we fill the gaps ! -

Progressing miniaturization of electrical inter-connections requires the use of HDI PCBs with blind microvias filled with electroplated copper. The newly developed copper electrolyte Copper SLOTOCOUP SF 30 enables the filling of blind microvias free from defects with a low copper thickness on the surface of the PCB at the same

time. This leads to a more efficient application of raw materials and to a more cost-effective manufacturing of PCBs. In future a new copper electrolyte should enable the filling of through holes - first trials at laboratory scale showed already promising results.

Article about filling of blind microvias by Dr. Michael Dietterle,

Dr.-Ing. Max Schlötter GmbH & Co. KG

[Published in WOMag 05/2014]

We fill the gaps!Increase of the integration density of PCBs by filling of blind microvias and through holes with electroplated copper.

www.schloetter.com

We fill the gaps! Increase of the integration density of PCBs by filling of blind microviasand through holes with electroplated copper.

Smartphone

Miniaturization is still the determining trend in micro electronics that means smaller and more efficient systems which should be more cost-effective than current systems. Their best-known representatives being, are smart phones and Tablet PCs. Their performance - despite of the same or even smaller unit size - was possible to increase significantly during the last years. The great attractiveness of these devices is reflected by high and constantly increa-sing sales figures.

A substantial contribution to the miniaturization is provided by HDI PCBs (High Density Inter-connect) which enable due to their high inte-gration density the signal fan out of highly functional electronic components onto the smallest area. For electrical interconnection of single PCB layers through holes are replaced by space-saving blind microvias. By filling of blind microvias with electroplated copper (Blind Microvia Filling) the integration density can be further increased. In the meantime the use of HDI PCBs isn’t only restricted to mobile electronics any more but it‘s also increasing in other appli-cations e.g. in the automotive sector.

The newly developed electrolyte Copper SLOTOCOUP SF 30 which in the filling of blind microvias compared to the previous generation of electrolytes, deposits only a very low copper thickness on the surface, enables a more resour-ces-, energy- and cost-efficient manufacturing of HDI PCBs. The performance of this electrolyte is introduced on the basis of laboratory results and pilot tests in a 7.200 litres vertical con-tinuous plating line. In the meantime Copper SLOTOCOUP SF 30 is applied in mass production of HDI PCBs.

Since the integration density of HDI PCBs can be further increased by the use of very thin core materials, currently the development of an electrolyte for filling of through holes (Through Hole Filling) is being intensified. Results of first laboratory tests are presented.

2

Miniaturization in the Field of Micro Electronics

Fig. 1: Comparison of the size between a smart phone processor and 1 Eurocent coin.

The continuous miniaturization in microelec-tronics enables the production of more powerful electronic mobile devices. Their best-known representatives being are smart phones and Tablet PCs. The great attractiveness of these devices is reflected by high and constantly increasing sales figures. In the year 2013, for the first time, more than one billion smart phones have been sold [1], for 2014 a sales volume of approx. 1.2 billion and for 2017 of approx. 1.8 billion devices is projected [2]. In the field of Tablet-PCs a sales volume of 271 billion devices is predicted. This corresponds in comparison to

the year before to an increase of almost 40 % [3]. The miniaturization in the field of micro-electronics is technically forwarded particularly by the development of increasingly more efficient highly functional semiconductor chips. These are installed in packages with very small sizes of the housing and very high pin-count, arranged in area arrays.Figure 1 shows the size comparison between a typical smart phone processor and a one Euro-cent coin. The back side of the processor shows 976 connections on an area of approx. 2 cm². This corresponds to around 5 connections/mm². The pin-pitch is only 400 µm.

