Solder Joint & Interconnect Technology and Failure Analysis

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Failure Analysis of Solder Joints
and Circuit Boards
By: Roger Devaney
Hi-Rel Laboratories
1
Typical types of solder joints
2
PWB ILCs still cause many failures
3
Laminate stack up
• A layer of uncured prepreg is placed on each
side.
• Outer cores with the internal layers patterned
are laid up in alignment jig.
• Layers are laminated under heat and pressure.
4
Laminate stack
• Laminate is now a single unified structure
5
Via holes are drilled
• Precision tungsten carbide drill bits are used to
drill holes where needed.
• Drilling results in a damaged layer that must be
removed by etching.
6
Detail view of hole drill damage
7
Drill Damage
8
Nail heading due to a dull drill bit
9
Drill damage removed by etching
• A “witches brew” of HF and H2SO4 is used to
removed damaged glass fibers and smeared
epoxy resin.
• Very critical step to ensure via reliability.
10
Positive Etchback
11
Positive Etchback
12
Electroless copper plating
• Electroless copper plating covers entire board, especially
drilled hole walls.
• Provides base for subsequent electrolytic copper.
13
Electrolytic copper plating
• This is the conductor layer of copper applied
over the electroless copper.
14
PWB Microvia Failure
15
BGA Solder Joints
16
Traditional Sn-Pb eutectic joints
Pb free solder joints
17
Head-on pillow (HoP) BGA joint
HoP is caused by:
•Solder paste printing and rheology issues
•Reflow temperature uneven or too low
•Board warping during reflow
•Process out of control
18
19
As BGAs get smaller they can be
more prone to failure
20
Failure of microBGA joint
21
Do Pb-free and Pb/Sn mix well?
22
BGA Dye & Pry Test
23
Dye & Pry Testing
• This is a quick/inexpensive way to look for
cracked or non-wetted BGA joints.
• Allows for simultaneous inspection of all of
the joints at once.
• Materials and equipment needed are
readily available
24
Dye & Pry Test Procedure
• Cut out device to be tested from the PCB
• Clean flux from under device and bake dry
• Immerse part in Dykem Red fluid under
partial vacuum
• Shake off excess dye and bake dry
• Pry off BGA using pliers and/or vise and
screwdriver
• Inspect for any dye on separated joints
25
Dye & Pry
26
Post-Pry inspection
27
Post-Pry inspection
28
Fatigue failure of Column Grid Array (CGA) solder joints
29
Flip Chip solder joints
30
Flip chip cracking due to flexure
31
Chip on chip with Au stud bumps
32
Au-Sn lid seal voids seen at x-ray; are they for real?
33
Au-Sn lid seal voids seen at x-ray; are they for real?
34
These voids are real!
35
Thermal Fatigue
• When the assembly is temperature or power
cycled the different materials in the attach want
to expand/contract according to their CTE’s.
• The attach material is (usually) the weakest
point in the assembly so it is expected to absorb
the stresses of thermal mismatch by yielding in
creep.
• The amount of creep an attach can endure is
limited, then it will begin to crack.
36
Solder fatigue in thru-hole joint
37
Thermal fatigue in gull-wing joints
38
Classic Solder Fatigue!
39
Temp cycle failure of a BGA joint
40
Black Pad failure
This only occurs on Electroless Nickel, Immersion Gold
(ENIG) finished devices & boards:
• ENIG has come into wide use with the advent of RoHS and the leadfree solders
• The ENIG process actually “corrodes” the top layer of the electroless
nickel-phosphorous as the gold is deposited in a displacement reaction
• This displacement reaction concentrates the phosphorous in the upper
nickel layer right under the gold, and sometimes gets out of control
• Normal electroless nickel will have 8-12% P, but black pad regions can
have up to 30% P!
• During soldering the very thin gold layer dissolves instantly leaving the
solder on top of the corroded, high P, nickel layer.
• This can result in dewetting and/or poor solder joint strength
• When the solder joints fail, the corroded nickel layer is exposed and it is
usually black in appearance; hence the name…
41
Black Pad failure
42
Black Pad failure
43
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