IPC-7095C-2013.pdf - 第155页

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 sold…

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

No reproduction or networking permitted without license from IHS

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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.

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9.2.8 Eggshell Void

9.3 BGA Interposer Bow and Twist

Plastic BGAs have a

tendency to warp during normal assembly reﬂow pro-

cesses. The warpage can occur on the BGA substrate or on

the product PCB. The result can be an open or short con-

dition at the solder joint that deals with the stresses. The

temperature (reﬂow proﬁle), BGA construction, solder

paste volume, and cooling condition all contribute to the

possible defect. Shorts in the corner balls, is an indication

of warpage in BGA where the corners of the BGA package

are bent inward (frowning BGA).

Solder shorts are caused in adjacent and/or opposite BGA

corners as the substrate bows downward (frowning) and

applies a stress to the corner balls. This same phenomena

can cause balls, away from the corner, to lift away from the

mounting substrate as the substrate changes from frowning

to smiling as shown in 9.3.1 and 9.3.2. As BGA substrates

and die are becoming thinner, package warpage is increas-

ing. In order to have a robust SMT process, checking to see

that sufficient solder paste has been added to the land is

recommended. The process should be closely monitored to

ensure that additional defects are not created such as solder

bridging, solder balling, etc.

9.3.1 BGA Interposer Warp

Opens in corner balls is an indication of warpage in BGA

where the corners of the package are lifted upward. Such

opens, as shown in 9.3.2, could be minimized by using an

extra amount of solder paste.

Adding excess solder is not a solution to the problem.

Determining the root cause and addressing the reasons for

the anomaly is more important toward establishing a robust

process. Modifying the stencil aperture to permit solder

paste deposits on the board should be used as a solution to

open corners only when certain process or component con-

ditions cannot be altered, such as when the reﬂow process

has already been optimized, the BGA package or the BGA

interposer cannot be redesigned, or the product board can-

not be redesigned. In addition, the anomaly must consis-

tently occur and solder and component inventory should be

considered prior to making any process changes. If the

decision is made to use excess solder paste to correct opens

in corner balls, the process should be closely monitored to

insure that additional defects are not created such as solder

bridging, solder balling, etc.

Microvia 0.65 mm @ 1000 cycles

Shows collapse of solder joint, intermittent contact fail-

ure resulted

Possible Cause

Entrapment of air or other gaseous species during reflow. The

air or other gaseous species could possibly have evolved from

the microvia in the PCB.

Potential Solution

Remove component and replace with new one.

Possible Cause

– Warpage of the BGA caused by thermal mechanical stresses

during reflow soldering.

– Check other corners for similar conditions.

– May be caused by frowning BGA interposer where the corner

becomes raised.

Potential Solution

– Increase ball size at corner or use glue-dot. In some

instances add in tape during the reflow process.

– Return package to supplier.

– Check coplanarity using shadow moiré at reflow temperature.

IPC-7095C January 2013

142

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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