IPC-7095C-2013.pdf - 第156页

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 o…

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

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

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

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

9.3.2 Solder Joint Opens Due to Interposer Warp

9.4 Solder Joint Conditions

The following sections deal

with the conditions of the solder ball in relationship to the

mounting structure and the component interposer. In each

instance, an explanation is provided as to the reason for the

occurrence of the condition.

9.4.1 Target Solder Condition

9.4.2 Solder Balls with Excessive Oxide

Possible Cause

– The failure appears to be due to insufficient paste release

from the stencil or insufficient ball size.

– Warped package (smiling BGA) with corner balls lifted up.

Potential Solution

– Check balls at incoming prior to assembly.

– Return package to supplier.

– Implement incoming inspection and/or source audit.

– Apply extra solder paste on corner lands.

Possible Cause

Solder balls exhibit a uniform shape and texture, and they are

symmetrically aligned to the attachment sites, showing ball col-

lapse on both top and bottom of BGA balls.

Potential Solution

Document process parameters and maintain a stable process.

Possible Cause

Solder ball oxides will form due to exposure to multiple reflow

solder cycles (topside or bottom side).

Potential Solution

This can be reduced significantly by correct flux selection,

reducing the number of production reflows to two and not

cleaning the PWB between reflows.

January 2013 IPC-7095C

143

Provided by IHS under license with IPC

Not for Resale

No reproduction or networking permitted without license from IHS

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