IPC-7095C-2013.pdf - 第110页

inspection, then the precision of any subsequent measure- ments will be severely compromised. For example, assume a4x4 array of 0.75 mm diameter balls (pitch 1.5 mm) and a7x7 array of 0.30 mm diameter balls (pitch 0.5 mm…

the X-ray source, utilizing the peripheral portion of the

cone of radiation to produce an oblique view with the high-

est achievable magniﬁcation.

Figures 7-29 and 7-30 illustrate the beneﬁts of oblique

viewing at high magniﬁcation with actual images of a

FBGA comparing a top down transmission view with an

oblique view.

The images in Figure 7-29 may be slightly different in tex-

ture and darkness for a lead-free application. The tube

intensity and/or power may need to be slightly reduced in

order to avoid over exposure.

7.3.3.3 Cross-Sectional X-Ray Technologies On double

sided boards, some subset of the solder joints will be inac-

cessible to the transmission X-ray technique due to over-

lap, whereas the cross-section technique will have greater

test access. Because the transmission X-ray technique cap-

tures information from the entire solder volume and the

cross-section technique captures speciﬁc slice information,

these techniques have both unique and common capabili-

ties to detect some types of solder defects. For more

detailed information, contact the manufacturers of AXI sys-

tems who can provide information about the capabilities of

their techniques and systems.

Both tomosynthesis, Figure 7-31, and laminography, Figure

7-32, are radiographic techniques that provide image

‘‘slices’’ of the device that can be viewed. With laminogra-

phy, the X-ray source and the X-ray image plane are

moved in a coordinated way with respect to the electronic

device being inspected. A clear image of only one layer or

slice of the device appears; all other layers in the image

plane are blurred out. Tomosynthesis collects several trans-

mission X-ray images from different angles during the

inspection and combines the digital data of those images

mathematically.

This enables virtual slices to be created at the desired plane

for analysis.

7.3.4 Analysis of the X-Ray Image An understanding of

the construction of the particular BGA device and of the

reﬂow process will aid in the interpretation and analysis of

the X-ray image of the ball bonds. The concurrent factors

that should be considered for X-ray image analysis might

include:

• Determine if the balls are collapsible (eutectic) or noncol-

lapsible (noneutectic).

• Deﬁne if noncollapsible balls have been placed in the

corners to retain coplanarity.

• Was reﬂow temperature maintained sufficiently to permit

full alignment and collapse?

• Does BGA package appear to have physically deformed

in some way during reﬂow?

These factors will add further insight into the interpretation

and analysis of the X-ray image.

7.3.4.1 Field of View In determining the criteria for

inspections it is also important to determine how much can

be seen of the BGA at any one time. As the pitch of the

BGA gets smaller, the ball size is also reduced. Several

magniﬁcation levels can be applied to the evaluation. Table

7-5 provides the different pitch and ball size characteristics

applicable to any ﬁeld of view. The magniﬁcation range

varies; however, should be between 30X and 50X. Depend-

ing on the ball size, the ﬁeld of view can be determined by

simply dividing the number 15 by the ball size. Thus, 15

divided by 0.75 (for the 0.75 mm ball) results in being able

to see 20 balls of that size for evaluation. As the ball size

decreases, a greater number of ball images are viewed for

quality evaluations.

The display of the ﬁeld of view from a real-time x-ray sys-

tem depends on the number of pixels available from the

x-ray detector and how this is displayed on the operator

screen. This can vary from 640 x 480 pixels to 1600 x 1200

pixels, or more, in commercially available x-ray systems.

As one of the quality measurements that is used in this

document is voiding, unless a minimum number of pixels

is suggested for the diameter of the solder balls when under

Figure 7-29 Top Down View of FBGA Solder Joints

FBGA

Solder Joints

Top Down

View

Figure 7-30 Oblique View of FBGA Solder Joints

Three solder joints are clearly open (No. 2,3,4).

The void (No.1) is located near the component pad.

The solder joint above right (No.6) is in contact to the

board but with insufficient wetting (no meniscus).

FBGA

Solder Joints

Oblique View

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inspection, then the precision of any subsequent measure-

ments will be severely compromised. For example, assume

a4x4array of 0.75 mm diameter balls (pitch 1.5 mm) and

a7x7array of 0.30 mm diameter balls (pitch 0.5 mm) are

shown fully on screen (similar to that shown in the table).

