Via hole plugging processes: an overview of the advantages and drawbacks to typical plugging methods.

The demand for HDI circuit boards has made via plugging popular. Yet, as it often turns out, the filling process itself and the subsequent process steps for creating a planar and metalized plug are not as easy to achieve as one should expect.

As a rule, the production steps shown in FIGURE 1 are observed for the optimum plugging of buried vias, independent of the mode of plugging.

[FIGURE 1 OMITTED]

Generally, there are four processes available to plug buried vias:

* Screen/stencil printing.

* Roller-coating.

* Vacuum plugging.

* Modified stencil printing.

Screen/stencil printing. When plugging by means of stencil printing, vias are filled by means of a corresponding stencil. This is achieved by placing a stencil drilled with the same pattern but with larger diameters underneath the PCB (FIGURE 2). The paste should be printed in such a manner that a small "hump" (nailhead) remains on the underside in order to ensure a complete filling. The printing parameters largely depend upon the aspect ratio of the vias (the ratio of material thickness to hole diameter) and must be determined by trials. Depending on the product used, an aspect ratio of tip to 12:1 can be realized.

[FIGURE 2 OMITTED]

Advantages of stencil printing include:

* Selective plugging; i.e., the only vias filled are those that need to be filled (positioning holes can remain free).

* The process is well known.

* The equipment already exists. Disadvantages include:

* A printing stencil is required for each layout.

* Difficult printing in case of substantially varying hole diameters on the PCB.

Modified roller-coating. The roller-coating process is performed by forcing paste on a roller through the vias (FIGURE 3). The pastes that are especially developed for this application differ from the screen/stencil printing types with respect to their lower viscosity and higher flow. The paste is kept in a trough while the roller-coater picks up the paste on its underside. The board is pressed against the roller-coater from above and the paste located between roller and PCB is pressed through the vias. Then the board is passed between two stripper squeegees so that the plugging paste residues are removed. In order to enable a slight nailhead on both sides of the panel, the practice is to position the stripping squeegees as flat as possible and slightly offset. The angle between squeegee and PCB should be <45[degrees] (FIGURE 4).

[FIGURE 3-4 OMITTED]

Advantages:

* Fast, secure filling of all vias.

* Simple process; only a few parameters have to be adjusted.

* No screen stencils are required (they are independent of layout).

* Less smearing and residues on the PCB.

* Depending upon stripping squeegee setting, a slight dip (plugging paste sink-in) or a nailhead can be realized.

* Depending on plugging paste used, aspect ratios can reach up to 17:1.

Disadvantages:

* Vias that are not to be plugged have to be masked.

* Cannot be used for structured PCBs because the squeegees cannot remove excess material from between the conductors.

* Only suitable for plugging materials that have a good storage stability at room temperature because the roller coater trough capacity is 8-10 kg (with customized trays the necessary amount of plugging paste can be reduced to 3-5 kg).

Vacuum plugging. A newer method of applying plugging pastes is performed using vacuum plugging equipment in which blind and buried vias are filled. While applying a vacuum onto the processing chamber, the plugging paste is pressed through the vias and into the blind vias by a special squeegee that travels over the panel (FIGURE 5). This squeegee, with a trough shape, is placed airtight onto the panel and fed with the pressurized plugging paste. The applied vacuum is supposed to prevent air inclusions in blind vias.

[FIGURE 5 OMITTED]

Modified stencil printing. This describes a novel process developed by Honeywell Advanced Circuits (now part of TTM) called "Fulfill" for filling vias with a high aspect ratio. This process is largely similar to screen printing, but the squeegee is replaced by a "trough" via in which the plugging paste is pressed into the vias. This process is set up with a special screen printer.

Advantages:

* Very. fast process on account of bi-directional filling (FIGURE 6).

[FIGURE 6 OMITTED]

* Positioning holes are exempted from filling.

* Suitable for an aspect ratio up to 13:1.

* Closed system prevents air from getting into the paste.

Disadvantages:

* Not suitable to plug "blind via holes" (risk of air inclusions).

* Partly suitable for partial plugging of PCBs.

* Not suitable to plug structured PCBs.

* A printing stencil (for placing underneath) is necessary for each layout.

Thermal Curing and Planarization

After thermal curing of the plugging paste--for most thermal setting plugging pastes this is carried out at 150[degrees]C for 30-45 min.--smearing and residues of the plugging paste are removed from the PCB by means of brushing and/or polishing, and the PCB and the via fillings are leveled.

Curing times are generally indicated as object holding times, meaning that the curing time is measured from the point when the panels reach curing temperature. The observation of the indicated curing conditions is of course mandatory to ensure the final properties are met, especially regarding Tg and a low CTE value.

