The Root Cause of Black Pad

7/29/2019 The Root Cause of Black Pad

1/52006 June JOM 75

Lead-Free SolderResearch Summary

This paper reports on a study of the

reaction of solder with theelectroless-

nickel with immersion gold (ENIG) plat-

ing system, and the resulting interfacial

structures. A focused-ion beam (FIB) was

used to polish the cross sections to reveal

details of the microstructure of the ENIG-

plated pad with and without soldering.

High-speed pull testing of solder jointswas performed to expose the pad surface.

Results of scanning-electron microscopy/

energy-dispersive x-ray analysis of the

cross sections and fractured pad surfaces

support the suggestion that black pad is

the result of galvanic hyper-corrosion of

the plated electroless nickel by the gold

plating bath. Criteria are proposed for

diagnosing black pad of ENIG plating.

INTRODUCTION

The plating system of electroless

nickel with immersion gold (ENIG) has

been widely used to finish solder pads of

printed circuit boards (PCBs), as well as

ball-grid array (BGA) and flip chip sub-

strates. It wets well by solder,13provides

a flat and uniform surface, and shows

high via strength, an important design

The Root Cause of Black PadFailure of Solder Joints withElectroless Ni/Immersion Gold Plating

Kejun Zeng, Roger Stierman, Don Abbott, and Masood Murtuza

consideration for thick PCBs with high

aspect ratio vias.4,5 Electroless nickel

plating Ni(P) often has a lower total cost

of ownership than electrolytic nickel

plating. The most attractive advantage

of ENIG over electrolytic Ni/Au plat-

ing is that it can be applied to fine-pitch

BGA substrates without complicating

the design layout.6,7

Any electrolyticprocess requires electric connection to

each pad. If the pitch is too small, the

electric connection (bussing) is difficult,

and processing costs become prohibitive.

Therefore, electrolytic Ni/Au is used

only for substrates with a sufficiently

large conductor pitch to permit busses

to each pad. Another disadvantage of

electrolytic Ni/Au plating is thickness

variation. The thickness of electrolytic

plating is sensitive to current density,

the voltage drop over the conductors,

and the geometry of the metal surface.

On some designs, thickness variations

can be as much as 1 m in the nickel

(for a nominal 5 m nickel thickness

specification) and 0.2 m in the gold

(for nominal 0.7m gold thickness). The

upper end of this gold thickness may

cause gold embrittlement in fine-pitch

BGA joints.8,9 For ENIG, the thickness

of both nickel and gold is much better

controlled. Usually, thickness is 50.5

m for electroless Ni(P) and 0.10.02

m or less for immersion gold.

However, ENIG finishes have exhib-

ited a black pad defect that can cause

brittle fracture at the interface between

the solder and metal pad.4,5,1016 The

failure typically occurs during mechan-

ical or thermal-mechanical testing. The

worst cases are BGA package solder

joint failure during a customers surface

mount assembly process, or in the

products final use by a consumer. To

the unaided eye, a solder joint that fails

from black pad shows a flat pad where

the solder ball separated from the pad.

Under an optical microscope, the flat

pad surface is observed to have little or

no solder remaining on it. In a scanning-

electron microscope (SEM), some small

crystals of tin-bearing intermetallic

compounds (IMCs) may be found on the

pad surface. However, no evidence for

the ductile fracture of the solder can beobserved. In cross sections of the failed

joint, Ni3Sn

4(for SnPb solder joints) or

Cu6Sn

5(for SnAgCu solder joints) is

found on the solder side, but a phospho-

rous content higher than that of the Ni(P)

plating is detected on the pad side.

Because of this observed high phospho-

rous content, many in the industry hold

that the ENIG black pad defect solder

joint failure is caused by the phosphorous

Figure 1. A 30-degree tilt view of a blackpad. The pad surface appeared clean.Only a small amount of fine IMC particles(gray) and little solder residue were present

on the pad.

50 m

50 m

a

b

Figure 2. (a) The top view and (b) side viewof mud-cracks in the entire pad surface.


2/5JOM June 200676

EXPERIMENTAL PROCEDURES

The test vehicle in this study was a substrate with SnPb solder balls and electrolessnickel with immersion gold plated pads. Substrates from two vendors, A and B, wereused in the investigation. It was known before the investigation that substrate A exhibitedblack pad but substrate B did not. Solder balls were attached to the pads by a standardreflow process for eutectic SnPb solder. Before thermo-mechanical testing, samples weresubjected to a standard preconditioning test that included 20 h of burn-in at 125C and

three reflows at 235C. The thermo-mechanical test of the samples was standard thermalcycling between 55C and +125C.