1 Eurocentsmart phone processor

14 m

m

16,2

5 m

m

Miniaturization in the Field of PCB

For space-saving and reliable electric connec-tion of processors with such a high pin-count, PCBs with correspondingly high integration density are needed. For this purpose the classic multi-layer PCB isn’t suitable since through holes are used for the electrical connection between the single layers of the PCB. These show relatively big diameters and extend through the entire thickness of the PCB. As a consequence, the space above and below the connection gets lost and cannot be used for

other structures e.g. conducting lines. Resulting from this, the low integration density of the multi layer PCBs is insufficient for the require-ments described above. Therefore a new highly integrated HDI generation has been developed a few years ago, the so called HDI PCB, which initially has been mainly applied for the produc-tion of mobile phones. When manufacturing HDI PCBs the single PCB layers are sequentially build-up (SBU, Sequential Build Up). So far, the build up layers have been mainly build-up on multilayer cores - with increasing tendency the build-up is performed also on thinner cores which will be further described in a later para-graph of this essay. The electric connections of

3


Fig. 3a: Copper plated blind microvias: Insufficient layer thickness inside the blind microvia

Fig. 2: Schematic structure of a 2-4-2 HDI PCB

the respective adjacent build up layers will be realized by laser-drilled blind microvias. Figure 2 shows the schematic structure of a 2-4-2

HDI PCB, that means the PCB is composed of a 4 layer multi layer-core and 2 build-up layers on each side.

Blind Microvias

In comparison to through holes, blind microvias show smaller diameters in the range of 50 - 150 µm and extend in Z-direction only over the thick-ness of one build-up layer (typically 50 - 150 µm). So they take up just so much space like it’s definitely required for the actual connection. HDI PCBs have a considerably higher integration density than multi layer PCBs and therefore they’re suitable for the signal fan out of highly functional electronic components in a small space. Furthermore, blind microvias show due to their significantly small size a higher signal

integrity than through holes which predestines them for the use in high frequency applications.

To make blind microvias conductive for electric signals, they have to be coated by electrolytic copper deposition (Blind Microvia Plating). Since available electrolytes didn’t deposit a sufficient copper layer thickness on the bottom of the blind microvias (Figure 3a) it was necessary to develop special copper electrolytes which allow the deposition of a sufficient copper layer thick-ness (Figure 3b).

Multi core-layer (4 layers)

Build-up layerBuild-up layer

Build-up layerBuild-up layer

Fig. 3b: Copper plated blind microvias: Sufficient layer thickness inside the blind microvia

11,8 µm

6,7 µm

86,1 µm

80,2

µm

10,1 µm

14,8 µm

89,9 µm

83,5

µm

4

Filling of Blind Microvias

A further increase of the integration density can be enabled by stacking of blind microvias (Stacked Blind Microvias). For this the blind microvias must be filled with an electrically conductive material. If instead of a conductive paste, electrolytic deposited copper is used, then the following advantages will be received:

· increased reliability (blind microvias contain only copper, no additional phase boundary)

· better thermal management (heat can be dissipated by the copper filled blind microvias)· a further increase of integration density (no additional pads for contacting of electrical components are needed)

The essential process steps for manufacturing of HDI PCBs with copper filled blind microvias are schematically shown in Figure 4. If a further layer should be build-up, the process sequence must be restarted beginning from process step 2 again.

1 core 2 dielelectric + copper 3 laser drilling

4 conductive layer 5 blind microvia filling 6 copper thinning

7 photoresist 8 copper etching 9 photoresist stripping

Fig. 4: The essential process steps for the manufacturing of HDI PCBs with copper filled blind microvias.

By stacking of copper filled blind microvias also non-adjacent build-up layers can, at minimum space requirement, be connected (Figure 5). All stacked blind microvia designes make the complete waiving of through holes possible.

Fig. 5: Stacked Blind Microvias

13,1 µm

86,9 µm

5

Established Electrolyte for Blind Microvia Filling


It ‘s typical that electrolytes for blind microvia filling contain a relatively high copper ion con-centration in the range of 40 - 60 g/l in combi-nation with a low concentration of sulphuric acid in the range of 10 - 50 ml/l as well as chloride ions. The organic electrolyte additives necessary for the control of the coating charac-teristics are different from supplier to supplier. But most frequently the components contain the following three components:

· starter (inhibitor)

· grain refiner (activator)

· leveller (inhibitor)

Furthermore, the processes of the different suppliers can differ among other things in the following main areas:

· type of plating line used

(standard vertical, vertical continuous, hori- zontal continuous)

· anode material (copper anode, insoluble anode)

· applied current form (direct current, pulse current, reverse pulse current)

· applicable current density

The processes for blind microvia filling offered by Schloetter operate without exception with direct current in standard vertical plating lines or vertical continuous plating lines. For anode materials either copper or mixed metal oxide (MMO) are possible.

