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ENIG Finishing and „Black Pad“ Phenomenon. Electronic Materials and Assembly Processes for Space Workshop – University of Portsmouth. G. Mozdzen, F. Rüdenauer, W. Costin, L. Schorn. Content. Metallographic investigations of PCBs with ENIG finishing with respect to “Black Pad” phenomenon. - PowerPoint PPT Presentation
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Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
Electronic Materials and Assembly Processes for Space Workshop – University of Portsmouth
G. Mozdzen, F. Rüdenauer, W. Costin, L. Schorn
ENIG Finishing and „Black Pad“ Phenomenon
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
Content
PCB – short time in use – detachment of BGA – failure analysis
-Investigation of BGA and corresponding area on PCB
-Investigation of conductive paths
Metallographic investigations of PCBs with ENIG finishing
with respect to “Black Pad” phenomenon
PCB after 100 thermal cycles in the temperature range between -10 and +60°C
PC boards have been delivered by ESA-ESTEC
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
Methods
High Resolution Scanning Electron Microscopy (HRSEM)
Light Microscopy (inverse metallographic microscope)
Energy Dispersive Spectroscopy (EDS)
Focused Ion Beam (FIB – CrossBeam Workstation)
HM 3/ 3 5h/ 600°C
0
100
200
300
400
500
600
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6
Di s tance (um)
Al Fe Cu Co W Ti
Al
C u
Fe
Ti W
C o
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
PCB BGA
Failure Analysis Assumption “Black Pad”
What is “Black Pad”?
BGA - Bonding traces on solder balls
PCB - Bonding traces on conductive pads
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
“Black Pad” – Definition
It causes poor solderability and consequently it is responsible for weakness of solder joints.
Origin of “Black Pad” - corrosion of Ni during the Nickel deposition (e.g. M. Walsh) or/and during the Gold Immersion process (e.g. R. A Bulwith, N. Biunno,..,) and formation of P-rich layer on the top of EN film. The corrosion process starts on the Nickel nodule boundaries causing spikes and propagates inside the nodules . On the corroded surface no or very poor IMCs developed during soldering. The corrosion due to IG process can be control by sufficient IG-bath parameters (e.g. pH, temperature, process duration).
“Black Pad” phenomenon – black or grey appearance of the conductive pads.
I: ENIG finishing – P enrichment on EN surface due to corrosion (Ni-bath, Au-bath)
II: Soldering - P- enrichment on the interface; EN-to-solder due to formation of IMCs
III: Solder joint - IMCs embedded in P-rich “underground” as a result of II or I+II
No “Black Pad” phenomenon
“Black Pad” phenomenon
Solder Joint and P - enrichment
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
BGA detached from PCB / EDS Surface Analysis
Location Element Incident Beam 20kV 10 kV Wt% Wt%
P 1 Ni 22 5 Sn 65 92 Pb 13 1 top of the
solder ball Si 1 P 8 45 Ni 88 15 top of the
pad Sn 4 40
20 kV – high activation volume
information from large volume
10 kV – low activation volume
information from area close to the surface
Q; How can we prove a P enrichment on the detached surfaces?
A; By using low and high accelerating voltages
PCB
BGA
Element Incident Beam 20kV 10 kV Wt% Wt%
P 1 Ni 22 5 Sn 65 92 Pb 13 1 Si 1 P 8 45 Ni 88 15 Sn 4 40
Conclusion
Failure (detachment) due to
“Black Pad” effect
To early for conclusions
an indication of “Black Pad”
but no an evidence
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
(At%)P 45 72Ni 15 12Sn 40 16
top of the pad10kV (Wt%)
Ni3P - Phase(At%)
2575
EDS Surface Analysis – Interface Composition
It has been reported that during reflow Ni3P has formed, consequently the black interlayer between IMCs and EN-film consists predominately of this phase (P. Snugovsky – Celestica Inc., Materials Science Lab.),
Q; Could we confirm such a finding taking into account our results of chemical analysis?
A; we can not confirm observation of P. Snugovsky
The top of EN film consists of P , only small amount of Ni3P possible
(At%)P 45 72Ni 15 12Sn 40 16
top of the pad10kV (Wt%)
Q; Did Sn enrich on the pads surface ?
A; According to EDS surface analysis - yes
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
BGA detached from PCB / Surface Analysis
PCBBGA
High density of elongated convex structures (width < 1µm)
High density of elongated concave structures (width < 1µm)
Elongated convex structures on the BGA side correlate with elongated concave structures on the PCB side
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
BGA detached from PCB / Surface Analysis Q; What is the chemical composition of the elongated structures?
A; BGA side – the elongated convex structures consist predominately of Pb.
BGA PCB
A; PCB side – the elongated concave structures are partly filled with Sn.
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
BGA – Solder Ball - Cross SectionQ; Do we observe IMCs ?
BGA Solder Ball
A; IMCs are present on the BGA surface, the width of IMC zone ~ 4 µm
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
~ NiSn (Ni3Sn4)
Q; Which phases have formed during
soldering?
~ Ni3Sn7
A; NiSn and Ni3Sn7 could be detected
by EDS analysis
Brittle phase of the type Me3Sn ((Ni+Au)3Sn) which is believed to be detrimental for the strength of the bonding has not been identified
BGA – Solder Ball - Cross Section
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
PCB - Cross Sections
The surface analysis revealed Sn inside the cavities as well as P and Sn enrichment on the surface (close to the surface)
Sn
Ni
Cu
P
Au
Pb
FIB cut
The elongated concave structures are partly filled with Sn.