After mechanical testing, both the fractured pads and cross sections of failed jointswere examined by scanning-electron microscopy (SEM) (JEOL 840). A quantitativeanalysis of the compositions of the phases involved was performed using a silicon-drifted lithium energy-dispersive x-ray analyzer (EDX), with the SEM running at 20kV. The software for quantitative analysis of phase compositions was Iridium by iXRFSystems. Quantitative calibration of the software was carried out using National Instituteof Standards and Technology-traceable standards. The interfacial phases were identifiedby their atomic ratios that were determined by EDX analysis.

An FEI-830 dual-beam focused-ion beam (FIB) was used to polish the mechanicallypolished cross sections so that no details of the microstructure would be hidden bysmeared materials on the polished surface. The polished cross sections for microanalysiswere not chemically etched. For some samples, no mechanical grinding was carried out,but cross sections were made by FIB cutting.

content of the Ni(P) plating.

The purpose of this paper is to clear

the confusion about the solder joint

failure caused by ENIG black pad defect.

The authors will demonstrate that a high

phosphorous content by itself cannot be

taken as evidence for black pad, and the

origin of black pad is not in the solder

or soldering process. Criteria will bedefined for identification of black pad

failure.

See the sidebar for experimental pro-

cedures.

RESULTS

Figure 1 presents an ENIG-plated pad

where the solder ball fell off after thermal

cycling. Though there were some bright

IMC crystals on the pad, there was virtu-

ally no solder residue. In a tilt view, the

pad appeared flat. At higher magnifica-

tion, a feature that looked like the

boundaries of the plating nodules was

observed in a top view (Figure 2a). In

the 30-degree tilt view in Figure 2b, the

boundary-like features appeared as

separations from the plating nodules,

hereafter called mud-cracks because of

their appearance. The pad surface was

rather clean. Energy-dispersive x-ray

(EDX) analysis of the area in Figure 2a,

which was about 765m2, found 81.0Ni,

5.2Sn, and 13.8P (wt.%). Assuming that

all the signals of tin were from the few

bright Ni3Sn

4crystals, and neglecting

this solder residue on the pad surface,

the atomic ratio of Ni:P in the pad surface

was calculated from these data to be

75.1:24.8, very close to the stoichiom-

etry of Ni3P. This indicates that the joint

was broken between Ni3P and Ni

3Sn

4,

and the mud-cracks were in the Ni3P

layer. In a cross section of the pad, the

mud-cracks shown in Figure 2 appeared

as spikes in the pad surface (Figure 3).

This is the conclusive evidence that the

ENIG plating had the black pad

defect.

To find out whether or not the mud-

cracks in the pad surface were created

by soldering, the cross section of the

as-received substrates with ENIG plating

from vendors A and B were polished by

FIB. It was found that the known-bad

substrate from vendor A had spikes inthe pad surface. The spikes were clearly

observed at 10,000X (Figure 4a). In

contrast, no such defects were found in

substrate B even at 35,000X. A thin layer

of gold plating was seen on the top

surface and the boundaries between

plating nodules were free of defects (see

Figure 4b). The details of the defect

region of substrate A are revealed in

Figure 5. It can be seen that the feather-

like structure around the spike or crack

is different from the normal Ni(P) plat-

ing. The defect region consists of two

different areas: a bright core with dark

surrounding material. Elemental map-

ping by EDX shows that the bright core

is rich in gold and the phosphorous

content in the defect region is higher

than the Ni(P) plating (Figure 5).

A typical ENIG-plated substrate from

vendor A that has not been subjected to

soldering is presented in Figure 6. The

pad had severe defect regions at the

periphery. In a top view, the boundaries

of the plating nodules appeared gray and

wide, decorated by a dark material, prob-

1 m

Figure 3. A cross section throughseveral cracks. Spikes in the crosssection are conclusive evidence for

black pad.5 m

2 m

a

b

Figure 4. The FIB cutting of the as-receivedENIG platings (without solder ing). (a) Mud-cracks are clearly seen in a black pad at10,000X. (b) No such defect was foundin a known-good ENIG plating at even amuch higher magnification.

