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