IPC-7095C-2013.pdf - 第153页

9.1.2 Solder Mask Defined Land on Product Board The main disadvantage of solder mask deﬁned land is that the stress concentrations created by SMD (solder mask deﬁned) solder joint can be the origin of solder joint fail- …

9 DEFECT AND FAILURE ANALYSIS CASE STUDIES

The following clauses identify possible assembly anoma-

lies related to the assembly of BGA components. The

descriptions include post process failures related to the

mounting structure characteristics and the variation in the

solder ball used as the BGA termination. In many instances

attachment metallurgy are discussed especially if the char-

acteristics contributed to the joint failure. Final joint con-

ﬁgurations are also analyzed.

9.1 Solder Mask Defined BGA Conditions There are

two ways BGA Lands are deﬁned: solder masked deﬁned

(SMD) where the land size is larger than solder mask and

the molten BGA ball touches the solder mask after reﬂow.

The other method for designing the BGA land is called

etched or nonsolder mask deﬁned (NSMD) where the mask

opening is larger than the copper land and hence the ball

does not touch the solder mask after reﬂow. This condition

is shown in 9.1.1 and 9.1.2.

Solder mask-deﬁned lands may be used on corresponding

noncritical or functional pins, since SMD lands can help

minimize pad cratering defects. However, it should be

understood that solder mask deﬁned lands create additional

stress initiation sites and should be avoided on both the

interposer and the printed board land.

9.1.1 Solder Mask Defined and Nondefined Lands

Possible Cause

Interposer is solder mask defined; board is metal-defined. If the

two areas are very different, the stresses are not uniform and

cracks may occur at the solder mask defined side. Board land

pattern too large.

Potential Solution

Area of the two attachment conditions should be similar or

identical. In addition, solder mask defined lands create addi-

tional stress initiation sites and should be avoided on both the

interposer and the printed board land.

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9.1.2 Solder Mask Defined Land on Product Board

The main disadvantage of solder mask deﬁned land is that

the stress concentrations created by SMD (solder mask

deﬁned) solder joint can be the origin of solder joint fail-

ures and reduced reliability. This condition is shown in

9.1.3. For equal solder joint height, increases in fatigue life

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

use of nonsolder mask deﬁned (NSMD) vs. SMD lands,

with the larger improvements for solder joints with the

more severe loading conditions.

There are three main disadvantages of SMD lands:

• Less real estate for topside breakout

• Loss of accuracy of land dimension

• Reduced reliability as it is the origin of early solder joint

failure

9.1.3 Solder Mask Defined BGA Failures

Possible Cause

Solder mask encroaches too far on land at board level. This

condition creates stresses in the ball that can propagate a

crack during temperature changes.

Potential Solution

Always design product boards using only metal-defined

(NSMD) lands, unless a solder mask defined land is needed to

reduce the incidence of pad cratering.

Crack starts in the solder and eventually travels down to

and through the intermetallic layer. Nickel buildup under

the solder mask is also evident.

Possible Cause

Crack starts in solder at sharp corner of solder mask. The con-

dition due to stresses in the ball caused crack propagation.

Potential Solution

Always design product boards using only metal-defined

(NSMD) lands, unless a solder mask defined land is needed to

reduce the incidence of pad cratering.

January 2013 IPC-7095C

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Not for Resale

No reproduction or networking permitted without license from IHS

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9.2 Over-Collapse BGA Solder Ball Conditions The

plastic BGA, generally, has solder balls that collapse to

about 625 µm from its original size of 750 µm. After the

package is soldered to the board, the ball collapses to about

500 µm. However, if there is a heat spreader or a heat slug

in the package for heat dissipation, the ball may collapse to

as low as 300 µm. As the ball ﬂattens there is a decrease in

reliability due to limited solder height and solder joint

compliancy. Also, solder ball spread may occur beyond the

desirable pitch clearance. A good approximation is that the

initial reﬂow reduces the height by about 10%; with the

added weight of a heat spreader this ﬁgure may increase to

25% of the original height (ball diameter). The land pattern

and solder mask clearance also play a part in the analysis.

The extremes of this condition are shown in 9.2.1 through

9.2.4.

9.2.1 BGA Ball Shape without Heat Slug 500 µm Stand-

off Height

9.2.2 BGA Ball Shape with Heat Slug 375 µm Standoff

Height

9.2.3 BGA Ball Shape with Heat Slug 300 µm Standoff

Height

Possible Cause

Weight of BGA does not over-collapse ball. This is the target

condition and becomes the measure of evaluation over other

BGAs or balls on the same BGA.

Potential Solution

Use spacer if more clearance is desired. It should also be

judged to check on the variation between ball collapse.

Possible Cause

Weight of BGA with heat slug causes ball to over-collapse. This

deformation may be acceptable depending on the component

pitch so that balls do not touch.

Potential Solution

Mandatory requirement to include spacer preventing ball col-

lapse.

Possible Cause

Weight of BGA with heat slug causes ball to over-collapse. This

is a definite poor quality condition and should be remedied.

Potential Solution

Mandatory requirement to include spacer preventing ball col-

lapse.

IPC-7095C January 2013

140

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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