The application of Pad-in-Via respectively Via-in-Pad designs leads to a further increase of the integration density since the connections of

Fig. 6: Via-in-Pad Design (schematically)

Component

solder ball

HDI PCB

components can be made directly on copper filled blind microvias and so no additional pads are required (Figure 6).

6

Fig. 7a: Blind Microvia Filling (schematically)

Fig. 7b: Blind Microvia Filling cross section

Dent

A B

C

21,9 µm 137,5 µm

101,

7 µm

93,2

µm

In the early days of blind microvia filling there were next to the standard requirements for electrodeposited copper coatings on PCBs (e.g. ductility, reliability) the following additional requirements:

· defect-free filling of blind microvias without the incorporation of electrolyte solution

· minimum filling degree respectively maximum permissible dent

These requirements could be met by specifically developed copper electrolytes (Figure 7a and 7b).

The crossection in Figure 7b shows a pre-rein-forced blind microvia with a diameter of 138 µm and a depth of 102 µm. During the filling process 93 µm Cu have been deposited in the blind microvia while only 22 µm (C) have been deposited on the surface. This results in the following figures:

Dent (A-B): 30.4 µm

Filling (B/A): 75 %

Metal distribution (B/C): 426 %

It’s particularly due to the mode of action of the leveller that copper isn’t preferentially deposited on the surface but inside the blind microvias, that means in the areas with low current density and low electrolyte exchange. The molecules of the leveller adsorb during the filling process mainly in the areas of high current density and strong electrolyte exchange. This inhibits the copper deposition in these areas and leads to a preferred deposition of copper in the blind microvias.

The electrolyte additives must interact perfectly with one another in order to achieve a good filling result. Figure 8a shows a blind microvia prior to the filling process as well as different filling results (Figure 8b - e). These results may occur only by the variation of elec-trolyte additives but same plating parameters.

Fig. 8a: Blind Microvia prior to the filling process

3,6 µm

74,1 µm

51,7

µm

7


Fig. 8d

Fig. 8cFig. 8b-e: Blind Microvia after the filling process and varia tions of electrolyte additives.

Fig. 8e

9,4 µm

44,2

µm

53,4

µm

85,7 µm10,0 µm

48,4

µm

30,9

µm

85,5 µm

24,8 µm

51,1

µm

8,5 µm

74,2 µm

8,1 µm

46,3

µm

8,3 µm

86,5 µm

New Electrolyte for Blind Microvia Filling

The integration density of the PCB can be further increased by reduction of line and space. In order to etch such fine conductive patterns the copper thickness on the panel must be low since a strong underetching may cause problems.

Like demonstrated in Figure 4, it’s possible to reduce the copper thickness on the surface after blind microvia filling by copper-thinning. But for this procedure additional process steps and plants are necessary. Furthermore, during copper thinning the previously deposited copper will be partly removed. This has a negative effect on resources-, energy- and cost efficiency of PCB manufacturing. For complete avoidance - or at

least for reduction - of the copper thinning process the copper thickness deposited during the filling process should be as low as possible.

During intensive development, the organic additive system was modified in such a way that a defect-free blind microvia filling in combi-nation with a very low copper surface thickness could be achieved. The newly developed elect-rolyte Copper SLOTOCOUP SF 30 has especially been developed for the operation with insoluble anodes as follows:

8

160 - 240 g/l Copper Salt BV

15 - 45 ml/l Sulphuric acid, conc.

50 - 70 mg/l Chloride

3 - 10 ml/l Additive SLOTOCOUP SF 31

0.2 - 1.0 ml/l Additive SLOTOCOUP SF 32

0.2 - 2.0 ml/l Additive SLOTOCOUP SF 33

The electrolyte is operated with current densities of max. 2 A/dm² in a temperature range between 18 and 22 °C.

In Figure 9a and 9b a blind microvia is shown prior and after a 60-minutes filling process withCopper SLOTOCOUP SF 30 performed in labora-tory scale.