- -


3/52006 June JOM 77

ably phosphorous. In the side view, the

boundaries looked deep. One boundary

in the photo was so deep that it looked

like a crack, as indicated by the arrow

in Figure 6b. Cross sectioning of this

area by FIB found that in addition to the

spikes between the plating nodules, the

nodules were porous (Figure 7).

Figure 8 presents another BGA pad

from vendor A. The boundary of thenodule at the edge of the pad has been

completely cracked. The neighboring

nodule has a large defect region, and

gold has penetrated into the crack as

indicated by the arrow (Figure 8a).

Nodules such as this will likely fall off

under low shear stress. Indeed, missing

nodules were observed in the substrates

from vendor A, as indicated by the arrow

in Figure 8b.

DISCUSSION

Diagnosis of Black Pad

When the black pad defect of ENIG

plating causes a solder joint failure, usu-

ally a high phosphorous content is

detected at the fractured pad surface.

Because of this, a high phosphorous

content at the pad surface is often citedin failure analysis reports as the evidence

for black pad and, accordingly, a lower

phosphorous content of the Ni(P) plating

is recommended as a fix. In fact, a phos-

phorous content of about 15 wt.% (25

at.%), higher than the original Ni(P)

plating deposit, is expected after solder-

ing, no matter whether the ENIG plating

suffers from black pad defect or not. To

understand this, we need to know the

solder reaction with the ENIG plating.

When solder melts on ENIG plating,the gold plating quickly dissolves into

the molten solder and tin will be in direct

contact with the Ni(P) plating.17,18

Depending on the type of solder, a layer

of IMC will form at the interface after

cooling. The IMC layer is Ni3Sn

4for the

joints of SnPb1,1921 and SnAg,22,23 but

(Cu,Ni)6Sn

5for SnAgCu solder joints.24

26 The solder reaction enhances the

crystallization of the amorphous Ni(P)

plating.19,27,28 For many commercially

produced substrates, the phosphorous

content in the Ni(P) plating is in the range

of 710 wt.%.29 After crystallization, the

amorphous Ni(P) plating in this compo-

sition range is converted into the mixture

of nickel and Ni3P.30,31 The nickel atoms

will be taken into the crystals of

(Cu,Ni)6Sn

5or react with tin to form

Ni3Sn

4. Therefore, after reflow, the crys-

tallized portion of the Ni(P) plating will

become Ni3P that has 15 wt.% (25 at.%)

phosphorous.19,32 Between these two

compound layers, there is another very

thin layer, about 100 nm thick, contain-

ing nickel, tin, and phosphorous.20,26,3338

The composition of this layer has not

been agreed upon yet in the literature.

For convenience, it will be referred to

as Ni3SnP according to Reference 36.

When the ENIG black pad defect causes

solder ball failure, the solder ball is usu-

ally separated from the pad between the

IMC layer (Ni3Sn

4or (Cu,Ni)

6Sn

5) and

the Ni3P layer. Thus, if the pad surface

is analyzed by EDX, the signals detected

will be mainly from the Ni3P layer and

a phosphorous content of about 15 wt.%

or 25 at.% is expected. This high phos-

phorous content in the fractured pad

surface, by itself, is not good evidence

for black pad defect.

Based on the authors experience and

the literature data, the following criteria

are proposed for identifying the black

pad defect of the ENIG plating. At low

magnification, either by optical micro-

scope or SEM, the failed pad appears

flat. There is very little solder, or no

solder, remaining on the pad (Figure 1).

At high magnification under SEM, the

pad appears dark. Some isolated IMC

crystals or solder residues may be visible,

but the pad is not covered by solder or

IMC layer. Nodule boundaries are clearly

seen in a top view of the pad surface

(Figure 2a). To verify that the nodule

boundaries are separated, approximately

30 degrees of sample tilt of the pad

surface should be taken (e.g., Figure 2b)

to view the mud-cracks. To obtain con-

clusive evidence, cross section the pad

to reveal spikes in the pad surface (Figure

3). When spikes are observed in the cross

section and they can be correlated to the

500 nma

b

c

Figure 5. (a) The FIB cutting of a mud-crack in the as-plated substrate (withoutsoldering) from vendor A. Elementalmapping indicates that (b) the corrodedarea is rich in phosphorous and (c) thegold atoms have penetrated into the corevolume of the corroded area.

