IPC-7095C-2013.pdf - 第141页

• Stif fening Mechanism • Solder V olume • 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…

The dynamic warpage of the PCB and/or FCBGA stack

also leads to varying shapes of solder joints that are accept-

able from a quality standpoint. Figure 8-14 illustrates

examples of acceptable solder joints. Most BGA solder

joints are convex solder joints, since the smallest angle (in

blue in the top left photo of Figure 8-14) of the solder joint

surface tangent with respect to either pad surface is ≤90° at

all points along the solder joint surface.

PCB and/or BGA stack dynamic warpage can lead to some

solder joints being stretched into a columnar shape. Figure

8-15 shows an acceptable columnar solder joint. For

columnar solder joints, the solder joint surface is normal to

either side of the PCB land.

8.4.7 Failure Signature-6: Mechanical Failure Mechani-

cal stress caused by board ﬂexing from in-circuit testing is

not uncommon in PCB assembly. As the size of the BGA

becomes larger, the stress experienced by corner joints

becomes more signiﬁcant. Even probing beneath and sur-

rounding the BGA is required. Mechanical stress caused by

probe pins and vacuum force is sometimes ignored. It is

important to note that excessive monotonic stress induced

by mechanical stresses will lead to solder joint failures.

Since the weakest interface is the one that breaks, this fail-

ure signature could be different. The crack could be within

the BGA ball or at the PCB or package interface or within

the PCB as a lifted land (pad ‘‘cratering’’). Figure 8-16

shows two examples of a lifted corner land caused by

excessive mechanical stress. This defect is also termed as

pad cratering.

Pad cratering can result in a failure of the joint due to elec-

trical opens. The initial crack weakens the joint mechani-

cally. As it progresses, the crack may intercept electrical

traces causing an open. This is shown in Figure 8-17. Fail-

ure by this signature is enhanced by the higher tempera-

tures of lead-free reﬂow temperatures, harder laminates,

and increasing land or trace densities.

The robustness of the BGA joints against mechanical stress

is a function of several factors:

• Location of the BGA

• Thickness of the PCB

• Stack Up

• Land Size

Figure 8-14 Examples of Acceptable Convex Solder Joints with Solder Joint Surface Tangents Shown in the Top Left Photo

Figure 8-15 Example of an Acceptable Columnar Solder

Joint

IPC-7095C January 2013

126

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--`,,```,,,,````-`-`,,`,,`,`,,`---

• 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.

January 2013 IPC-7095C

127

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--`,,```,,,,````-`-`,,`,,`,`,,`---

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

IPC-7095C January 2013

128

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

--`,,```,,,,````-`-`,,`,,`,`,,`---