IPC-7095C-2013.pdf - 第139页

mismatch creates a generalized ‘ ‘bi-metallic strip’ ’ ef fect when the BGA package is heated and cooled during the SMT Reﬂow Soldering process. This ef fect creates a ‘ ‘dynamic’ ’ warpage of the package. The FCBGA pack…

BGA ball and the solder paste which indicates that the

paste did not reach proper reﬂow temperature. In some

cases, the solder paste may not have coalesced. It should be

noted that for most lead-free solder, the surfaces of the sol-

der joints will appear grainy.

8.4.3 Failure Signature-2: Land, Nonsolderable Con-

tamination on the PCB land will cause nonsolderable inter-

face between the PCB land and the BGA ball. The solder

will wet to the BGA ball but not to the land. There might

be a partial or complete open with electrical contact. A

failure with this signature could be a result of faulty nickel

plating on the PCB with ENIG as a surface ﬁnish. One type

of failure, known as ‘‘black pad,’’ is shown in Figures 8-8

and another type of failure is shown in Figure 8-9. It also

could be a result of PCB supplier rework process and reap-

plying solder mask to the BGA area.

8.4.4 Failure Signature-3: Ball Drop This is an open sol-

der joint which forms between the solder ball and the BGA

component substrate. This causes the BGA ball to drop and

creates elongated solder ball with round or a ﬂat top. Ball

drop is a failure caused by a high topside temperature dur-

ing wave solder (>Liquidus-20°C). The Liquidus tempera-

ture of a solder alloy is the temperature above which the

whole solder volume is liquid. During wave soldering, the

BGA balls are softening due to the high topside tempera-

ture. Thermo-mechanical stress is causing the balls to pull

away from the component substrate and create an open

joint as shown in Figure 8-10. Ball drop can also be caused

by high peak temperature and dwell time during reﬂow.

8.4.5 Failure Signature-4: Missing Ball Missing solder

ball during ball attached process or due to handling dam-

age is shown in Figure 8-11. This defect signature is usu-

ally very clear and is easy to detect with X-ray or ICT

techniques.

8.4.6 Failure Signature-5: PCB and BGA Stack War-

page

As both the package substrate and the PCB thick-

ness have become thinner with the advent of the recent

boom in handheld products, such as smart phones and tab-

lets, the magnitude of this dynamic warpage has increased.

This increase in dynamic warpage causes new, various sol-

der joint shapes.

This signature occurs when the PCB and/or BGA stack is

warping during the reﬂow process as shown in Figure 8-12.

For ﬂip chip BGA Packages, the silicon die expands much

less than the package substrate laminate. This expansion

Figure 8-7 Grainy Appearing Solder Joint

Figure 8-8 Nonsolderable Land (Black Pad)

Figure 8-9 Land Contamination (Solder Mask Residue)

Figure 8-10 Solder Ball Drop

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mismatch creates a generalized ‘‘bi-metallic strip’’ effect

when the BGA package is heated and cooled during the

SMT Reﬂow Soldering process. This effect creates a

‘‘dynamic’’ warpage of the package. The FCBGA package

may become concave in shape, with negative (-) warpage,

when heated to the SMT reﬂow temperature, above 220°C,

but then become less warped when cooled down after exit-

ing the reﬂow soldering oven. Similarly, the PCB also

exhibits dynamic warpage behavior and may go from a

being relatively ﬂat to becoming convex or concave (as

shown in Figure 8-12).

The FCBGA package and board warpage can affect the

solder joint formation. Various solder joint defects can

occur during SMT reﬂow soldering due to the increase in

PCB and/or BGA stack warpage, under un-optimized SMT

process. These include Head-on-Pillow (HoP), Head-on-

Pillow Open which is similar to HoP but the solder ball

does not make any contact with the solder on the PCB

land, Non-wet Open (NWO) where there is no solder on

the PCB land and Solder Bridging. Figure 8-13 depicts the

case of a severely warped PCB and BGA stack causing

these solder joint defects across the ball array of the pack-

age. SMT process optimized solutions are listed in Section

9.4.

Figure 8-11 Missing Solder Ball

IPC-7095c-8-12

Figure 8-12 Example of Dynamic Warpage of Flip Chip BGA Packages and PCBs

At Room Temperature / Before

Reflow Soldering

PCB

Relatively Flat

PCB

Positive (+) Warpage

Convex in Shape

PCB

Negative (-) Warpage

Concave in Shape

When Heated to Reflow Temperatures

(OR)

FCBGA Package

Positive (+) Warpage

Convex in Shape

FCBGA Package

Negative (-) Warpage

Concave in Shape

FCBGA Package

Negative (-) Warpage

Concave in Shape

IPC-7095c-8-13

Figure 8-13 Example of a Severely Warped BGA Package and PCB After Reﬂow Soldering in an Un-Optimized SMT Process

Die

Package Substrate

Board

Non-Wet Open

(NWO)

Head-on-Pillow

(HoP)

Solder bridging Head-on-Pillow

(HoP) Open

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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

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