Fig. 9a: Blind Microvia filling with Copper SLOTOCOUP SF 30 in laboratory scale prior to

Fig. 9b: and after the filling

99,6 µm

93,2

µm

Figure 9b shows a pre-reinforced blind microvia which showed prior to the filling a diameter of approx. 100 µm and a depth of approx. 93 µm. During the 60-minutes filling process 90 µm copper have been deposited inside the blind microvia while on the surface only 4 µm have been deposited. This results in the following figures:

Dent: 5.1 µm

Filling: 90 %

Metal distribution: 2376 %

In comparison to the previous electrolyte gene-ration the copper thickness deposited on the surface was extremely reduced. This is demons-trated by the metal distribution which shows an extremely high value of > 2000 % (Figure 9b).

Copper SLOTOCOUP SF 30 has been tested under pilot production conditions in a 7.200 litres vertical continuous plating line in cooperation with our Taiwanese partner AGES in the “Schloetter-AGES PCB Development Center”, Taipeh (Figure 10).

3,8 µm 106,3 µm

97,0

µm

90,3

µm

9


Typical results of these tests are shown in Figure 11 - 13.

Figure 11a shows a pre-reinforced blind microvia which showed prior to the filling process a dia-meter of approx. 108 µm and a depth of approx. 71 µm. During the filling process 74 µm copper have been deposited inside the blind microvia while on the surface only 10 µm have been deposited. This results in the following figures:

Dent: 7.0 µm

Filling: 91 %


Figure 11b shows a further copper filled blind microvia which is from the same PCB than the blind microvia shown in Figure 11a.It has to be noted that despite of a not ideal BMV geometry a very good filling result can be achieved.

Figure 11c shows a not pre-reinforced blind microvia with a diameter of approx. 98 µm and a depth of approx. 68 µm. During the filling process 76 µm copper have been deposited

Fig. 10: Vertical continuous plating line with a volume of 7.200 litres in the “Schloetter-AGES PCB Development Center” in Taiwan.

Fig. 11a-c: Test results Copper SLOTOCOUP SF 30: filling of blind microvias

9,7 µm 108,5 µm64

,1 µ

m

9,7 µm 97,8 µm

68,0

µm

81,1

µm

10,3 µm 106,0 µm

10

Fig. 12a: Test results Copper SLOTOCOUP SF 30: filling of blind microvias close to each other.

Fig. 12b: Copper SLOTOCOUP SF 30: detailed view Fig. 13: Test results Copper SLOTOCOUP SF 30: filling of very flat blind microvias

6,9 µm104,2 µm

9,1 µm102,1 µm 103,6 µm

86,3

µm

inside the blind microvia while on the surface only 10 µm have been deposited. This results in the following figures:

Dent: 5.0 µm

Filling: 96 %


Copper SLOTOCOUP SF 30 also enables the defect-free filling of blind microvias close to each other with a low surface copper thickness at the same time. Figures 12a and 12b show the filling result at a blind microvia pitch of only 200 µm.

Very flat blind microvias which occur in extremely thin dielectrics can also be completely filled with Copper SLOTOCOUP SF 30 but in this case a slightly higher copper surface thickness is obtained (Figure 13).

The tests under pilot production conditions confirmed that Copper SLOTOCOUP SF 30 enables a very good blind microvia filling in combination with extremely low copper surface thickness. After successful sample plating for several Asian PCB manufacturers, the electrolyte meanwhile is used in mass production of HDI PCBs.

11


Through Hole Filling

A further integration density increase of HDI PCBs can be achieved by the replacement of the commonly used relatively thick multilayer cores by considerably thinner cores with a thickness between 100 and 200 µm. Since the thickness of such an innerlayer core is very similar to the thickness of build-up layers, it’s also called coreless build-up (Figure 14).

The all stacked blind microvia build-up depicted in Figure 14 contains a 100 µm core, and copper filled blind microvias.

Very thin cores may also exhibit through holes. Until now, these through holes have to be plated with copper, filled with paste, planarized and copperplated again. Therefore the production of these cores is very complex and expensive. Furthermore the use of paste can lead to relia-bility issues.

In analogy to blind microvia filling, the reliabi-lity can be increased and the thermal manage-ment can be improved by filling of through holes with electroplated copper.The build-up of two build-up layers on each side of a thin core with copper filled through holes is schematically demonstrated in Figure 15.

