Problems related to conductive pastes as fillers have led to the development of electrolytes.
To etch fine conductive patterns (75 µm lines/spaces) of the panel plated copper without excessive underetching, the total copper thickness (Figure 1) before etching must be correspondingly low. While copper thickness can be reduced by performing one or more copper thinning cycles, as shown in Figure 2, this increases costs and reduces productivity.
Productivity can be increased and costs reduced by eliminating the copper thinning step or, at least, reducing the number of copper thinning cycles. To permit this, the total thickness of the copper layer (Cu laminate + Cu strike layer, if present + Cu blind microvia filling) on the PCB must be reduced. A number of PCB manufacturers therefore specify a maximum copper layer thickness of 25 µm. This means that only approximately 18 µm of copper may be deposited on the PCB surface during the blind microvia filling process step. Besides the advantages in terms of productivity and costs, this also implies less waste of material because less copper has to be plated and etched subsequently.
Processes for blind microvia filling by copper electroplating have been used in volume for a number of years, principally in Asia. To achieve the required coating properties, these processes use high-leveling sulphuric acid copper electrolytes containing organic bath additives alongside copper sulphate, sulphuric acid and chloride. Several types of blind microvia filling electrolytes are now commercially available from a number of suppliers. The various processes differ in the following areas:
- Organic additive system.
- Anode material (copper anodes or insoluble anodes).
- Applied current form (direct current or reverse pulse plating).
- Type of plating line used (standard vertical, vertical
- continuous or horizontal continuous).
- Applicable current density.
A typical filling result achieved with current generation electrolytes results in a plated copper thickness of 22.7 µm, exceeding the limit of 18 µm required for 75 µm/75 µm lines and spaces. To meet this limit, new electrolytes offering improved filling performance needed to be developed. The objective is to deposit less copper onto the surface of the PCB than previously, while achieving at least comparable filling. This is often referred to as “superfilling.” In addition, plating times must not be longer than in established processes, and continued use of present plating equipment should be possible.
Figure 3 shows the filling achieved with this new electrolyte in a blind microvia with dimensions of 100 µm θ and 65 µm depth. Copper thickness was reduced from 22.7 µm to 10.7 µm for a plating time of 60 min. at a current density of 1.5 A/dm2. The dent is 7.6 µm.
The novel electrolyte significantly reduces copper thickness, while also achieving a slightly better filling. (The dent is shallower.) The plating time was shortened by 8 min. In addition, the novel electrolyte can be used in the same plating plant as its predecessor and does not require any alterations to equipment.
The performance of the novel electrolyte will first be demonstrated for the requirements dent < 10 µm and copper thickness < 18 µm. PCBs (size: 500 mm x 400 mm) with blind microvias (Ø: 100 µm, depth: 80 µm) were copper-plated in a 1400-litre test module equipped with insoluble anodes. This test module is identical in construction to the electroplating module of a vertical continuous plating line introduced into market a few years ago. Unless otherwise stated, all tests were carried out using PCBs that were treated with electroless copper.
Figure 4 shows the filling achieved after plating for 75 minutes at 1.2 A/dm2. The blind microvia is completely filled with copper; the copper thickness is 15.8 µm, and the dent is 0 µm.
The filling achieved with these parameters is excellent, but at a current density of 1.2 A/dm2, productivity is insufficient. Completion of blind microvia filling within a maximum of 70 min. was therefore requested. In the subsequent test, plating time was reduced from 75 to 60 min., and current density increased from 1.2 to 1.5 A/dm2, so the same amount of electric charge was available for copper-plating in both tests. Figure 5 shows that complete blind microvia filling was achieved, even with a shorter plating time of 60 min. The dent increased from 0 µm to 9.5 µm, while the copper thickness remained practically unchanged. This result shows the new process is able to fulfill the requirements regarding dent and copper thickness outlined above.
Figure 6 shows the filling result achieved when a copper strike was applied. With a copper strike, the dent is reduced from 9.5 µm to 6.3 µm.
