Technical Paper12

Journal of the HKPCA / 2015 / Autumn / Issue No. 57

Dr. Norbert L tzow, Gabriela Schmidt, Atotech Deutschland GmbH

Waimun Wong, mer Erdogan, Atotech (China) Chemicals Ltd.

ü

Ö

ABSTRACT

INTRODUCTION

The demand for ever finer circuitry especially for IC-substrate

manufacture has lead the way away from the traditional

subtractive circuit formation to additive, semi-additive, and

(advanced) modified semi-additive technology. These

technologies provide many challenges to conquer during

production.

Except for the fully additive process, which remains a niche

technology, a copper seed layer is required in order to be able

to pattern plate the desired circuitry. This seed layer has to be

removed through etching to finalize the circuit formation. This

process step is commonly described as flash etching or

differential etching.

This paper describes the newly developed ferric sulfate based

etchant for flash/differential etching. The focus will be on the

etch performance in comparison to hydrogen peroxide etchants.

In addition regeneration equipment designed for this application

will be illustrated and discussed, especially under economical

and ecological aspects.

During the semi-additive processing (SAP), modified semi-

additive processing (MSAP), and advanced modified semi-

additive processing (aMSAP) a copper seed layer is being used

onto which the conductors are being plated. In SAP this seed

layer consists of a layer of electroless copper, with a thickness

ranging from 0.3 m to 2 m, depending on the design and

manufacturer. Therefore after pattern plating and resist stripping

only the thin electroless copper seed layer needs to be

removed for final circuit formation. Considering MSAP several

different variations exist. The seed layer can consist of a single

µ µ

EcoFlash 100 - Recyclable Anisotropic

Etchant for Advanced Flip Chip Manufacturing

™

copper type, i.e. half-etched CCL, or a combination of different

copper layers, i.e. electroless copper with strike plating. The

differentiation between MSAP and aMSAP lies within the

thickness of the copper seed layer, with aMSAP typically having

a total copper thickness in the SAP range, i.e. by using

sputtered copper or ultra-thin copper foil, while MSAP samples

exhibit copper thickness from 2-10 m.

Nevertheless for both SAP and mSAP the copper seed layer

has to be removed through etching to finalize the circuit

formation. Typical etching solutions contain sulfuric acid and

hydrogen peroxide in addition to organic stabilizers and banking

agents. Two draw-backs have been observed with peroxide

based etchants, first the solution requires feed and bleed

operation to maintain the maximal copper content and to

replenish spent oxidizer, and second peroxide based etchants

etch three-dimensionally with the same etching speed no mater

if sprayed or in immersion. The first draw back has economical

as well as ecological effects, since considerable amounts of

chemical waste is being generated and thereby requires waste

treatment. The second drawback has functional effects, since

the three-dimensional etching causes undercut of the

conductor tracks of several micrometers, thereby affecting the

mechanical stability of the track as well as the electrical

properties (i.e. impedance control).

In order to prevent these two draw-backs of the typical

peroxide based etchants a totally different etchant system has

been developed. The novel etchant is based

on ferric sulfate and thereby offers the possibility to regenerate

the solution in by-pass equipment, therefore eliminating the

need for feed and bleed operation. Furthermore, besides

regenerating the oxidizer pure copper is plated, which can

either be re-used internally or sold to recyclers. In addition this

µ

EcoFlash 100™

Technical Paper 13

www.hkpca.org

ferric sulfate based etchant causes minute to none undercut

eliminating the second draw back of peroxide based etchants.

EcoFlash 100 was developed as a drop-in replacement and

therefore requires only one process step. The etchant is based

on ferric sulfate together with sulfuric acid and is therefore

compatible with stainless steel 316 and titanium horizontal

spray equipment, in contrast to sulfuric acid/hydrogen peroxide

systems, which are not compatible with titanium and cupric or

ferric chloride etchants, which are not compatible to stainless

steel. Deciding on developing a ferric sulfate based etchant

further provided the unique opportunity to develop a waste

water free etchant, enhancing Atotech's strive towards

environmental friendly Green Technologies.

By utilizing the redox system of ferric and ferrous sulfate the

opportunity was provided to use electrolytic cells to regenerate

the consumed oxidizing agent while at the same time removing

the etched copper as pure solid copper.

Cu +2 Fe + Cu + 2Fe

Figure 1 shows the etching mechanism of copper using ferric

sulfate. Two moles of ferric sulfate are reduced in order to

oxidize one mole of copper. This clearly shows that a typical

feed and bleed operation would be highly uneconomical due to

the high amount of ferric sulfate required and the high price of

ferric sulfate. Therefore an electrolytic cell is being used to fully

regenerate the consumed ferric sulfate as shown in Figure 2.

