IPC-7095C-2013.pdf - 第111页

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 ef fects, etc.) and lowers the action level still furthe…

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

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this technology is the inability to view interior rows with

the same level of quality and clarity. It is sometimes pos-

sible to focus on interior solder joints but not at the same

level of detail as the exterior rows. It is usually not possible

to see paste on the second or third row of balls. The anal-

ogy would be the inability to see trees in the center of a

forest from the outside.

Lens design is a distinguishing feature of this technology.

The highly advanced lenses are able to focus and redirect

an image 90° using a mirror or prism. A high-resolution

CCD camera and monitor are used to capture and display

the image. Magniﬁcation, depending on working distance,

ranges from 50x to 200x (see Figures 7-35 and 7-36).

Lighting is a critical factor. The image quality will be poor

if the light source does not properly illuminate the solder

joint being inspected. Front lighting permits frontal inspec-

tion of a solder joint while back lighting is useful for

detecting solder shorts and other obstructions. Back light-

ing also displays the solder joint outline which makes it

easy to view the overall shape.

A robust positioning system that provides adequate support

and protection for the lens and CCD camera is essential. It

must eliminate motion due to shock and vibration and it

must be adjustable through the desired range of motion.

Analytical software is also desirable. In addition to display-

ing a real time image of the solder joint it is useful to have

features such as image capture and measurement. Some

systems provide reference photographs of acceptable and

unacceptable solder joints. These images can be compared

simultaneously with the image being evaluated, which

reduces subjectivity during inspection.

7.3.7.1 Protocols for BGA Assembly Reliability and

mechanical strength of SMT joints are very important,

especially in high reliability applications. Voids and other

defects may hurt thermal cycle reliability. However, there

is temperature cycling data that exhibit voids, improving

the test results.

Voids in solder joints are common and unavoidable at low

levels. Voids in BGAs are very difficult to characterize

quantitatively, nondestructively. X-ray is the only practical

solution, but it’s tricky!

Void quality protocols must apply to all assembly situa-

tions. Rework, even when ‘‘required,’’ can make matters

worse. In-process and lot-acceptance protocols must be

developed. Out-sourcing/extended supply-chains pose

additional challenges.

Several early studies document the inevitable occurrence of

voids. Testing included some void characterization of con-

ventional SMT joints, as well as an objective to link abnor-

mal failures to abnormal causes, (e.g., extreme voids).

Figure 7-34 Endoscope Example

Figure 7-35 Lead-Free 1.27 mm Pitch BGA Reﬂowed in

Nitrogen and Washed Between SMT Passes

Figure 7-36 Lead-Free BGA Reﬂowed in Air and Washed

Between SMT Passes

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