Defect analysis workshop

Nordson Test and Inspection
Defect Analysis Workshop
John Travis

X-ray Inspection Criteria & Common Defect Analysis

4
Recognize &
Identify Common
Defects
Today's Mission

Why X-ray Inspect?
 Non-destructive test for optically hidden joints
(e.g. BGA’s, QFN’s, Connectors) & internal
package details (Ball Bonds, Wedge Bonds)
 Field failure analysis
 Process development
 Production quality control
 Set-up changes
 New Product Introduction (NPI)
 Verification for Rework

 High magnification at Oblique Views
 Application Support
 Digital Processing
 Sub Micron/High Power
 High resolution (1.3 MP or more)
 Good separation of similar grey levels within the
x-ray image (65,000 Levels)
 Oblique views around point of analysis (Rotation)
 Ease of use
Suggested Requirements for X-ray Inspection

7
NORMALVIEW What are we seeing Here??

8
OBLIQUEVIEW What Can we see Here??

9
PCB Pad
BGA
Ball
Solder Paste
Pad
PCB
2 x circles on the x-ray
image show BGA
connection are OK
PCB
Solder Paste
BGA
Ball
If open connection, the
circle on X-ray image
will not be seen
X-Ray
“OPEN Connection”
Inspection Logic

Proper BGA Wetting Principles
1. Solder ball is more dense than PCB
pad
2. If proper wetting occurs the lighter
density PCB pad shares its density
with the ball.
3. That being the case, the pad will
show up as a lighter color than the
ball if a homogenous bond is created.
4. If proper wetting does not occur then
the pad will carry it’s own density
properties and will show up as a dark
mass outside the perimeter of the
BGA.

When the two masses do
not share a common
density, the PCB pad is
exposed and an open
solder joint is detected as
indicated in the image on
the left.
BGA Open

 Strength of x-ray inspection is being able to look
for opens, voiding, fractures and generally monitor
production quality.
 Also - able to prove to your customer that the
reflow quality is good - BGAs are often forgiving of
process variations - incl. Pb-free
 Combat the frequently heard phrase from your
customer - “The boards you sent me do not work! It
must be how you soldered the BGA!”
Using XRAY for BGA Inspection

Common BGA Opens
 Cold Profile or Cold spots (visible solder-paste spheres)
 Hot Tears; caused by rapid cool down using lead free
paste
 Head in Pillow
 Exposed Pad on PCB……..Indicative of Profile not quite
hot enough to create an inter-metallic bond. Could also
point to contaminated components.
 Excessive Voiding Cracks or Fractures due to stress.
 Clogged Aperture on stencil
 Bridging

Copyright © Dage 2006
Transition from Hot to cold

21 | 00 Month 2009 | Confidential
Hot Tears or Fissures

Sample
Detector Rotates
X-ray
Tube
principle – Limited Angle CT or PCT
 2D images captured with detector
at an angle
 No sample size limitation as far
as it fits in the X-Ray sample tray-
No need to cut the sample
 Small area of the sample imaged
 Similar reconstruction method to
full µCT – produces a CT model
with 240 CT slices
 No Limitations compared to full
µCT
© NordsonDAGE 2012

Slices of BGA with Hot Tears

Importance of Identifying Void Location

Solder voiding only reduced the reliability when the voids
were located in the crack path

Types of Common Voids

Example of Champagne Voiding

Recommended BGA Inspection Procedure
 Always inspect at an Oblique Angle for Opens
 Check corners – shows lifting or Potato Chipping
 Work your way into the centre.
 This will Most likely to be the last point to reflow if any
substantial ∆T is present
 All terminations should be reasonably circular and
consistent in size
 Voiding, opens (non-reflow), HIP, Partial Wetting
 Inspect whole of BGA after rework – gives confidence in
the process

 Increasing voiding
 Check fillets
 Solder balling
TARGET ACCEPTABLE DEFECT
Inspection Criteria Chip
Components

 Increasing voiding > 20%
 Check fillets
 Insufficient solder
Inspection criteria (LGA)

 Heel fillet voiding
 Joints reflowed
 Insufficient solder
Inspection Criteria Gull Wings

 Increasing voiding
 Joints reflowed
 Missing interfaces
Inspection Criteria BGA

Inspection Criteria – Pin-in-Hole Fill
 Oblique view
 > 75% fill required
 Voiding in hole possible

Acceptable QFN Solder

Cracks & Breaks
 Copper tracks or in Solder Joints
 Die cracks:
 Fatigue from thermal cycling
 External stress (dropping)
 See an absence of material in x-ray image (from lower density)
 May need oblique view to see

Cracks & Breaks
Normal
(Top Down)
View
Oblique
View

Fillet Tearing in Lead-Free
Joints

These devices were
blowing up!
Dendrites

The standard micro sectioning methods
could not find the problem

Dendrites between Pad and lead-frame on SOT 89
The bond wire is 20 microns diameter
so the dendrites are only a few microns

Dendrites at High Magnification

CT Image of
Dendrites at High Magnification

Movie of Dendrites
at High Magnification

Dendrites

Defective Traces

Through Hole Solder

Defect analysis workshop

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