Failure Analysis of Solder Joints
and Circuit Boards
By: Roger Devaney
Hi-Rel Laboratories
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Typical types of solder joints
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PWB ILCs still cause many failures
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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.
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Laminate stack
• Laminate is now a single unified structure
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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.
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Detail view of hole drill damage
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Drill Damage
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Nail heading due to a dull drill bit
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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.
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Positive Etchback
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Positive Etchback
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Electroless copper plating
• Electroless copper plating covers entire board, especially
drilled hole walls.
• Provides base for subsequent electrolytic copper.
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Electrolytic copper plating
• This is the conductor layer of copper applied
over the electroless copper.
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PWB Microvia Failure
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BGA Solder Joints
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Traditional Sn-Pb eutectic joints
Pb free solder joints
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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
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As BGAs get smaller they can be
more prone to failure
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Failure of microBGA joint
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Do Pb-free and Pb/Sn mix well?
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BGA Dye & Pry Test
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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
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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
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Dye & Pry
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Post-Pry inspection
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Post-Pry inspection
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Fatigue failure of Column Grid Array (CGA) solder joints
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Flip Chip solder joints
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Flip chip cracking due to flexure
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Chip on chip with Au stud bumps
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Au-Sn lid seal voids seen at x-ray; are they for real?
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Au-Sn lid seal voids seen at x-ray; are they for real?
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These voids are real!
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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.
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Solder fatigue in thru-hole joint
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Thermal fatigue in gull-wing joints
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Classic Solder Fatigue!
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Temp cycle failure of a BGA joint
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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…
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Black Pad failure
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Black Pad failure
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