Fe + Fe + e-

As stated in Figure 2 a possible side reaction could occur at the

anode in which oxygen is formed. By providing sufficient

amount of ferrous sulfate this side reaction can be prevented.

Therefore a sufficiently high content of ferrous sulfate in the

™

0 3 2 + 2 +

2 3 +

�

�

�

Figure 1: Copper etching using ferric sulfate

Figure 2: Regeneration of ferric sulfate at the anode

[possible side reaction: 2H O O + 4e- + 4 H ]2 2

+

PROCESS DESCRIPTION

solution is needed in addition to optimization of the fluid flow

inside the regeneration unit.

The electrolytic cell offers besides the regeneration of the

oxidizer also the benefit of removing the etched copper from

the etchant as pure copper. Figure 3 shows the plating

equation of copper on the cathode of the electrolytic cell.

Hydrogen formation occurs only at high current densities and at

low copper content.

Cu + 2e- Cu

Due to the fact that the entire consumed oxidizing agent is

regenerated no dosing of the ferric sulfate is needed. In addition,

since the etched copper is plated out as pure copper the

typical feed & bleed operation, in which the amount of

chemistry is removed (bleed) and replenished by copper free

working solution (feed) to maintain a constant copper content in

the etchant system, is not needed. For comparison, a typical

hydrogen peroxide etchant has a copper loading capacity of

25g/l therefore the required bleed amount can be calculated for

a 1 m/min line and an etch amount of 1.5 m:

As the calculation in Figure 4 states, a standard etchant

employing hydrogen peroxide requires a bleed amount of 1.07

l/m2 of cut board which sums up to 4.5 m of solution wasted

during one week of production (20h/day, 6 days/week). This

bleed is fully functional chemistry that has to be wasted,

increasing chemical costs for the process and waste water

costs for the waste water treatment.

Due to the fact that an electrolytic regeneration unit can be

employed for the EcoFlash 100 process no bleed is required,

not only reducing the process costs, but also decreasing the

environmental footprint. In addition pure copper is obtained

2 + 0

3

�

� �[possible side reactions: Fe + e- Fe and 2H + 2e- H ]3+ 2+ +

2

Figure 3: Reduction of copper at the cathode

µ

™

1.5 * 8.94*2=26.8 g Cu/m cut board; 26.8/25=1.07 l/m bleed

1m/min=> 34.8 m /h => 933.2 g Cu/ h => 37.3 l/h bleed required

2 2

2

Figure 4: Feed & bleed calculation hydrogen peroxide etchant

directly at the process, which can be further utilized on-site or

sold off site reducing the process costs additionally by 10-20%.

An example calculation is presented in Table 1.

Just like hydrogen peroxide based etchants also the ferric

sulfate etchant requires process specific additive packages.

Figure 5 shows a selection of defects commonly confronted

with during differential etching. Undercut at the bottom of the

track is a common defect seen with hydrogen peroxide based

etchants. Copper footing or trapezoidal etching of the

conductor is a defect seen with ferric sulfate etchants.

Therefore proper additive packages are needed to ensure that

the ferric sulfate etchant is capably of providing even etching at

the top and bottom of the conductor to ensure the proper/ideal

rectangular conductor shape as shown in Figure5.

Without the proper additive package the ferric sulfate etchant

will provide highly trapezoidal conductors with the tendency of

copper footing of which neiter are accepted for differential

etching. Therefore extensive screening of a variety of additives

has been conducted to find the proper combination of additives

which will prevent the trapezoidal etching and copper footing

and do not interfere with the regeneration process. As Figure 6

illustrates ferric sulfate etchants etch at the bottom of the

conductor considerably slower than at the top of the conductor,

which results in strong trapezoidal shapes tracks. The target

was to find an additive package which protects the conductor

and increases the etching speed at the bottom of the conductor.

With the proper additive package and conditions the resulting

conductor shape will be rectangular.

The EcoFlash 100 process is the result of this extensive

research. Three chemical packages are being provided for this

process, EcoFlash 100 100, used only for make-up,

EcoFlash 100 200, and EcoFlash 100 300, used only to

compensate for drag-out losses. Besides the three EcoFlash

100 packages sulfuric acid and DI water are required. Table 2

provides details of the make-up:

™

™

™ ™

™

Table 1: Cost Saving Calculation for EcoFlash 100™

Figure 5: Typical defects found during differential etching

Figure 6: Etching mechanism without and with additive for a ferric sulfate

etchant.