The pre-reinforcement of the conductive layer (electroless copper or direct metallization) with an electroplated copper strike is an ideal base layer for subsequent filling. A thin copper layer with a thickness of only 2 to 5 µm is sufficient. Appropriate pre-treatment (e.g., acid cleaner) produces an active copper surface that facilitates a very quick onset of the blind microvia filling. With all other parameters unchanged, this results in an improved filling. Application of a copper strike frequently permits blind microvia filling to be carried out at higher current densities, leading to shorter plating times. In the tests described herein, the copper strike was produced using an electrolyte used for copper plating blind microvias in volume PCB production.
If lower requirements for blind microvia filling (e.g. dent < 25 µm) and copper thickness (e.g. copper thickness < 25 µm) are possible, very good results can be achieved with the new electrolyte, even with short plating times and high current densities. For blind microvias (Ø: 100 µm, depth: 80 µm) a dent of 13.3 µm, a copper thickness of 22.3 µm is achieved with a current density of 2.0 A/dm2 and a 55-min. plating time (Figure 7a). Increasing the current density further to 2.5 A/dm2 produces a dent of 20.9 µm and a copper thickness of 23.3 µm over a plating time of only 45 min. (Figure 7b).
The new electrolyte can also produce a good filling in large size blind microvias. These have a larger volume, meaning more copper is required for filling, and the plating time for a current density of 1.5 A/dm2 needed to be prolonged to 90 min. For a blind microvia (Ø: 100 µm, depth: 100 µm), a dent of 4.4 µm and a copper thickness of 24.6 µm was achieved (Figure 8a). For a significantly larger blind microvia (Ø: 150 µm, depth: 100 µm), a dent of 20.2 µm and a copper thickness of 23.7 µm could be achieved (Figure 8b) using the plating parameters as mentioned above.
Direct metallization. The new blind microvia filling electrolyte can also be used for PCBs that have been treated with direct metallization. Figure 9 shows the filling achieved on a PCB treated using a graphite-based direct metallization process. However, with unchanged plating parameters, slightly worse filling was achieved compared to the PCB treated with electroless copper (Figure 5).
Copper strike deposition improves blind microvia filling considerably, even on PCBs treated with direct metallization. Figure 10a shows a blind microvia (Ø: 110 µm, depth: 60 µm) in a PCB treated with graphite-based direct metallization after deposition of a copper strike. It is clearly apparent that the copper strike was not deposited conformally, and that the copper thickness is greater in the area of the capture pad/dielectric transition. In combination with the active surface of the copper strike, this geometry provides ideal conditions for subsequent blind microvia filling and permits a higher current density and shorter plating time to be used. Figure 10b shows very good blind microvia filling after deposition at 1.9 A/dm2 over 50 min.
Pattern plating. The novel electrolyte can also be used for pattern plating (Figure 11), but few experimental data are currently available.
Through-hole plating. The novel electrolyte can also be used for plating through-holes (Figure 12). However, it should be noted that good throwing power between 70 and 100% can only be achieved with thin PCBs and low aspect ratios. As with pattern plating, few experimental data are currently available.
Alongside electrolyte and plating parameters, the blind microvia filling result is also strongly dependent on the size and shape of the unfilled blind microvias. The best filling results are achieved with conical blind microvia shapes. However, blind microvia shapes encountered in practice often deviate significantly from this ideal. Depending on the dielectric type and the laser drilling parameters, recessions can result (see circles in Figure 13a and b).
The new electrolyte for blind microvia filling also permits overhangs and recessions to be filled with copper without defects (see circles in Figure 14). A copper strike was used in this case.
Reliability. The reliability of electroplated copper layers is an important quality criterion in PCB production, and appropriate reliability tests must be performed on an ongoing basis. The copper layers deposited using the novel electrolyte exhibit an elongation of about 20% and pass the reliability tests in Table 1.
The electrolyte is made up with copper sulphate, sulphuric acid and hydrochloric acid, and contains three organic bath additives alongside the inorganic ingredients. The concentration ranges of each of the electrolyte components are in Table 2. To ensure a good blind microvia filling, it is quite common that the copper ion concentration in blind microvia filling electrolytes is significantly higher in comparison to other copper electrolytes for PCB production.
