High-Frequency Laminates are when an electrical conductor is covered with a thin, molecularly thin insulating layer, forming an electrically conductive surface with excellent damping properties.
Making high-frequency laminates creates a Rogers RT/duroid 6202 High-Frequency Laminates. These laminates are for several electrical applications used by Rayming PCB & Assembly. For example, they include the damping of inductors. As a result, an impedance measuring bridge is a useful tool in determining the correct laminate. This paper will explain how to make the laminate and use the impedance bridge.
A Rogers RT/duroid 6202 High-Frequency Laminate consists of several layers of metal (or composite) foil, a thin dielectric spacer, copper foil, another dielectric spacer, and finally, another dielectric spacer, a layer of metal (or composite) foil. Additionally, the outermost layer has a thickness of one-eighth the skin depth at the frequency desired. In addition, there is an air gap at the middle metal foil layer to prevent any current flow or eddy currents seen in the innermost RF layer at HF frequencies.
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Handling
High-frequency laminates, once cut, needs careful handling. You must never touch the adherent side. This is because it will pick up a charge and stick to other objects. When removing the laminate from the work table or circuit board, do so with the adherent side facing down or use a pair of roller nippers.
Some of the essential factors in determining the impedance of a high-frequency laminate are The gauge of the metal foil, the thickness of the dielectric spacer, frequency, and inter-laminar spacing.
The metal foils used in high-frequency laminates are either nickel or copper foils. Nickel has a higher electrical conductivity than copper. Generally, the higher the conductivity of the metal, the better it performs. The thickness of the dielectric spacer is also significant. The thinner the spacer, the lower is its impedance and the greater amount of RF current that can flow through it.
Single- and Double-Sided PCB Fabrication Guidelines
We can produce PCB cartesian with a single-sided or double-sided core. Single-sided PCB cartesian cores are preferred for RF applications to reduce ground currents, reduce EMI effects, and simplify the layout. We should place ICs as close to the die as possible and close to the top of the board.
- Surface Preparation:When laminating high-frequency laminate on a single-sided board, the top surface of the board must be smooth. Use fine-grit sandpaper to remove any cuts, nicks, or discolorations.
Wear a dust mask to protect yourself from the dust generated during the surface prep process.
- Tooling Holes:Use a drill and carbide bit to drill tooling holes. A 1/8 inch bit is sufficient for all high-frequency laminates. The Holes should be 1/8″ from the edge of the board to accommodate imperfections caused by machine drills. Drill deeper if necessary.
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Drilling
You should drill the hole with a drill press only. Do not use a power drill.
If the hole is oblong, it will be necessary to drill it out with a larger bit so that we can press the laminate into place. The hole must be free from any burrs or sharp edges.
Press the laminate into place using a roller tool. Ensure that you press the laminate onto the board and no air pockets.
Avoiding Smear:
Recall that we ground the laminate on top. This prevents a smear from occurring when rolling the core over the laminate. Overheated tools can cause the laminate to stick together and move. To avoid this, use as gentle a touch as possible. Make sure the roller is clean and dry.
Stack Construction:
You should stack the laminates from the lowest frequency to the highest frequency. Use as few layers of laminates per board as possible. We can drill the core in stacks or individually. In this case, we use a stacked construction. The highest frequency RF layer can usually be up to 2 layers thick.
Drill Type:
We recommend using the drill-n-fill method for cores with single-sided circuits. This is a process to drill the core in stacks. You will stack the drilled core on top of a previously drilled circuit board, which must be pre-cut to match the hole sizes of the stack of laminate just laid down. During this process, make sure that you center all the holes and not too deep so that they will run into the other side of the board.
Drill Parameters:
The drill used must be a solid carbide drill bit and should be sharp.
This tool comes with the cartesian and will be one of the few tools you will use in this process. Do not let this tool get damaged. To remove the drill, place a pin or nail through the hole left by the drill. Hook the head of the nail with a pair of pliers or another similar tool to remove it from your circuit board.
Deburring:
Once the laminate is saturated, it tends to shrink. If a component is not tight enough in each hole, this can cause the component to be pulled away from the circuit as the laminate shrinks. The minimum inter-laminar spacing should be 0.002 inches (50µm). Any less than this will result in an effect called cross-modulation. Cross modulation occurs because of capacitance between layers of laminate and any adjacent traces. This capacitance can result in unwanted coupling or crosstalk.
Hole Preparation:
The hole preparation process is a critical step in producing a PCB. Unfortunately, incorrect drilling of the holes has led to many problems such as short circuits, solder bridging, and circuit failure.
Metallization
The best way to manufacture a PCB is to use the in-mold technique. Packaging can be done by using aluminum blanks or through injection molding. In-mold laminating is a process where a precision metal foil mesh is applied directly to the laminate. Molding involves pressing the laminate in between two metal plates called molds. The metalized laminate is then pressed between two metal plates and forced into a die under pressure and heat. To ensure a suitable mold, it is necessary to maintain a temperature range of 134-152 degrees Fahrenheit to maintain the highest level of production and yield.
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Resist Strip,
Resist is a thin metal film coated on the inner side of a printed circuit board. The resist coating can be a thin, continuous film or can probably shape into thick strips. If you make the strips, they should be cut and properly sized to fit all holes in the laminate of the circuit board. At least one millimeter thick is ideal, but thicker may be used depending on the size of the hole to fill and the type of material used.
Soldermask Application
This section outlines the steps necessary to apply the Soldermask onto the raw laminate board. The first step is the cleaning of the laminate board. You should remove and clean all dirt, oils, and grease with a cleaning solution.
The second step is applying a thin layer of inorganic solid solder reflowable material onto all surfaces of the laminates using a soldering iron and solder paste solder gun.
The third step involves applying the other side of the solder mask on top of the laminate. Again, we use a squeegee to push out excess amounts of solder paste solder mask. Furthermore, there should be no voids between or around any holes during this process.
The fourth step is applying a copper-base metal reflowable material to all laminate surfaces. These materials are not suitable because there is no way for the laminate to conduct electricity in a vacuum environment, so this can cause problems with production and testing. In addition, depending on the thickness of the base material, it will drip onto the laminate due to gravity. This can cause a bigger mess of melted solder masks if it drips down into any voids in between or around any holes or during this process.
Final Circuitization
This step aims to apply an acrylic reverse-side coating on the raw laminate circuit board. We do this by placing an adhesive layer on top of the surface of the laminate. Then we flip the board, place a paper mask over this adhesive, and put it in place. Finally, dispensing solder onto the paper mask and lifting excess amounts using a squeegee will create a final circuit pattern.