Table 2: Make-Up for EcoFlash 100™

Technical Paper14

Journal of the HKPCA / 2015 / Autumn / Issue No. 57

As mentioned before no bleed operation is needed. In addition

the consumption of the individual components is low governed

mainly by drag-out losses. Therefore no bleed operation is

needed. In Addition the low dosing amount provide the

opportunity to use small chemical containers for the dosing unit

instead of the usually several hundred liter containing dosing

tanks used for peroxide based processes. This provides the

opportunity to free floor space for the regeneration unit.

For analyzing the etching performance of a differential etching

system the conductor shape before and after etching needs to

be investigated. Therefore cross-section investigations were

done for all the tests and light microscopy images were taken.

The conductor size of the investigated samples was 50 m

down to 10 m thereby requiring high quality cross-sectioning

procedures. Therefore the samples were coated with

electroless nickel, employing Atotech's Aurotech CNN process.

The nickel layer prevents air gaps between the resin and the

copper, which could be filled with grinding and polishing

residues and falsify the conductor size and quality. For most

cross-section investigations white light microscopy images

were taken, due to the ease and speed.

The etching performance of the EcoFlash 100 process was

compared to sulfuric acid/hydrogen peroxide based etchants

using the same etching equipment. Test samples included SAP,

MSAP, and aMSAP boards. Figure 7 shows cross-section

images of a SAP sample with 16 m line and 19 m space.

The electroless copper layer is seen in the image of the

untreated sample. Employing the peroxide based etchant

considerable undercut at the bottom of the conductor was

found (~1.5 m) while on the EcoFlash 100 etch sample no

µ

µ

™

µ µ

µ ™

PROCESS PERFORMANCE

Figure 7: Cross-section of 16 m SAP trackµ

undercut was observed. In addition the conductor surface

appears rougher on the peroxide etched sample. Massey and

Zee [1] have shown that the signal integrity of high frequency

data lines is influenced by the copper surface roughness. The

degree of undercut seen with hydrogen peroxide based

etchants varied depending on the sample type and the copper

plating conditions. In Figure 8 cross-section images of aMSAP

samples are shown. The seed layer was sputtered copper and

the track width was 10 m with 15 m spacing. The images

clearly show the strong undercut caused by the hydrogen

peroxide etchant. The untreated sample already shows a small

wedge most likely caused by a dry film foot prior to pattern

plating. This wedge was still seen after etching with EcoFlash

100 but no additional undercut. The strong undercut seen for

the hydrogen peroxide etchant treated aMSAP sample in Figure

8 significantly changes the cross-section area of the conductor

thereby increasing the risk to the signal integrity.

On MSAP samples undercut was observed occasionally even

when employing hydrogen peroxide based etchants. Most

MSAP samples showed a slight trapezoidal shape after copper

plating, most likely due to the dry film imaging. Figure 9

compares the cross-section images of a MSAP sample prior to

etching, after EcoFlash 100, and after using a hydrogen

peroxide based etchant. Employing EcoFlash 100 the

conductor shape is more rectangular than that of the hydrogen

peroxide based etchant. Additionally the track surface on the

EcoFlash 100 sample appears smoother.

µ µ

™

™

™

™

Figure 8: Cross-section images of 10 m aMSAP tracksµ

Figure 9: Cross-section images of 50 m MSAP tracksµ

Technical Paper 15

www.hkpca.org

REGENERATION UNIT & ENVIRONMENTAL BENEFITS

The strength of this ferric sulfate based etchant is the fact that

electrolytic regeneration equipment can be utilized reclaiming

the copper and regenerating the oxidizer. Collie [2] describes

chemical regeneration of oxidizers e.g. cupric chloride, ferric

chloride, and ferric sulfate. For chemical regeneration of ferric

sulfate the usage of hydrogen peroxide was mentioned. In the

case of ferric sulfate the reaction with hydrogen peroxide is also

known as Fenton's reaction, and known in the waste water

treatment for removal of organic contaminations [3]. For the

EcoFlash 100 the usage of hydrogen peroxide was not

advisable, due to the highly exothermic reaction and the

destruction of the additives required for the etchant. Instead

electrolytic cells [4] seem promising, due to the fact that the

ferric sulfate could be directly regenerated and in addition the

etched copper could be directly reclaimed.

The redox system of ferric and ferrous sulfate has been in use

in copper plating baths in conjunction with inert anodes for a

while. Therefore similar equipment setup could be used for the

regeneration unit. The prototype unit consists of vertical plating

tanks employing coated titanium anodes. A centrifugal pump

delivers continuously etchant from the horizontal etcher to the

regeneration unit. At the inert anodes the regeneration of ferric

sulfate occurs, while on the cathode the copper is being plated.