Fig. 14: Coreless build-up

Fig. 15: Coreless build-up (schematically)

Core with copper filled through holes

Build-up layer

12

New Electrolyte for Through Hole Filling

Similar to blind microvia filling, the mass trans-port inside the hole becomes more difficult with an increasing aspect ratio, that means decreasing diameter of the hole and/or increa-sing z-expansion of the hole. This complicates a defect-free filling of the through holes.

By modification of the electrolyte composition it was finally possible to fill the through holes with electroplated copper. The filling results shown in Figure 16a und 16b of not pre-rein-forced through holes (diameter approx. 50 µm/depth approx. 160 µm) are from the same PCB section, plated with direct current.

Fig. 16a: Through Hole Filling with electrolyte inclusion.

Furthermore, during the electrodeposition no change of the electrolyte solution has been per-formed.

As can be seen, no reliable defect-free filling of the through holes can be performed yet.The electrolyte solution entrapped in the defect (Figure 16a) expands when the HDI PCB is heated. During components mounting by soldering or during a later thermal load, this expansion could lead to a crack of the connection. This may lead to a failure of the whole system. Therefore, the focus of the further development is on reliable defect-free filling of through holes (Figure 16b).

Fig. 16b: Through Hole Filling without electrolyte inclusion.

13,5 µm 74,2 µm

185,

2 µm

46,0 µm

15,6 µm 78,0 µm

187,

3 µm

50,2 µm

13


References[1] International Data Corporation (IDC), "Worldwide Quarterly Mobile PhoneTracker", January 2014.[2] NPD DisplaySearch, "Smartphone Quarterly report", November 2013.[3] Gartner "Forecast: PCs, Ultramobiles, and Mobile Phones, Worldwide, 2011-2018, 1Q14 Update", March 2014.

AcknowledgementThe author would like to acknowledge the support of AGES Group (Taiwan) and particularly Mr. Albert Yeh, in this project.

Summary

Due to their high functionality combined with small dimensions modern microprocessors con-tribute significantly to the progressive miniatu-rization in the field of microelectronics. The HDI PBCs allow the reliable fan-out of high pin-count electronic components using minimal space. Filling of blind microvias with electroplated copper leads to a further increase of the integ-ration density of HDI PCBs. The newly developed electrolyte Copper SLOTOCOUP SF 30, which is already applied in HDI PCB mass production enables defect-free filling of blind microvias combined with a low copper thickness on the

surface. As a result the integration density can be further increased and a more ressources-, energy- and cost-efficient manufacturing of HDI PCBs becomes possible. By the use of so-called coreless designs based on very thin core materials with through holes the integration density can be increased even more. Results of first laboratory tests show, that such through holes can be filled with electro-plated copper by direct current plating, but until now no reliable defect-free through hole filling can be achieved. The key aspect of the future development will therefore be the over-coming of this problem.

14

Bath- Name panel pattern RPP DC horiz.- vertical standard Blind Microvia anode no. plating plating plating continuous vertical Filling technology line plating line plating line

Summary of the current processes

– – Cu

– – – – Cu

– – – MMO

– – MMO

– – MMO

– – Cu

– – Cu

– – – Cu

– – Cu

03019 Copper

SLOTOCOUP CU 50

03026 Copper

SLOTOCOUP HL 10

03033 Copper

SLOTOCOUP SF 30

03105 Copper

SLOTOCOUP BV 50

03311 Bright Copper

SLOTOCOUP BV 110

03810 Copper Bath

SLOTOCOUP PRT 120 D

03812 Copper

SLOTOCOUP PRT 120

03814 Bright Copper

SLOTOCOUP CU 140

03821 Copper

SLOTOCOUP CU 210

= standard = possible Cu = copper anodes MMO = insoluble mixed metal oxide anodes

Copper Electrolytes for the PCB Manufacturing

15

11/2

014

Dr. - Ing. Max Schlötter GmbH & Co. KGTalgraben 3073312 Geislingen/SteigeGermanyT + 49 (0) 7331 205 - 0F + 49 (0) 7331 205 - [email protected]

DIN EN ISO 9001: 2008DIN EN ISO 14001: 2004

we fill the gaps ! -

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