2.0 ma

2.0 mb

Figure 6. (a) The top view and (b) side viewof defected boundaries of the Ni(P) platingnodules from vendor A. The sample wasas-received without soldering. A deep andwide groove is indicated by the black arrow.A dark substance is seen on the walls ofthe deep boundaries.


4/5JOM June 200678

1 m

Figure 7. An FIB cutting through thedefected peripheral area of the pad inFigure 6. Deep corrosion of the Ni(P)plating was revealed.

500 nma

5.0 nmb

Figure 8. (a) An FIB cutting of the padperiphery without soldering. The Ni(P) layerwas cracked and a large area beneath thecracks was defected. (b) The top view ofthe pad periphery after the solder ball waspulled off. Samples from vendor A.

mud-cracks in the tilted view of the pad

surface, then it can be concluded that the

ENIG plating has black pad defects.

It should be pointed out that while the

present work uses cross sections polished

by FIB, the spikes can also be revealedby traditional metallographic tech-

niques.39,40

Origin of Black Pad Defect

Since the black pad defect is usually

found when the BGA package falls off

the PCB during assembly, it is logical to

suspect that soldering caused the black

pad defect. However, this was not found

to be true. The as-received (before sol-

dering) substrates from two vendors, A

and B, were analyzed. Before analysis,

it was known that the ENIG plating by

vendor A had black pad defect but that

of vendor B did not. Cross sections in

Figure 4 show that A had spikes that

corresponded to mud-cracks, but even

at much higher magnification B did not

show any defect and the nodule bound-

aries were good.

The root cause for the black pad defect

of the ENIG plating has been discussed

in the literature and several models have

been proposed.4,7,13,19,4144 Of these

models, Biunnos42 is supported by the

results of the present work. His model

suggests that black pad defect is the result

of galvanic hyper-corrosion of the Ni(P)

plating by the immersion gold bath.

Earlier experimental results confirmed

the validity of this model.7,40,45 The

immersion gold process is a controlled

corrosion (displacement) process during

which nickel atoms on the surface of the

Ni(P) plating are replaced by gold atoms.7

In principle, it is a self-limiting process

because once the surface of the Ni(P)

plating is covered by the gold, the dis-

placement reaction stops. However, if

the process is out of control, hyperactive

corrosion may happen. For instance, the

surface of the electroless Ni(P) plating

has a nodular structure. There are bound-

aries and crevices between the nodules.46

If a boundary or crevice is too deep and

thus the supply of gold atoms to the

crevice is slowed down, the gold con-centration in the crevice will be different

from that of the plating bath. Conse-

quently, a galvanic cell will be set up

between the crevice and the surface,

resulting in heavy corrosion in the crev-

ice.

Also, reducing agents can be added

to immersion gold baths to deposit the

gold more quickly. A poor choice of or

poor control of these reducing agents

may produce the inconsistent nature of

the black pad defect (i.e., not every padon a substrate shows the same degree of

defects). Figure 5 provides evidence for

this model. The boundary between the

two nodules is voided and its opening to

the surface is very narrow. The consumed

gold of the plating solution in the void

cannot be replenished quickly. The

concentration difference leads to gal-

vanic corrosion of the Ni(P) plating

around the void. The corrosion converts

the dense, amorphous Ni(P) into a

porous, micro-crystallized structure into

which the gold atoms have penetrated,

shown by the elemental mapping images

in Figure 5.

Another fact that should be noted is

that, because of the depth of the void

and the near-closure of its opening to

the surface, after the plating process the

plating solution was trapped in it. The

rinsing process after plating could not

effectively remove the residual plating

solution. Therefore, corrosion would

continue until the residual solution was

exhausted.

Failure Mechanism of Solder

Joints with Black Pad

As mentioned previously, after the

molten solder spreads onto the ENIG-

plated pad, the gold plating dissolves

into the solder. The molten solder follows

the gold that has penetrated deeply into

the corroded area, making the corroded

area more porous (see the gold-rich area

in Figure 5c). It can be assumed that,

even if the corroded area is still solder-

able, the bonding will be very weak due

to the porosity of the corroded Ni(P). A

low shear stress would be enough to

crack the joint.

Previous work has found that Kirken-

dall voids formed between the main IMC

(Cu6Sn

5or Ni

3Sn

4) and the Ni

3P layer

(i.e., in the thin layer of Ni3SnP).34,35,47

Because of the formation of this voided

layer, the interfacial bonding of solderto the ENIG plating is by nature weak

even if the Ni(P) plating does not suffer

from hyperactive corrosion.

