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: 1. Adhesion to the dielectric base 2. Resistance to print-through processes 3. Resistance to chemicals and chemical reactions 4. 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
Page|23 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.
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- Fall '18
- Gaurav Beniwal
- Printed circuit board, Circuit Boards, layer PCB, Single Layer PCB