Inner layer processing is a critical stage in multilayer printed circuit board manufacturing, requiring precise control of surface preparation, imaging, developing, etching, and resist stripping. The right chemistries and careful process control can help you produce consistent circuit patterns, minimize defects, and prepare inner layers for successful integration into the finished board.
The inner layers of a multilayer printed circuit board (PCB) are where the board’s internal circuitry takes shape. Processing these layers precisely is essential for the performance and reliability of the finished board. Choosing the right chemistries to support each stage of inner layer processing contributes to the success of your end product.
Seacole’s technical sales team can help you identify the chemistries you require for surface preparation, photoresist and imaging, developing, copper etching, and resist stripping. Our products perform optimally without disrupting pH, resist loading, etch conditions, or filtration.
Why Inner-Layer Process Control Matters
Inner layers become part of a multilayer board’s construction. Once they are completed and incorporated into the board, correcting defects in the inner layer can be difficult or impossible. Process control during develop, etch, and strip are particularly important to the overall quality and performance of the finished product and to reduce rework down the line.
Careful control of your chemistries, equipment, and operating conditions can help you produce consistent circuit patterns and minimize defects at the inner layer stage, preparing the way for successful outer layer, solder mask, and final finish processes.
Preparing the Copper Surface
Surface preparation ensures your photoresist adheres consistently to the copper and your circuit image transfers accurately. Cleaning is particularly important because contaminants or residues can interfere with resist adhesion and can contribute to defects later in the process.
Several chemical treatments may be used to prepare the copper surface. Two that the Seacole team may recommend for your process include CHROME/Clean AC-1S and Cu/Scrub 4F-9.1.
- CHROME/Clean AC-1S: Concentrated, non-etching, free-rinsing acid cleaner designed to remove chromate and zincate anti-tarnish coatings from copper foil.
- Cu/Scrub 4F-9.1: Acidified pumice scrub designed to promote cleaning and micro-roughening of copper laminate.
Once the copper is properly prepared, it is ready for a layer of photosensitive dry-film resist to be laminated onto the panel to prepare for the imaging step. Accuracy is critical when imaging the inner layer, as it creates the foundation for the circuit pattern and determines which portions of the copper will remain on the board after etching.
Developing the Circuit Pattern
Developer chemistries remove appropriate portions of the unexposed photoresist to reveal the underlying copper in the areas that will be etched away in the next step. The developing process should create clean, well-defined circuit features.
Seacole carries a variety of chemistries to serve this step of the process, including:
- PC-450C: Develops fully or semi-aqueous dry films and/or liquid photo imageable solder masks. Contains chemical stabilizer and cleaner.
- PC-750: Develops fully or semi-aqueous dry films and/or liquid photo imageable solder masks. Contains chemical stabilizer. Increases pH of solution to obtain 15–20% more throughput per run.
- Anti/Foam CR 98: Non-silicon, non-solvent anti-foam for the developing and stripping process. Rinses completely, leaving no residue behind on board surfaces or equipment.
- Anti/Foam CR 98S: Organic defoamer eliminates foam during the developing and stripping process. Rinses completely, leaving no residue behind on board surfaces or equipment.
- Anti/Foam CR 99: Non-silicon, non-solvent anti-foam for the developing and stripping process. Rinses completely, leaving no residue behind. Especially effective with high pressure and high solution throughput equipment.
Process conditions such as pH, resist loading, equipment performance, and development time can affect the result of your developing process. Inadequate development can leave resist residue behind, while overly aggressive conditions can damage the image. Work with the Seacole team to find the balance that is right for your process.
Etching Away Unwanted Copper
Etching removes the copper that was exposed during the development step. Etchants attack the copper that is not protected by the photoresist, leaving behind the circuit pattern. Etching conditions must be controlled carefully to maintain line widths and circuit geometry.
For this delicate step, the Seacole technical team may recommend one of the following cupric etchant chemistries.
- Re/Gen CC-40: Sodium chlorate solution regenerates cupric chloride etching systems using VIS-U-ETCH and a secondary specific gravity control. Eliminates need for chlorine gas storage. Replacement for chlorine gas systems.