Obviously, if the periphery is severely

corroded (e.g., Figure 6), cracking is

easily initiated and propagates through

the voided Ni3SnP layer, leading to

fracture of the joints between the main

IMC layer and the Ni3P layer. The draw-

ing in Figure 9 schematically shows this

fracture process. It can be seen that, after

fracture, two different kinds of regionsshould be observed on the pad side by


5/52006 June JOM 79

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Kejun Zeng, Roger Stierman, Don Abbott, and

Masood Murtuza are with Texas Instruments in

Dallas, Texas.

For more information, contact Kejun Zeng, TexasInstruments, Packaging Reliability, 13536 N. CentralExpressway, MS 940, Dallas, TX 75265; (972) 995-2202; fax (972) 995-2658; e-mail [email protected].

top view. One is the exposed Ni3P and

mud-cracks and the other one is the

fractured Ni3SnP layer. Indeed,the

authors have seen these two different

regions in Figure 2athe dark regionsare Ni

3P and the gray regions are

Ni3SnP.

CONCLUSION

Since mud-cracks are often observed

in the ENIG-plated pads after the solder

joints fail, there have been many discus-

sions on whether or not the black pad

failure of solder joints was caused by an

improper soldering process. In the pres-

ent work, the authors found that when a

BGA substrate has black pad failure of

solder joints, it has mud-cracks or spikes

in the nickel plating before soldering,

and the material around the cracks is

corroded. These obervations support the

theory that the black pad deficit in ENIG

plating is the result of hyperactive cor-

rosion of the electroless nickel plating

by the immersion gold plating bath.

Mud-cracks are created by the soldering

process. Interfacial failure of solder joints

with the ENIG plating is the combined

effect of hyper-galvanic corrosion of the

electroless nickel during gold plating

and Kirkendall voiding in the Ni3SnP

layer after reflow. To avoid the black pad

failure of solder joints, the key is to avoid

hyper-galvanic corrosion of the electro-

less nickel plaating during the immersion

gold plating process.

ACKNOWLEDGEMENT

The authors would like to thank B.

Holdford for assistance in microanalysis

of FIB-polished cross sections. Valuablediscussions with Kuldip Johal, Atotech

USA, and R.J. Coyle, Lucent Technolo-

gies, are gratefully appreciated.

References

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1016.6. D. Cullen, Proc. IPC National Conf.: A Summit onPWB Surface Finish and Solderability(Bannockburn,IL: IPC Association, 1998), pp. 4458.7. R.J. Coyle et al., IEEE Trans. Comp. Packag.Technol., 26 (2003), pp. 724732.8. C.E. Ho et al., J. Electron. Mater., 29 (2000), pp.11751181.9. C.H. Zhong et al., Proc. 50th Electr. Comp. Technol.Conf. (Piscataway, NJ: IEEE, 2000), pp. 151159.10. K. Banerji and E. Bradley, Proc. Surface MountTechnol. Inter. (Edina, MN: SMTA,1994), pp. 584595.11. E. Bradley and K. Banerji, Proc. 45th Electr. Comp.Technol. Conf. (Piscataway, NJ: IEEE, 1995), pp.10281038.12. T.I. Ejim et al., Proc. 21st Inter. Electr. Manuf. Symp.

(Piscataway, NJ: IEEE, 1997), pp. 2531.13. Z. Mei et al., Proc. Pacific Rim/ASME Inter.Intersociety Electr. Photonic Pack. Conf., Adv. Electr.Pack. (New York, NY: ASME, 1997), pp. 15431550.14. P. Johnson et al., Proc. SemiCon West(San Jose,CA: SEMI, 1999), pp. G19.15. D. Cullen et al., Proc. IPC Works(Bannockburn, IL:IPC Association, 2000), p. S03-2.16. G.M. Wenger et al., Proc. 26th Inter. Symp. Testingand Failure Analysis (Materials Park, OH: EDFAS/

Figure 9. A schematicillustration of a solder jointwith ENIG that was corroded

during the gold platingprocess. Black pad failure isthe result of propagation ofthe mud-cracks in the Ni(P)plating through a voided thinlayer of Ni-Sn-P between IMCand Ni

3P layers.

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The Root Cause of Black Pad