IPC-7095C-2013.pdf - 第138页

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…

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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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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