ﬁrst. Board design and

Figure 8-19 Solder Mask Inﬂuence

Figure 8-20 Reliability Test Failure Due to Very Large Void

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