- 09 January 2006 -
Intelligent Design
By Jim Watkowski, Director of Metallization Technology and Donald DeSalvo, Applications Manager for Metallization, MacDermid Inc., Waterbury, Conn.; and Naomi Ando, senior research and development scientist, MacDermid Inc., Yokahama, Japan

The growing demand for higher performance has pushed designs to incorporate via in pad designs and stacked microvias.

Figure 1: Voids in solder. (Click image to enlarge)

During assembly, solder paste is applied and must fill the blind microvia (BMV). If air or volatiles are trapped within the BMV, it can cause voids (see Figure 1) and subsequent outgassing during the reflow process.

To enhance reliability and prevent the formation of voids in the solder joint and/or outgassing during the assembly operation, the BMVs can be filled. Blind microvia can be filled with conductive pastes, but the method of application of these pastes, and the difference in the thermal co-efficient of expansion between the pastes and the surrounding copper barrel, make them difficult to manage.

Electrodeposited-copper via filling offers many benefits:

• Electrodeposition of copper is a well-known and established technology in the PCB fabrication industry.
• Sulfate-based electrolytic copper is cost effective.
• Copper-filled vias possess the same electrical properties as the circuitry and are more conductive than organic, conductive pastes.
• Copper-filled vias enable the use of stacked via designs.
• Copper-filled vias have the potential to improve thermal conductivity and heat dissipation in PWBs.

Capability Vehicle
The test vehicle used in the capability evaluation was a CO₂-drilled blind microvia design. Varying hole diameters, in a 4 x 4-in. grid pattern, enabled all variations of diameter and depth to be plated simultaneously.

On side 1, the via diameters were 75 µm, 100 µm, 125 µm, and 150 µm by 50-µm depth. On side 2, the via diameters were 100 µm, 125 µm, 150 µm, and 175 µm by 75-µm depth.

Figure 2: RCC was used for 50 -µm depth.

Figure 3: Prepreg was used for 75-µm depth.

Dielectric materials have an effect on filling capabilities. Generally, non-glass reinforced materials are easier to fill. The uniformity of the laser-ablated sidewall enhances solution movement and chemical replenishment at the bottom of the via, thus allowing a uniform deposition.

Protruding glass fibers have a deleterious effect on filling capability. These fibers challenge the initial metallization process. Poor initial metallization coverage or voiding beneath the fibers can cause plating folds or incomplete fill. Chemical replenishment in the deep recesses is more difficult—the end result being the via plates shut before they can be filled and entrap volatiles.

Both RCC and prepreg were utilized in the test vehicle (see Figures 2 and 3). RCC was incorporated into the construction for the 50-µm depth and prepreg was used for the 75-µm depth.

Via Filling Criteria
Product development protocol defined the objectives as:

• Maximize the via-filling capabilities across the range of geometries utilized on production scales;
• minimize the amount of copper deposited on the surface of the PWB;
• and meet thermal reliability criteria (IPC 2.6.8).

Figure 4: Acceptance criteria is filling ratio > 80%. Fill ratio = B/A x 100.

Table 1: Prcoess Parameters

The metric used for defining filling capability is the industry-accepted fill ratio. Fill ratio is the thickness of copper plated, as measured from the capture pad to the bottom of the dimple (B), divided by the total thickness as measured from the capture pad to copper plated surface (A) times 100 (see Figure 4).

Via Filling Bath Components
Sulfate-based copper systems are a well-established technology in the PWB fabrication industry. Due to their wide use and low operating cost, development efforts concentrated on utilizing the sulfate system to accomplish the goal of filling blind microvias.

Organic additives are required to modify the electrolytic copper deposit. A combination of suppressors, anti-suppressors, and levelors facilitate microdistribution enhancement, grain structure, leveling characteristics, physical properties, and cosmetics. Identifying the proper organic species, understanding their interactions, and optimizing concentrations for robust performance is critical to developing a successful system. (See Table I.)

The inorganic components of the bath were investigated to determine their impact on filling vias. Unlike typical high-throw electrolytes, lower acid levels and increased copper levels proved to be key to filling vias.

The primary inorganic contributing factor for filling success is the copper sulfate concentration. Conversely, microdistribution of through holes decreases as a function of increasing copper concentration.

Figure 5: Type 2 suppressor or levelors.

The organic components of the system are a combination of suppression and anti-suppression additives. One type of suppressor is adsorbed onto the copper surface of the substrate, in effect, decreasing the plating rate, or suppressing plating.

The anti-suppressor increases the plating rate by selectively desorbing the suppressor.

The second type of suppressors present in this system is responsible for the filling mechanism. The type 2 suppressors (Figure 5), or levelors, displace the anti-suppressors in the high current density areas, such as the substrate surface. The lower current density areas, such as the bottom of the blind microvia, has less concentration of the levelor, hence the plating rate at the bottom of the via is faster than at the surface.

As the via fills, more levelor displaces the anti-suppressors and the plating rate slows.

Solution Movement
Variations in solution movement and fluid delivery systems were investigated. Low, medium, and high air agitation, as well as 90º direct impingement, and cathode movement were tested for their impact and filling performance. Unlike typical pulse plated copper, where increased solution agitation is beneficial, the results of the increased agitation, whether it resulted from direct impingement or increased air, were the same.

The more vigorous the agitation, the less filling capability the process exhibited. It is theorized that the increased agitation disrupts the interaction between the levelor and the anti-suppressor in the high current density areas, as well as increases the concentration of levelor at the bottom via and, in effect, decreasing the plating rate.

Figure 6: The via fill process.

Figure 7: Filling capablities remain constant beyone 300AH.

Capabilities
Utilizing the capability test vehicle described earlier, and using a fill ratio of >80% for acceptability, the capability of the process is quite extensive. The via fill process, when operated within specified parameters, can achieve acceptable fill with aspect ratios of 0.4:1 up to 1:1. The hole diameter range of 75 µm up to 175 µm and depths as deep as 75 µm were successfully filled (see Figure 6). These geometries cover the vast majority of the production work presently being built. Plating time required to achieve fill results varied depending on the diameter and depth of the via being plated as well as the sidewall formation profile.

Process capability was evaluated as a function of bath life. Filling characteristics were verified to establish the ampere-hour per liter where the capability fell below our minimum acceptable criteria of 80% fill ratio. Filling capabilities remain constant beyond 300AH/l (see Figure 7).

Conclusion
The DC copper electroplating system has been developed to consistently fill blind microvias over a range of geometries. No special rectification, nor fluid delivery system is required, resulting in low capital expenditure to accomplish the filling goals. Existing electrolytic tank designs and specifications enable the PWB fabricator to adopt the technology and provide enhanced reliability for via in pad design and to expand the technology to a stacked microvia package.
Development continues to include a pattern plate process as well as increasing the fill capabilities to include deeper vias (100 µm and deeper), and aspect ratios in excess of 1:1. Investigation continues into the interaction of complex wave rectification, new electrolyte compositions and their impact on via filling, high aspect ratio plating, and thermal cycling performance.

References

1. Singer, A., Chouta, P. et al., "Effect of Via in Pad Via-Fill on Solder Joint Formation."
2. Takahashi, K., "Novel Build-up PWB for Latest Mobile Phone," IPC Fall Meeting; Sept. 2003.

Jim Watkowski is director of metallization technology and can be contacted at (e-mail jwatkowski@macdermid.com); Donald DeSalvo is the applications manager for metallization; and Ron Blake is senior application engineer. They are based in Waterbury, Conn. Naomi Ando is a senior research and development scientist based in Yokahama, Japan.