P-enrichment close to the surface,
Presence of Sn on the surface
Q; Does analysis of the cross section confirm the results of the surface analysis ?
A; Results of the surface analysis have been confirmed by the results of the cross section analysis
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
BGA detached from PCB / Summary
BGA side
Presence of IMC – ~NiSn, ~Ni3Sn7
No presence of the brittle phase - Ni3Sn
High density of the elongated convex structures (width < 500nm)
Elongated convex structures are predominately composed of Pb
P-enrichment close to the EN surface (~300nm layer)
P rich layer may contain small amount of Ni3P
Presence of Sn on the surface (~ 100nm film)
Presence of elongated concave structures
Elongated concave structures are partly filled with Sn
PCB side
Elongated convex structures on the BGA side correlate with elongated concave structures on the PCB side
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
Additional Information needed for Interpretation and Conclusions
Investigation of conductive paths present on PCB
Area with middle stage of corrosion,
distinct nodule boundaries
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
Additional Information needed for Interpretation and Conclusions
Investigation of conductive paths present on PCB
Corroded nodule boundaries correlate with spikes - corrosion continuous from surface towards interior
FIB cut
Black bands (Black Pad phenomenon) below Au film visible
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
Investigation of conductive paths present on PCB
P-enrichment - “Black Pad” phenomenon occurs locally
Black bands (“Black Pad” phenomenon) below Au film <~ 100nm
Au film ~ 300nm
Conductive paths on the investigated PCB exhibit mostly an early stage of “Black Pad” phenomenon
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
BGA detached from PCB / Interpretation
Elongated concave structures on the surface of conductive pads correspond with spikes which have been formed due to corrosion during GI process.
Corrosion caused besides formation of the spikes also P -enrichment on the top of EN film - “Black Pad” phenomenon
Elongated structures are an indirect proof for “Black Pad” phenomenon because P-enrichment on the interface EN-to-solder could be an effect of IMC formation as well.
The separation of BGA from PCB refers to “Black Pad” phenomenon
This conclusion is supported by the “Black Pad (Band)” phenomenon which could be identified at the conductive paths from the corresponding board.
spike elongated structure
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
BGA detached from PCB / Mechanism
Corrosion due to EN deposition or/and GI process - formation of spikes and P rich layer of the top of conductive pads – “Black Pad” phenomenon
Soldering – extension of P-rich zone due to formation of IMCs, formation of a certain amount of Ni3P possible
IMCs embedded in a very weak “underground” – no or poor bonding
Reaction of Sn with Ni is impaired (stopped) because of P enrichment and of a deficit of Ni (small amount of Ni in P-rich layer is bonded to Ni3P)
Further diffusion of Pb and Sn towards the interface, (Sn diffusion rate higher than the Pb), Sn partly deposits on the interface; IMCs-to-EN and partly into the spikes.
Formation of elongated structures (convex – side of solder balls, concave- on the conductive pads) which provide a weak mechanical bonding between BGA and PCB
BGA detached from PCB during service due to e.g. even
very slight vibration
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
ENIG Finishing and „Black Pad“ Phenomenon/ Discussion
Strength of Bonding
Pros ConsComposition uniformity,
Corrosion resistance,
Good solderablility ,
Cheap, well established –opitimised, ……..,
Ni-bath, Au-bath – in order to avoid “Black Pad” phenomenon a huge number of parameters has to be balanced therefore the possibility of “Black Pad” is very high.
Early stage of “Black Pad” almost always present
Even an early stage of “Black Pad” can be very detrimental for a bonding strength if it sums up with P-enrichment caused by IMCs formation during soldering.
Formation of IMCs and bonding
strengthening
P-enrichment on the interface; IMCs-to-EN as a
natural effect of the bonding
Two coupled processes which are running in opposite directions
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
ENIG Finishing – example of „Black Pad“ Phenomenon
Example of locally occurred “Black Pad” phenomenon, smooth surface, spikes, no adhesion
PCB after thermal cycling (100 cycles -10 and + 60°C) Conductive pad after pulling off of an IC strand
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
„Black Pad“ Phenomenon - Conclusions
Even an early stage of “Black Pad” can be detrimental for the strength of a bonding if it sums up with P-enrichment caused by IMCs formation during soldering.
Early stage of corrosion (early stage of “Black Pad” phenomenon) can only be recognised at high magnification (SEM analysis necessary). This impedes significantly a quality control of PCB.
Formation of IMCs does assure a good bonding since even more or less well developed IMCs will separate from conductive pads if the interface between the IMCs and an EN film is weak (interface which predominately consists of P or of P + Ni3P).
Formation of interleaved elongated structures due to presence of spikes in an EN film (solder ball - convex, EN surface - concave) may simulate a good bonding. An assembly with such a bonding is difficult to sort out.
The risk of a failure of PCBs with ENIG finishing increases dramatically if the boards are exposed to vibration, mechanical shock or thermal cycling.
Detachment of BGA solder ball from Ni surface
Bruce Hough “ Solving the ENIG Black Pad Problem: An INTRI Report on Round 2”
Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen
AMAT – Advanced Materials and Aerospace Technology
Thank you for Attention