1 Failure Analysis of Solder Joints and Circuit Boards By: Roger Devaney Hi-Rel Laboratories.

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Failure Analysis of Solder Joints and Circuit Boards

By: Roger DevaneyHi-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

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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 lead-free 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