AMAT – Advanced Materials and Aerospace Technology

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

Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen

AMAT – Advanced Materials and Aerospace Technology

Content

Metallographic investigations of PCBs with ENIG finishing with respect to “Black Pad” phenomenon PCB – short time in use – detachment of BGA – failure analysis -Investigation of BGA and corresponding area on PCB -Investigation of conductive paths 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

Light Microscopy (inverse metallographic microscope) High Resolution Scanning Electron Microscopy (HRSEM) Energy Dispersive Spectroscopy (EDS) Focused Ion Beam (FIB – CrossBeam Workstation) Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen

PCB

Failure Analysis

AMAT – Advanced Materials and Aerospace Technology

Assumption “Black Pad”

BGA What is “Black Pad”?

PCB - Bonding traces on conductive pads Annual ESA School Meeting, Portsmouth University, 17- 02-10 BGA - Bonding traces on solder balls Grazyna Mozdzen

AMAT – Advanced Materials and Aerospace Technology

“Black Pad” – Definition

“Black Pad” phenomenon

– black or grey appearance of the conductive pads. 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).

II: Soldering -

P enrichment on the interface; EN-to-solder due to formation of IMCs

No “Black Pad” phenomenon III: Solder joint -

IMCs embedded in P rich “underground” as a result of

II

or

I+II I: ENIG finishing

– P enrichment on EN surface due to corrosion (Ni-bath, Au-bath)

“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

Q;

How can we prove a P enrichment on the detached surfaces?

A;

By using low and high accelerating voltages PCB

20 kV – high activation volume information from large volume 10 kV – low activation volume information from area close to the surface

Annual ESA School Meeting, Portsmouth University, 17- 02-10 BGA top of the solder ball top of the pad To early for conclusions “Black Pad” effect Grazyna Mozdzen

AMAT – Advanced Materials and Aerospace Technology

EDS Surface Analysis – Interface Composition

It has been reported that during reflow Ni 3 P 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?

top of the pad

P Ni Sn 10kV (Wt%)

45 15 40

(At%)

72 12 16 Ni 3 P - Phase

(At%)

25 75 Q;

Did Sn enrich on the pads surface ?

top of the pad

P Ni Sn 10kV (Wt%)

45 15 40

(At%)

72 12 16 A;

we can not confirm observation of

P. Snugovsky

The top of EN film consists of P , only small amount of Ni 3 P possible

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

BGA PCB Elongated convex structures on the BGA side correlate with elongated concave structures on the PCB side High density of elongated convex (width < 1µm) structures Annual ESA School Meeting, Portsmouth University, 17- 02-10 High density of elongated concave structures (width < 1µm) Grazyna Mozdzen

AMAT – Advanced Materials and Aerospace Technology

BGA detached from PCB / Surface Analysis

Q;

What is the chemical composition of the elongated structures?

BGA PCB

A;

BGA side – the elongated convex structures consist predominately of Pb. Annual ESA School Meeting, Portsmouth University, 17- 02-10

A;

PCB side – the elongated concave structures are partly filled with Sn. Grazyna Mozdzen

Q;

Do we observe IMCs ?

BGA Solder Ball AMAT – Advanced Materials and Aerospace Technology

BGA – Solder Ball - Cross Section

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

BGA – Solder Ball - Cross Section

Q;

Which phases have formed during soldering?

~ NiSn (

Ni 3 Sn 4

) ~ Ni 3 Sn 7

A;

NiSn and Ni 3 Sn 7 could be detected by EDS analysis Brittle phase of the type Me 3 Sn ((Ni+Au) 3 Sn) which is believed to be detrimental for the strength of the bonding has not been identified 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)

Q;

Does analysis of the cross section confirm the results of the surface analysis ?

FIB cut

A;

Results of the surface analysis have been confirmed by the results of the cross section analysis

Sn Ni Cu P Au Pb

The elongated concave structures are partly filled with Sn. P-enrichment close to the surface, Presence of Sn on the surface 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, ~Ni 3 Sn 7 No presence of the brittle phase - Ni 3 Sn High density of the elongated convex structures (width < 500nm) Elongated convex structures are predominately composed of Pb

PCB side

P-enrichment close to the EN surface (~300nm layer) P rich layer may contain small amount of Ni 3 P Presence of Sn on the surface (~ 100nm film) Presence of elongated concave structures Elongated concave structures are partly filled with Sn 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

FIB cut Corroded nodule boundaries correlate with spikes - corrosion continuous from surface towards interior 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

Conductive paths on the investigated PCB exhibit mostly an early stage of “Black Pad” phenomenon Au film ~ 300nm P-enrichment “Black Pad” phenomenon occurs locally Annual ESA School Meeting, Portsmouth University, 17- 02-10 Black bands (“Black Pad” phenomenon) below Au film <~ 100nm Grazyna Mozdzen

AMAT – Advanced Materials and Aerospace Technology

BGA detached from PCB / Interpretation

spike elongated structure Elongated concave structures on the surface of conductive pads correspond have been with spikes which formed due corrosion during GI process.

to caused besides formation of the spikes also P enrichment on the top of EN film -

identified at the conductive paths from the corresponding board.

Elongated indirect structures proof for are an “Black Pad” phenomenon

because P-enrichment on the interface EN-to-solder could be an effect of IMC formation as well.

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 conductive pads – “Black Pad” phenomenon the top of  Soldering – extension of P-rich zone due to formation of IMCs, formation of a certain possible amount of Ni 3 P  IMCs embedded in a very weak or poor bonding “underground” – no  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 Ni 3 P)  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 PCB – side of solder balls, concave- on the conductive pads) which provide a weak mechanical bonding between BGA and Annual ESA School Meeting, Portsmouth University, 17- 02-10

BGA detached from PCB during service due to e.g. even very slight vibration

Grazyna Mozdzen

AMAT – Advanced Materials and Aerospace Technology

ENIG Finishing and „Black Pad“ Phenomenon/ Discussion

Pros

Composition uniformity, Corrosion resistance, Good solderablility ,

Cons

Ni-bath, Au-bath “Black number Pad” of – in order to phenomenon parameters has a to avoid huge be balanced therefore the possibility of “Black Pad” is very high.

Cheap, well established – opitimised, ……..,

Early stage of always present “Black Pad” almost

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.

Two coupled processes which are running in opposite directions Formation of IMCs and bonding strengthening P-enrichment on the interface; IMCs-to-EN as a natural effect of the bonding

Strength of Bonding Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen

AMAT – Advanced Materials and Aerospace Technology

ENIG Finishing – example of „Black Pad“ Phenomenon

PCB after thermal cycling (100 cycles -10 and + 60 °C)

Conductive pad after pulling off of an IC strand Example of locally occurred “Black Pad” phenomenon, smooth surface, spikes, no adhesion 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 recognised at high magnification “Black Pad” phenomenon) can only be (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 + Ni 3 P).

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 boards are exposed to increases dramatically if the 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

Annual ESA School Meeting, Portsmouth University, 17- 02-10 Grazyna Mozdzen