- Re/Gen CC-45: Sodium chlorate solution regenerates cupric chloride etching systems. Safer alternative to chlorine gas as a regenerative oxidizer. Eliminates need for chlorine gas storage. Replacement for chlorine gas systems.
These chlorate solutions help you maintain carefully controlled etching conditions that improve safety while achieving accurate circuit geometry.
Stripping the Resist
Once imaging and developing are complete, it is time to remove the photoresist layer. Resist strippers dissolve the remaining resist to expose the finished inner-layer circuitry.
The Seacole technical team may recommend one of these effective resist strippers for this stage of the PWB manufacturing process:
- POLY/SOLV RS-40: A good economical dry film stripper. Strips all fully aqueous dry film resists down to 0.2–0.5 cm to be flushed away during filtration. Less odorous and easier to waste treat than other products due to lack of sodium, potassium or ammonium hydroxide, glycol ethers, or alcohols.
- POLY/SOLV RS-50: Another option for a good economical dry film stripper. Strips all fully aqueous dry film resists down to 0.2–0.5 cm to be flushed away during filtration. Blends copper and solder inhibitor to reduce the attack of solder on hard boards. Less odorous and easier to waste treat than other products.
- POLY/SOLV RS-70: Builds on the technology of RS-40 and RS-50. Strips all fully aqueous dry film resists down to 0.2–0.5 cm to be flushed away during filtration. Reduces entrapment of dry film in areas exhibiting tin or tin/lead over-plate. Less odorous and easier to waste treat than other products.
- POLY/SOLV RS-720: Cornerstone product for resist stripping of smaller boards. Advanced formula strips all fully aqueous dry film resists down to 0.2–0.5 cm to be flushed away during filtration. Reduces entrapment of dry film in areas exhibiting tin or tin/lead over-plate. Unique blend of inhibitor compounds minimizes corrosion of tin-lead or solder on hard boards. Less odorous and easier to waste treat than other products.
- POLY/SOLV RS-740: Developed for the challenges of today’s boards with ultra fine lines and tight spacing. Strips all fully aqueous dry film resists down to 0.2–0.5 cm to be flushed away during filtration. Penetrating additives reduce entrapment of dry film in areas exhibiting tin or tin/lead over-plate. Minimal chemical and/or galvanic corrosion attack on solder. Less odorous and easier to waste treat than other products.
Once the photoresist has been stripped away, the inner layer manufacturing process is complete, and the printed circuit board is ready for outer layer manufacturing.
Seacole’s technical team can help you identify the chemistries appropriate for your specific processes. Our goal is to help you design a reliable, efficient process that produces high-quality finished boards.
Bring us your printed circuit board manufacturing challenges, and we’ll deliver solutions. Contact us today for expert technical advice on your PCB process.
Inner Layer Processing FAQs
Why is process control important during inner layer manufacturing?
Once an inner layer is incorporated into a multilayer board, correcting defects can be difficult or impossible. Controlling chemistry, pH, resist loading, etching conditions, equipment performance, and other process variables helps maintain consistent circuit patterns and reduce defects and rework.
What chemistries are used for PCB inner layer processing?
Inner layer processing may require a range of chemistries, including copper cleaners and surface-preparation products, photoresist developers, anti-foams, cupric etchant chemistries, and dry-film resist strippers. The appropriate products depend on the specific manufacturing process and equipment.
What process variables affect inner layer etching performance?
Etching performance can be affected by factors such as etchant concentration, temperature, spray pressure, solution flow, specific gravity, copper loading, and exposure time. Maintaining these variables within the appropriate operating range helps control etch rate, minimize undercutting, and maintain the intended circuit line widths and geometry.
What happens after the inner layer is completed?
After the photoresist is stripped, the completed inner layer moves on to the next stages of multilayer PCB manufacturing. The finished inner layer will eventually be incorporated into the multilayer construction before outer-layer processing, solder mask application, and final finishing.
