NASA-DoD Lead-Free Electronics Project
Automated X-Ray of Circuit Card Assemblies
Machine:
YESTech YTX-5000 model in-line x-ray machine
X-Ray Tube: Sealed reflection target
130 Kv, 5 micron spot size
39-watt max. output
X-Ray FOV 0.2” to 1.5” variable
Program settings:
All x-ray inspections are performed at 75kV and 40
A.
Inspection set-up:
QFN
1. Inspect each lead bank for joint presence and bridging
Threshold = 172
2. Inspect center pad for voids (flag if > 25%)
Threshold = 151
CSP-100
1. Inspect blocks of balls (5 x 5) for presence and bridging
Threshold = 120
2. Inspect shape of individual balls for consistency
Threshold = 120
Shape Limit = 1.45
3. Inspect size of individual balls
Threshold = 120
Size Range = .125 mm² - .175 mm²
Translates to Æ range of approx. 0.40 mm – 0.47 mm
Loose component ball size is 0.46 mm
Average x-ray ball size is 0.45 mm
4. Inspect individual balls for voiding (flag if > 10%)
Threshold = 100
BGA-225
1. Inspect blocks of balls (3 x 3 and 3 x 4) for presence and
bridging Threshold = 95
2. Inspect shape of individual balls for consistency
Threshold = 95
Shape Limit = 1.45
3. Inspect size of individual balls
Threshold = 95
Size Range = .400 mm² - .525 mm²
Translates to Æ range of approx. 0.71 mm – 0.82 mm
Loose component ball size is 0.75 mm
Average x-ray ball size is .81 mm
This range was extended to .390 mm² - .550 mm² as
various batches were run – a single batch tended to push
one end of the range or the other, but would not vary over
the entire range
4. Inspect individual balls for voiding (flag if > 10%)
Threshold = 75
Shape Limit: For a perfect circle, the shape value will be 1. As a shape deviates from a
perfect circle, the shape value will increase. It is unknown exactly how the value is
calculated, but the equipment/software vendor recommends using a shape limit of 2 or
less.
Ball Size: The measured size of the ball in the x-ray image is dependent on several factors:
· Original component ball size
· Amount of solder added at assembly
· Weight of component (squish factor)
· X-Ray parameters (power)
· Background (extent to which ball contrasts with surrounding image)
· Threshold value (ball edges are ‘feathered’, and affected by small changes in threshold)
With some degree of effort, the measured balls size from the x-ray image may be adjusted to match the actual ball size by manipulating the power and threshold settings. This is not necessary, however, and setting relative limits to detect defects on an optimized image is suitable. The limits are not necessarily at the defect level, however, and many balls flagged by the system were in fact not defects. The automated x-ray system applies inspection algorithms to binary images to determine the presence of defects. The inspections include:
· Looking for the expected number of distinct shapes within the inspection area,
within a range of the expected size, and at the expected locations (pitch)
· Checking the overall size (diameter / pixel count) of the shapes found
· Checking the ‘roundness’ of the shapes found
· Looking for and evaluating size of voids within those shapes found
The algorithm requires that a threshold value be set to define what is, and what is not, solder. This will be used to create the binary images from the raw x-ray images. This value is determined by the programmer.

download full report
http://acqp2.nasa.gov/LFS%20Reliability/...esults.pdf