It is necessary to mass-produce electronic circuit boards in a highly mechanical manner for ensuring the lowest cost of manufacturing. Traditional through-hole electronic components with leads did not lend themselves to this approach. Therefore, since the 1980s, virtually all electronics hardware is being mass-produced using surface mount technology (SMT). Compared to the through-hole technology (THT) used earlier, the surface mount devices (SMD) associated with SMT offer several advantages in terms of manufacturability and performance.
- Hot Air Solder Leveling (HASL)
- Organic Solderability Preservatives (OSP)
- Immersion Silver (ImAg)
- Immersion Tin (ImSn)
- Electroless Nickel/Immersion Gold (ENIG)
- Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
- Material used for the PCB
- Placement of traces
- Placement of planes
- Component interconnections
Materials Used for RF PCBs
- Stability of dielectric constant—Varying from lot to lot and over frequency
- Loss factor—Depending on surface contamination and the hygroscopic nature of the material
- Ability to withstand processing temperatures—Lead-free processing temperatures are higher than regular soldering temperatures
- Thermal conductivity—Even low-power RF circuits can produce a lot of heat
Therefore, selecting a suitable material for making a PCB operating at high frequencies depends on the above factors and the product cost. The choice could range from the low-cost FR-4 material, with its higher loss and not tightly controlled dielectric constant, to FR-4 derivatives with better specifications, or to other specialized low-loss RF material with their well-specified dielectric constant.
- Separate all RF, low-level analog, and digital sections.
- Divide the RF section into circuit groups (amps, LO, VCO, etc.).
- Place all the high-frequency components early in the layout, as this helps to minimize the length of the RF routes (in RF PCBs, functional orientation is more important compared to DFM).
- Place the components carrying the highest frequency next to the connectors.
- Never place unrelated inputs and outputs next to each other. For instance, multi-stage windings should never be placed adjacent.
- When long input or output to RF amplifiers is unavoidable, choose to make the output longer.
- As the trace impedance is a critical factor when trying to control reflections, always match the impedance between the driver and the load, except where the trace is shorter than 1/20th of the wavelength.
- When using pull-up inductors or resistors at the outputs of open-collector devices, always place the pull-up component next to the output pin it is pulling up.
- In addition to decoupling the main power pins of the IC, decouple the pull-up also.
- Inductors usually have large magnetic fields around them-
- Never placed them close together, when in parallel (unless the intention is to couple their magnetic fields)
- Separate all inductors by 1x times the body height (minimum) OR
- Place inductors perpendicular to one another
- Confine “ALL” routes to the section or stage to which they are assigned –
- Digital traces in the digital section
- Low-level analog traces in the low-level analog section
- RF traces in the RF section
- Routing traces into adjoining sections is not recommended
- Route all short RF traces on the component side of the PCB, rout them to eliminate vias
- Place a ground layer below the RF traces.
- Minimize the vias in the RF path, as this reduces the breaks in the ground plane(s) and –
- Minimizes inductance
- Helps contain stray magnetic and electric fields.
- Long controls lines are acceptable, but take care to route them away from RF inputs.
- Keep RF lines away from one another by a minimum distance to avoid unintended coupling & crosstalk.
- Minimum spacing is a function of the acceptable level of coupling, and is good for crosstalk, directional couplers, crosstalk, differential lines coupled in even or odd modes.
Frameless Stencils which are loose foil not with rigis Frames have the advantage of being extremely economical and save tremendous amount of space as well. For Limited Production runs and where budgets are a factor, these are very useful.
1. Patterned border
2. Basic outline border.
Frameless Stencils are laser sliced weld glue stencils intended to work with stencil tensioning frameworks otherwise called Reusable Stencil Frames. Frameless Stencils are noteworthy more affordable than Framed Stencils and lessen storage room necessities.
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