The planarization process involves some critical aspects. For instance, the edges of the copper barrels are not removed. Moreover, it has to be ensured that the plugging paste is not partially removed from the vias. Practical experience shows that hard brushes with a ceramic surface give very good results in order to ensure a reproducible and perfectly even surface.

Furthermore, vertical belt grinding machines used in metalworking as well as oscillating grinders have in principle withstood the test. The main advantage of oscillating grinders is their relatively low cost compared to other processes. It is not clear how they will perform in a high-volume environment; data are nor available. However, this planarization method should be interesting for lower volume environments. The vertical belt grinding machine permits excessive material to be removed with high precision and throughput.

In order to facilitate brushing, various adjustments of plugging paste are available. Of interest are products that can be cured in two stages. In the first stage, the material is cured by UV radiation. At this curing stage the plugging paste is still "soft" and easy to brush. After leveling, the thermal curing process is carried out and thus the ultimate properties are achieved.

Standard Metalization

An essential step in HDI technology is the metalization of the plugged vias. This step employs standard metalization processes. Prior to electroless copper deposition, a desmear step is required that ensures the cleaning and activation of the resin matrix. The desmear process is a) swelling of resin matrix, and b) etching and roughening of the surface reduction.

The requirements for a user-friendly, reliable plugging paste are generally as follows:

One-pack system, Using a two-pack system would inevitably lead to air-bubble entrapment when mixing both components, which--when the material is transferred into vias during application--reduces the reliability of a plugged hole due to air pockets.

Solvent-free resp. low in VOCs, to avoid shrinkage during cure and resulting adhesion problems as well as to avoid the creation of non-planar surfaces.

Good flow characteristics, to ensure easy filling of different via diameters without the risk of cavernous or dipped surfaces.

Air bubble-free. While the danger of including air into the plugging paste is at its largest while bringing it into the vias, application of a vacuum during manufacturing can largely reduce the general risk of air bubble occurrence.

Possibly high Tg. Tg must be seen in conjunction with the CTE value. With temperature stress (e.g., temperature cycling tests) carried out below Tg, a constantly low CTE value is ensured.

Easy brushing, to avoid multiple brush passes while avoiding the risk of creating cavernous or dipped surfaces.

Resist soldering operations. No cracking or delamination; typical test is 10 cycles at 288[degrees]C for 10 sec.

Resist process chemicals. This mainly refers to the resistance of electroless or electrolytic plating in combination with a good adhesion of the applied metallization.

Good adhesion between the plugging paste and copper. A common test method is 1000 cycles @ -55[degrees]C/125[degrees]C.

Though not required for all applications, meeting UL 94 flammability standards has to be seen as a more or less basic requirement of a plugging paste. Besides these basic requirements and properties, additional points are mandatory for certain applications. One example: outgassing tests and requirements according to ASTM E595, which have to be met to order to qualify for NASA spacecraft applications, for instance.

Test criteria for ASTM E595 are total mass loss (<1.00 %TML) and collected volatile condensate material (<0.10% CVCM). This testing is decisive for the use of products in a vacuum, because outgassing materials can condense on sensitive components and cause problems and failures. For non-space applications this testing increases user confidence in a product due to the fact that shrinkage on account of volatile ingredients is excluded.

TABLE 1. Stencil Printing Plugging

Printing stencil    Stencil is stuck on V2 steel fabric, then the
                    fabric is removed in the printing area. Stencil
                    holes should be minimally larger than the vias of
                    the plated-through holes (PTHs).
Printing underlay   E.g., base material where vias are considerably
                    larger than the diameter of the PTHs
Setup               Only possible by pinning the reference holes
Print               Minimum snap-off without any lift
Squeegee speed      2.0 m/min
Printing squeegee   75 Shore
Squeegee profile    45[degrees], slightly broken
Flooding            Must be well filled
Flood squeegee      75 Shore (no metal)
Squeegee profile    90[degrees], sharp

BIBLIOGRAPHY

VDE/VDI--GMM, et al, The European Technology and Trend Report 2001/2002 on HDI-High Density Interconnected PWBs, October 2001.

"RCF," Galvanotechnik 89, no. 7, 1998.

Plugging processes Filling of buried vias by means of plugging processes, Lackwerke Peters technical information, 2002.

J. Pedigo, et al, "A Review of Filling High Density, high Aspect Ratio Vias in a High Volume Production Setting, IPC Annual Meeting Proceedings, October 2001.

Trace Laboratories test report, November 2001.

SVEN E. KRAMER is manager, application technology at Lackwerke Peters GmbH + Co (peters.de). He can be reached at [phone]; [e-mail].