RCC

Resin Coated Copper (RCC) is the combination material of a layer of uncured B-stage resin and a layer of copper foil, which is a unique dielectric material for building multilayer circuitry especially in HDI printed circuit boards (PCBs). Many resin coated copper films are available in several configurations with various copper and dielectric thickness, offering balances between the need to fill underlying circuitry while still maintaining the desired dielectric thickness.

In the manufacturing process of HDI PCBs, RCC takes place of the role of traditional bonding sheet and copper foil as insulating medium and conducting layer, then non-mechanical drilling method is used such as laser so that micro-via interconnect can be formed. Besides, RCC materials are compatible with conventional PCB fabrication process, like layup, lamination, DES, and are amenable to mass blind microvia formation methodologies.

 

Background of the Birth of RCC for HDI Mirovias

 

Increased density not only reduces layer counts but enables faster processing speeds. The time it takes electrical signals to travel from device to device dramatically decreases with the increase in board density. While the capacitance of mechanically drilled thru-holes is approximately 1pF, the capacitance of laser ablated (or laser drilled) blind vias is approximately 0.05pF, about one-twentieth that of through holes. Dense boards which demand such electrical performance will require many blind and buried vias. Economical construction of large numbers of blind vias will require new material solutions, such as those offered by resin coated copper (RCC). The all resin construction of resin coated copper allows the capability to create ultra-small vias, called microvias.

<6 mil as well as decrease the line and space circuit features both of which enable increased circuit density. An advantage of some of the microvias formation techniques is that they allow for mass hole formation. The mass, microvia formation techniques being explored laser ablation.

 

Microvia Forming Technique -Laser Ablation

 

Laser ablation is a microvia forming technique that can be used with nonreinforced dielectrics to create small holes, especially for microvia with dimeters equal to or less than 6mil (0.15mm), 5mil (0.12mm), 4mil (0.1mm) and 3mil (0.075mm). Fiberglass is typically an impediment in forming vias using a laser. Laser also holds advantages over conventional drilling in speed as well as cost reduction of consumables. Laser ablation on RCC to form microvias through using directed laser beams to break down copper and dielectric on RCC into particulate and/or gaseous matter.

Depending upon the type of laser that is used, the outer layer copper may also be ablated with the dielectric layer, including low flow PP and polyimide. But RCC material is expensive, and that’s why many HDI PCB fabricators use 106 PP and copper foil to replace RCC when making HDI boards.

 

What’s the Ideal Microvia Diameter to Laser on RCC?

 

When design a board with blind microvias, and this board require via-in-pad treatment process, we need to choose the best microvia diameters, and it’s better to use same diameters to improve laser drilling efficiency.

• If microvia diameter is small, the copper deposited and plated onto the microvias’ walls will be poor, while copper cap to form planarized pads will be poor (i.e. dimple is large).
• If microvia diameter is large, the copper deposited and plated onto the microvias’ walls will be good, while copper cap to form planarized pads will be good (i.e. dimple is ok).

With years’ experience of laser drilling RCC and “copper + 106 PP”, we understand a 4mil microvia diameter is always the ideal choice in HDI boards, even if the board requires ultra-fine pitch PoP component soldering.