The cathode can be a stainless steel panel, copper mesh,

copper foil, or as done in our tests copper clad laminate (CCL).

After passing through the regeneration cell the regenerated

etchant flows back to the etching chamber (Figure 10). The

recovery/regeneration unit is designed to permit usage of the

™

cathodes for several days during production before the

reclaimed copper thickness reaches a safety limit.

To ensure high process stability the utilization of an online

controller is mandatory. For this purpose a photometer device

has been used to analyze the Fe and Cu content. The online-

controller adjusts the current setting of the rectifier accordingly

to ensure a consistent etching speed. The process stability was

verified during loading tests in the Guangzhou TechCenter and

continued over a time period of three months. Periodically

solution samples were analyzed for Fe , Fe , and Cu content.

In addition the etch rate was verified periodically. Figure 11

shows the process stability diagrams for the etch rate and Fe

measurements. The grayed out data points were taken before

turning on the online analyzer. The black data points were taken

during operation with online analysis.

A statistical analysis of the data obtained during the loading test

showed high process stability as shown in Table 3. High c and

c values were determined for the etch rate and the Fe3+

content. In addition throughout the loading test the etching

3 + 2 +

3 + 2 + 2 +

3 +

P

PK

Figure 10: Schematic of the regeneration hook-up

Figure 11: Process stability diagrams

Technical Paper16

Journal of the HKPCA / 2015 / Autumn / Issue No. 57

performance, as observed by cross-section investigations,

remained constant. In addition no precipitation was seen in the

solution; the filter cartridges used in the etching chamber were

clean and required no replacement during the duration of the

test.

The advancement in printed circuit board design requires new

technologies to deliver the desired products. One of these new

technologies is the differential etching required for removal of

the copper seed layer during SAP, MSAP, and aMSAP. The

current process of record (POR) employs hydrogen peroxide as

oxidizer. One drawback seen with peroxide etchants is the

considerable undercut of the copper tracks, negatively

influencing the mechanical stability but also the electrical signal

quality and reliability. In addition hydrogen peroxide etchants

require feed & bleed operation producing several cubic meter of

waste water each week.

A newly developed ferric sulfate based etchant was introduced

which offers several key advantages. First, this ferric sulfate

etchant does not exhibit undercut as seen with the POR.

Second advantage is that the copper track surface roughness

This stable operation with the copper regeneration unit has

clear environmental benefits. Through oxidizer regeneration

these processes basically eliminates the need for feed & bleed

operation. Therefore standard Differential Etchants do generate

large volumes of copper contaminated waste water, this is not

the case with EcoFlash 100. A standard line running

20.000m production per monh would generate more than

7000 liters of waste water, this is eliminated with EcoFlash

100. At the same time this process allows for the recovery and

reuse of copper that would normally be lost in feed & bleed yet

another clearn environmental benefit vs. conventional

Differential Etchants.

™

™

2

SUMMARY & CONCLUSION

remains smooth, which is integral for high signal quality. The

third advantage is the fact that by utilization of an electrolytic

regeneration unit the process becomes nearly waste water free.

No feed & bleed operation is required to maintain constant bath

parameters in contrast to the POR, with a big impact on the

overall process costs. The small dosing required is mainly

governed by the drag-out loss. In addition the etched copper is

reclaimed on-site as pure copper and can be used on-site or

sold externally to copper recycling companies, considerably

reducing the process costs.

This unique and new ferric sulfate based etchant does not only

improve the etching performance of the differential etching

process, it also reduces the process costs and in addition it

reduces significantly the environmental impact of the production

site. Several thousand liters of chemistry and chemical waste

water can be saved in comparison to the POR. This new

process is in line with Atotech's global commitment to

sustainable development to help customers to reduce their

environmental footprint.

[1] R. Massey, A. Zee, "Use of Non Etching Adhesion

Promoters in Advanced PCB Applications," SMTA

Proceedings 2010

[2] M.J. Collie ed., , Park

Ridge: Noyes Data Corporation, 1982, ch. 7

[3] L . Har t inger,

Munich: Carl Hanser Verlag, 1991, ch. 3

[4] L . Har t inger,

Munich: Carl Hanser Verlag, 1991, ch. 7.

Etching Compositions and Processes

Handbuch der Abwasser - und

Recyclingtechnik f r die metallverarbeitende Industrie.

Handbuch der Abwasser - und

Recyclingtechnik f r die metallverarbeitende Industrie.

ü

ü

REFERENCES

Table 3: Process Stability Analysis

Technical Paper 17

www.hkpca.org