These guidelines are designed to give fabricators essential information for processing double-sided and multi-layer printed circuit boards (PCBs) with RO4835?laminates. For further details on inner-layer preparation and multi-layer bonding, please consult the RO4400 bondply processing guidelines.

Storage: Fully clad RO4835 laminates should be kept at room temperature (between 50-90°F / 10-32°C). It is advisable to implement a first-in-first-out inventory system and to track material lot numbers throughout the PCB processing and delivery of finished circuits.

INNER LAYER PREPARATION:

Tooling: RO4835 laminates work well with various pinned and pinless tooling systems. The choice between round or slotted pins, external or internal pinning, standard or multiline tooling, and pre- or post-etch punching will depend on the capabilities and preferences of the circuit facility, as well as the final registration requirements. Generally, slotted pins, a multiline tooling setup, and post-etch punching will satisfy most needs.

Surface Preparation for Photoresist Processing and Copper Etching: The preparation of copper surfaces for photo imaging varies based on the thickness of the RO4835 core. For thinner cores, a chemical preparation process involving cleaning, micro-etching, rinsing with water, and drying is recommended. Thicker RO4835 cores can be prepared using mechanical scrub systems.

RO4835 materials are compatible with most liquid and dry film photoresists, and once patterned, they can be processed using develop, etch, and strip (DES) systems typically utilized for FR-4 materials.

Oxide Treatment: RO4835 cores can undergo any copper oxide or alternative oxide treatment to prepare for multi-layer bonding. The optimal treatment option is usually the one suggested in the guidelines associated with the selected prepreg or adhesive system.

Multi-Layer Bonding: RO4835 laminates are compatible with various thermosetting and thermoplastic adhesive systems. It is advisable to refer to the guidelines for the adhesive system to determine the appropriate bond cycle parameters.

DRILLING:

Drilling Considerations: Standard entry materials (such as aluminum or thin pressed phenolic) and exit materials (like pressed phenolic or fiber board) can be used when drilling RO4835 cores or bonded assemblies, whether in single or multi-up stacks.

RO4835 materials can accommodate a wide range of drilling parameters. However, it is advisable to avoid drilling speeds exceeding 500 surface feet per minute (SFM). For mid-range and larger diameter tools, chip loads greater than 0.002” per inch are recommended, while smaller drills (less than 0.0135” in diameter) should use lower chip loads (less than 0.002” per inch). Generally, standard geometry drills are preferred over undercut styles, as they more effectively remove debris from the holes during drilling. Hit counts should be assessed based on the inspection of plated-through holes (PTHs) rather than the condition of the tools. Drilling RO4835?laminates may lead to increased drill wear; however, the quality of the hole wall is influenced by the size distribution of the ceramic powder rather than the drill bit's cutting edge. A hole wall roughness of 8 to 25 µm is expected and should remain consistent throughout the initial and subsequent drilling cycles.

PTH PROCESSING:

Surface Preparation: Thick multi-layer and double-sided constructions can be processed using conveyorized deburring equipment equipped with oscillating nylon brushes to abrade the copper surfaces. For thinner layers, pumice scrubbing by hand, conveyorized abrasive spray processing, or chemical preparation may be necessary. Generally, the material thickness and registration requirements should guide the choice of the most suitable deburring and surface preparation method.

Desmearing is usually unnecessary for drilled holes in double-sided boards, as the high glass transition temperature of the resin system (greater than 280°C / 536°F) reduces the likelihood of smear. However, multi-layer boards may require desmearing based on the requirements of the bondply or prepreg layers. If desmearing is needed, a single or double pass through alkaline permanganate or a CF4/O2 plasma process can be employed.

Rogers Corp. advises against etchback of RO4835?laminate layers, as this may lead to excessive etching of the resin near copper layers and cause loosening of filler particles on the hole wall.

Metal Deposition:

RO4835 materials do not require any special treatments before metallization and are compatible with electroless copper processing, as well as the direct deposition of ionic and colloidal conductive layers. For boards with high aspect ratio holes, applying a copper flash plate (0.00025”) prior to imaging may be beneficial.

COPPER PLATING & OUTER-LAYER PROCESSING:

RO4835 laminates can be processed using standard acid copper and electrolytic tin or tin/lead plating for both panel and pattern processing. Once plated, these laminates can undergo any standard strip/etch/strip (SES) process.

It is important to preserve the post-etch surface of RO4835?laminates, as this surface provides excellent bonding with direct screened and photo-imageable solder masks.

Final Metal Finishes:

RO4835 laminates are suitable for organic solderability preservatives (OSPs), hot air solder leveling (HASL), and most chemically deposited or electroplated finishes.

FINAL CIRCUITIZATION:

Circuits made with RO4835 laminates can be individualized through dicing, sawing, shearing, routing, or punching. V-scoring and breakaway tabs can be employed to facilitate the separation of circuits after automated assembly.

Recommendations for Routing:

SHELF LIFE:

Rogers’ High Frequency laminates can be stored for long periods at ambient room temperatures (55-85°F / 13-30°C) and humidity levels. At room temperature, the dielectric materials are inert to high humidity, but metal claddings like copper may oxidize when exposed to high humidity. This oxidation can be easily removed with a standard micro-etch process. Additionally, after extended storage (>5 years), the exposed dielectric along the panel edges may show some oxidation. However, considering standard tooling hole and trim loss, such minimal oxidation is not expected to penetrate the usable portion of the laminate.

It is important to note that since each application is unique, Rogers cannot guarantee that its products are suitable for any specific end use. Therefore, Rogers recommends that circuit designers and/or end users test the properties and performance of these materials in their intended applications to assess their suitability over the entire product life.