The methods used for analysis of the individual electrolyte components are summarized in Table 3. The leveler content of the electrolyte can be evaluated by plating a Hull cell panel. A cyclic pulse voltammetric stripping (CPVS) method for leveler analysis is currently under investigation.
Operating conditions. The main operating conditions for the new electrolyte are shown in Table 4. The electrolyte is used under direct current conditions and with insoluble anodes at a maximum temperature of 22°C. Replenishment of copper ions is performed by dissolution of copper oxide in a separate dissolving unit and subsequent addition to the plating tank. It is preferable to operate the electrolyte in a vertical continuous plating (VCP) line. This equipment combines the advantages of horizontal continuous plating lines with those of standard vertical plating lines. The electrolyte may also be used in standard vertical plating equipment, but VCP lines generally produce somewhat better results.
The novel electrolyte for blind microvia filling has been used in a VCP line in mass production of PCBs (line/space 75 µm/75 µm respectively 60 µm/60 µm) for about nine months. Filling of blind microvias (Ø: 110 µm, depth: 60 µm) is carried out at a current density of 1.5 A/dm2 over a plating time of 52 min. The dent is less than 10 µm. The resultant copper thickness is approximately 15 µm, and this can be reduced to the final thickness required for the subsequent tent and etch process by a single copper thinning cycle.
Before the new electrolyte was available, its predecessor was operated in the same plant. Using this previous electrolyte, complete blind microvia filling was achieved with the same current density of 1.5 A/dm2, but a plating time of 68 min. However, the copper thickness achieved was approximately 20 µm, requiring the copper thinning process to be repeated multiple times.
Thus the new electrolyte permits both the blind microvia filling process and copper thinning process to be carried out more quickly.
Summary
The superior filling performance of the new electrolyte permits blind microvias (Ø: 100 µm, depth: 80 µm) to be completely filled with electroplated copper (dent < 10 µm), while producing a lower copper thickness (< 18 µm) on the PCB surface. This enables blind microvia filling and line/space 75 µm/75 µm respectively 60 µm/60 µm via tent and etch process without requiring multiple copper thinning cycles. This increases productivity, reduces costs and leads to less waste of copper.
In case of lower dent and copper thickness requirements, blind microvias (Ø: 100 µm, depth: 80 µm) can be completely filled with copper over even very short plating times. Even very large blind microvias (Ø: 150 µm, depth: 100 µm) can be completely filled with copper over reasonable plating times.
The new process can also produce HDI PCBs treated with graphite-based direct metallization processes. A copper strike can be applied to increase the filling further, both with direct metallization and with electroless copper.
The electroplated copper layers meet the common reliability test requirements for PCBs.
Experience with the electrolyte in mass production of HDI PCBs to date shows that the new electrolyte permits stable and reliable blind microvia filling.
The electrolyte can also be used for pattern plating and through-hole plating, but only a small amount of experimental data is currently available.
Acknowledgments
The author would like to acknowledge the support of AGES Group (Taiwan), and particularly Albert Yeh, in this project.
Bibliography
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2. J.W. Stafford, “Semiconductor Packaging Technology,” Printed Circuits Handbook, 5th edition, McGraw-Hill, 2001, pp. 2.1 - 2.22.
3. C.F. Coombs and H.T. Holden, “Electronic Packaging and High-Density Interconnectivity,” Printed Circuits Handbook, 5th edition, McGraw-Hill, 2001, pp. 1.3 - 1.22.
4. M. Carano, “Electrodeposition and Solderable Finishes for HDI,” The HDI Handbook, 1st edition, ed. by H. Holden, BR Publishing, 2009, pp. 355 - 397.
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Ed.: This article is adapted from a presentation at SMTA International, October 2010, and is published with permission of the author.
Michael Dietterle, Ph.D. is with Max Schlötter GmbH (schloetter.de); This email address is being protected from spambots. You need JavaScript enabled to view it..
