IPC-7095C-2013.pdf - 第154页

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…

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

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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

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Provided by IHS under license with IPC

Not for Resale

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9.2.4 Critical Solder Paste Conditions The amount of

solder paste deposited for plastic BGA attachment is help-

ful, but not very critical in formation of good solder joint

since the ball itself can be the source of solder. However,

in the case of ceramic BGA (CBGA), it is very important

that enough solder paste is deposited. The recommended

volume of solder paste for 890 µm CBGA is 0.12 cubic

mm and 0.08 cubic mm minimum. If sufficient solder paste

is not deposited, as shown in 9.3.1, the reliability of solder

joint may be questionable. The reason solder must be

added to the high temperature solder ball or column is that

there is no contribution of solder volume from the package

termination to the solder joint.

9.2.5 Overly Thick Paste Deposit

9.2.6 Void Determination Through X-Ray and Cross-

Section

Transmission X-ray can detect void presence

(light areas) and the associated X-Y location. The tech-

nique can also detect uneven or missing solder balls (vari-

ous dark image diameters). An example of this condition is

shown in 9.2.7. However, cross-section X-ray is required to

determine the vertical (Z axis) location of the void in the

solder joint.

9.2.7 Voids and Uneven Solder Balls

There are many reasons for formation of voids in BGA.

However, the presence of voids does not pose any reliabil-

ity risks. Voids, as shown in 9.2.8, can survive 1000 ther-

mal cycles (no shock, 0-100°C), although more common

voids are shown in 9.2.7. Even though voids do not reduce

the fatigue life results in some tests, the presence of exces-

sive voids in solder joints is an indication of design,

process or materials issues. Product reliability should also

be veriﬁed.

Possible Cause

Thicker paste was intended for a ceramic noncollapsible ball,

not a plastic BGA ball.

Potential Solution

Reduce stencil thickness; microetch down at BGA area; reduce

stencil opening.

Possible Cause

– Excess voiding in solder ball attachment.

– Via in pad design (voids related to via in pad design are not

considered a defect as per IPC-A-610).

– Fast ramping profile.

– Forward compatibility scenarios (tin/lead BGA ball with Lead

Free Solder Paste).

Potential Solution

– Evaluate structural strength of joint through thermal stress or

microsection.

– Use reflow profile with long soak.

– Avoid situations noted under potential causes.

January 2013 IPC-7095C

141

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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