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

Typical types of solder joints

PWB ILCs still cause many failures

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.

Laminate stack Laminate is now a single unified structure

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.

Detail view of hole drill damage

Drill Damage

Nail heading due to a dull drill bit

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.

Positive Etchback

Positive Etchback

Electroless copper plating Electroless copper plating covers entire board, especially drilled hole walls. Provides base for subsequent electrolytic copper.

Electrolytic copper plating This is the conductor layer of copper applied over the electroless copper.

PWB Microvia Failure

BGA Solder Joints

Traditional Sn-Pb eutectic joints Pb free solder joints

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

As BGAs get smaller they can be more prone to failure

Failure of microBGA joint

Do Pb-free and Pb/Sn mix well?

BGA Dye & Pry Test

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

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

Dye & Pry

Post-Pry inspection

Post-Pry inspection

Fatigue failure of Column Grid Array (CGA) solder joints

Flip Chip solder joints

Flip chip cracking due to flexure

Chip on chip with Au stud bumps

Au-Sn lid seal voids seen at x-ray; are they for real?

Au-Sn lid seal voids seen at x-ray; are they for real?

These voids are real!

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.

Solder fatigue in thru-hole joint

Thermal fatigue in gull-wing joints

Classic Solder Fatigue!

Temp cycle failure of a BGA joint

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…

Black Pad failure

Black Pad failure