On an X-ray system with a 640 x 480 pixel detector

(assuming 1:1 presentation on the display), then for the

shortest screen axis, each type of BGA ball will be 68 and

44 pixels in diameter respectively. With an error of linear

measurement of 2 pixels (one at either edge), for example,

each BGA has an error of 2/68 and 2/44, or 3% and 4.5%

respectively. Should these values then be used to calculate

the area of the balls for the crucial void percentage calcu-

lation, then the error in the area will be 9% and 20%

respectively. In contrast, the values for an x-ray system

with a 1600 x 1200 detector would be 171 and 109 pixels

with a linear error of 1.2% and 1.8% and an area error of

1.5% and 3.25% respectively. For Class 3 products, a

maximum of 4% voiding by area is proposed. In the worst

case above (but as devices continue to shrink then the

lower spec X-ray systems will progressively worsen) the

minimum action level through system measurement will

Figure 7-31 Tomosynthesis

IPC-7095c-7-32

Figure 7-32 Scanned Beam X-Ray Laminography

Rotating X-ray Beam

Rotating X-ray

Detector

Result

Focal Plane

Table 7-5 Field of View for Inspection

Nominal Ball Diameter (mm) Pitch (mm)

Magniﬁcation

Recommendation Field of View

0.75 1.50, 1.27

30X - 50X

15/0.75 = 20 Balls

0.60 1.00 15/0.60 = 25 Balls

0.50 1.00, 0.80 15/0.50 = 30 Balls

0.45 1.00, 0.80, 0.75 15/0.45 = 34 Balls

0.40 0.80, 0.75, 0.65 15/0.40 = 38 Balls

0.30 0.80, 0.75, 0.65, 0.50 15/0.30 = 50 Balls

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have to be nearer 3%, and this is before any other errors in

X-ray measurement are taken into account (from grayscale

sensitivity variation, geometric magniﬁcation effects, etc.)

and lowers the action level still further.

7.3.5 Scanning Acoustic Microscopy Scanning acoustic

microscopy (SAM), also called scanning acoustic tomogra-

phy (SAT), is a nondestructive failure analysis tool. It uses

sound waves to scan the internal layers of an assembly. It

is commonly used in the semiconductor packaging ﬁeld to

detect delamination or voids located inside an electronic

assembly. It can locate delamination or voids inside a BGA

package. It can also locate similar anomalies in underﬁll

after the BGA has been attached to the substrate.

The resolution of delamination or void detection depends

on the acoustic frequency used for analysis. Resolution

increases with increasing frequency. A 230 MHz transducer

can bring detection resolution down to around a 25 µm gap.

A single point observation is called a SAM, a line scan is

called a b-SAM, and an area scan is called a c-SAM. The

c-SAM image in Figure 7-33 shows the location of voids

in underﬁll in a ﬂip chip assembly. The sample being

inspected needs to be in water during the SAM analysis.

Voids or delaminations that are open to water ingress can-

not be detected with this method.

7.3.6 BGA Standoff Measurement Feeler gauges pro-

vide a nondestructive method of determining the approxi-

mate ﬁnished standoff of the BGA after reﬂow. Feeler

gauges can be used at each corner after reﬂow and the

combined results can be used to determine an average

standoff measurement. This method is not as accurate as

cross-sectioning, but it is much less expensive and nonde-

structive provided that the operator does not attempt to

force a feeler gauge under the device. Doing so could result

in breaking solder joints. It does require adequate space

around the BGA for the feeler gauges to be inserted. The

standoff height of a BGA can give some indication that the

solder balls reﬂowed completely and uniformly. The stand-

off of a typical PBGA with a 0.75 mm ball is approxi-

mately 0.60 mm prior to reﬂow and drops to 0.45 mm

(including solder paste) after reﬂow. Other BGA packages

have their own stand-off height characteristics based on the

ball size, alloy used, and whether the BGA contains a metal

heat spreader. Since each package has its own stand-off

characteristics, users should develop a part assembly pro-

ﬁle in order to make the use of feeler gauges applicable.

7.3.7 Optical Inspection Endoscopy is an optical inspec-

tion method that permits visual inspection of tiny objects in

a small, conﬁned area. This technology has been adapted

and applied to BGA solder joint inspection. BGA solder

joints can be inspected and analyzed for a variety of criti-

cal factors such as:

• Overall Solder Joint Quality – evidence of proper wetting

• Solder Joint Shape – evidence of proper reﬂow

• Solder Joint Surface Texture – smooth vs. irregular

• Overall Solder Joint Appearance - ﬂux residue, etc.

• Solder Joint Defects - solder shorts, opens, cold solder

• Missing balls

This technology is best suited for inspecting exterior row

BGA solder joints as shown in Figure 7-34. A limitation of

IPC-7095c-7-33

Figure 7-33 Scanning Acoustic Microscopy

Reflected

Pulse

Fluid

Input

Pulse

Sample

- Application

Zone

T - Transducer

- Ultrasound

January 2013 IPC-7095C

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