Although manufacturing printed circuit boards through additive processes resolves some of the difficulties presented by subtractive manufacturing, additive methods also introduce their own levels of complexity and challenges in producing circuit boards of consistent quality. In essence, additive processes are based on introducing material to an assembly rather than subtracting from it, and in the case of printed circuit board manufacturing, this process can come in a variety of forms, including methods for working with three-dimensional circuits in molded arrays. Among planar circuit boards, swell-etch and adhesive methods are the most common types of additive processes, with both catalytic and non-catalytic laminate adhesion serving important roles. The difference between catalytic and non-catalytic laminates centers on whether the laminate is seeded with a catalyst material for electroless copper deposition or not.
Examining swell-etch and adhesive methods can help illustrate the fundamental principles that distinguish additive processing from other techniques, as each method provides a separate course for effectively bonding conductors to a dielectric base. In subtractive processing, an equivalent procedure involves the use of elevated temperatures and pressures. The copper foil in subtractive laminates is formed with protrusions on one side, and this roughened surface can support adhesion to a dielectric. By contrast, swell-etch processes physically duplicate the mechanical properties of a subtractive laminate by chemically producing cavities in the dielectric and filling them with copper. On the other hand, the adhesion process relies on applying an adhesive layer to bond conductors to the dielectric.
Laminate Adhesion and Catalysts
The quality and strength of a conductor’s adhesion to a substrate is an important factor for most additive processes. While the adhesion level at room temperature for subtractive laminates tends to fall within a certain range, such as 8 to 10 pounds per inch, variations within additive processes are often wider and can be unique to the specific method used. In addition to standard circuit board laminate requirements, additive laminates must also meet specific adhesion and catalyst parameters.
A laminate used in swell-etch processes may need extra resin covering its glass material to prevent rooting structure formation from exposing the glass reinforcement. A laminate used in adhesion processes, on the other hand, features an adhesive coating designed to activate at a specific stage of the production sequence, allowing the board to be handled normally until it is triggered. To fulfill catalyst requirements, the laminate must be sufficiently seeded with catalyzing agents so that a drilled hole will expose enough catalyst to undergo a reaction. However, the amount of catalyst should not exceed the level at which it would begin to degrade the laminate’s electrical properties.
Imaging in Additive Processes
Depending on the specific production method employed, the primary imaging for an additive process may have to meet some or all of a range of criteria, including:
• Adhesion to the dielectric base
• Resistance to print-through processes
• Resistance to chemicals and chemical reactions
• Resistance to catalyzing reactions
Print-through is caused by small amounts of light passing through the substrate and reaching the laminated layer on the opposite side of the dielectric. This layer is known as the resist, and it is used to transfer circuit patterns onto a substrate. Bounce-back is a similar problem involving light that is reflected from the dielectric at an improper angle from the glass reinforcement, sending it back to the target resist but preventing it from returning to its original point. Print-through and bounce-back problems can impede the development of conductor traces onto a functioning resist, but the use of a laminate with higher opacity can reduce the risk.
Most imaging resists are designed to bond to copper and it is usually possible to adjust the resist’s processing parameters to achieve a suitable level of adhesion. However, the resist may also be exposed to a variety of chemicals and chemical reactions that can undermine its integrity. After the circuit pattern has been developed, baking and supplementary exposure can be used to form additional cross-linking and thus increase the chemical resistance. This should also help prevent the resist from undergoing a catalyzing reaction that would hinder its ability to be stripped in the future.
Copper Deposition
Some additive processes rely on two electroless copper baths, with one serving as a standard flash bath that responds to the catalyst, while the second is responsible for depositing a copper coating that will improve thermal shock resistance. In an additive process, the thickness of the conductors tends to be highly uniform across the panel assembly, which means that multiple boards placed on a single panel will feature the same copper thickness regardless of their positions. The slow rate of the electroless copper deposition process can be a disadvantage, as the deposition may take as long as 24 hours to complete. However, the rate remains constant and easier to control and predict than a faster technique. Depending on the additive process used, the panel may not include a resist at the electroless copper coating stage to channel the deposition. In the absence of a resist, the copper is deposited at the same rate along horizontal and vertical axes, and an allowance may be needed to ensure there is enough space between adjacent parts.
