2. It is used in fields involving high frequency (frequencies greater than 300 MHz or wavelengths less than 1 meter) and microwave frequencies (frequencies greater than 3 GHz or wavelengths less than 0.1 meter).
3. This PCB is manufactured on a microwave substrate copper-clad board, employing either part of the process used for ordinary rigid circuit board manufacturing or using a specialized processing method.
4. Generally speaking, a high-frequency board can be defined as a circuit board designed to operate at frequencies above 1 GHz.
1. With the rapid advancement of science and technology, equipment designs are increasingly applied in the microwave frequency band (>1 GHz) and even the millimeter wave field (30 GHz). This trend signifies that frequency requirements are rising, which in turn escalates the demands on PCB substrate materials. For instance, substrate materials need to exhibit excellent electrical properties, good chemical stability, and minimal loss at higher signal frequencies, highlighting the significance of high-frequency boards.
2. Application fields for PCB high-frequency boards include mobile communication products, power amplifiers, low-noise amplifiers, passive components like power splitters, couplers, duplexers, and filters, as well as automotive anti-collision systems, satellite systems, and radio systems. The trend in electronic equipment development is towards higher frequencies.
3. Classification of high-frequency board materials includes:
A. Powder ceramic-filled thermosetting materials:
– Manufacturers: Rogers 4350B/4003C, Arlon’s 25N/25FR, Taconic’s TLG series.
B. PCB circuit board processing methods:
– The process is similar to epoxy resin/glass woven cloth (FR4), though the sheet is more brittle and prone to breakage. Drilling and milling tools experience a 20% reduction in lifespan.
4. PTFE (polytetrafluoroethylene) material:
A. Manufacturers: Rogers’ RO3000 series, RT series, TMM series; Arlon’s AD/AR series, IsoClad series, CuClad series; Taconic’s RF series, TLX series, TLY series; Taixing Microwave’s F4B, F4BM, F4BK, TP-2.
B. Processing methods:
1. Cutting: Keep the protective film during cutting to avoid scratches and creasing.
2. Drilling:
– Use a new drill bit (standard 130), ideally one by one, with a presser foot pressure of 40 psi.
– Use an aluminum sheet as a cover plate, and secure the PTFE plate with a 1mm melamine backing plate.
– After drilling, blow out dust with an air gun.
– Utilize the most stable drilling rig and parameters; generally, smaller holes require faster drilling speeds, smaller chip loads, and reduced return speeds.
3. Hole treatment: Plasma treatment or sodium naphthalene activation enhances hole metallization.
4. PTH copper sinking:
– After micro-etching (controlled to 20 microinches), the PTH enters the board from the de-oiler cylinder.
– If needed, proceed with a second PTH starting from the expected cylinder.
5. Solder mask:
– Pre-treatment: Use acidic plate washing instead of mechanical grinding.
– Bake the plate after pretreatment (90°C, 30 min), then brush with green oil and cure.
– Bake the plates in three stages: 80°C, 100°C, and 150°C for 30 minutes each. If the substrate surface is oily, rework by washing off the green oil and reactivating it.
6. Gong board:
– Place white paper on the PTFE board’s circuit surface, and clamp with FR-4 substrate or phenolic base plate (1.0 mm thick) to remove copper. Carefully trim burrs on the back by hand to prevent damage, then separate with sulfur-free paper and inspect visually. The gong board process’s quality is crucial.
5. Processing flow:
– NPTH PTFE sheet processing: Cutting – Drilling – Dry Film – Inspection – Etching – Erosion Inspection – Solder Mask – Characters – Spray Tin – Forming – Testing – Final Inspection – Packaging – Shipment.
– PTH PTFE plate processing: Cutting – Drilling – Hole Treatment (plasma or sodium naphthalene activation) – Copper Immersion – Board Electricity – Dry Film – Inspection – Diagram Electricity – Etching – Corrosion Inspection – Solder Mask – Character – Spray Tin – Molding – Test – Final Inspection – Packaging – Shipping.
6. Summary of difficulties in PCB high-frequency board processing:
– Immersion copper: Difficulties in copper adhesion to hole walls.
– Line gaps and sand holes: Challenges in map transfer, etching, and line width control.
– Green oil process: Issues with green oil adhesion and foaming.
– Board surface scratches: Strict control needed at each process stage.
