WILD RIVER TECHNOLOGY LLC Connector Launch Design Guide For Vertical Mount RF Connectors James Bell, Director of Engineering 4/23/2014 This guide will information on a typical launch design procedure, focused on compression attached vertical RF connectors. There will also be special tips Wild River Technology has learned over several years of our own design projects.
Table of Contents Pre-design Activity... 2 Choosing Your RF Connectors... 2 Choosing Your Connector Launch Designer... 2 Mechanical Considerations... 4 The Launch Design Checklist... 4 PCB Landing Pad Design and Simulation... 5 Launch Verification Measurement - Optimization... 6 About Wild River Technology... 8 1
More than ever, launch designs need to be considered once the signal groups are being allocated in the stack-up definition phase. Why? Because they are Critical to the success of the high-speed paths Easy to get wrong Prone to causing board fabrication spin cycles if the measurements don t match simulation. The following guide will help you to become more aware of what it takes to design a good high speed connector launch, and also help you decide how to best approach the effort; to do the design work in-house or to have an outside service design it for you. Although this guide is focused on designing launches for vertical connectors, most of the principles and methods apply to launch designs for any high frequency connector. Pre-design Activity Choosing Your RF Connectors The goal of any RF connector design is to eliminate or reduce reflections, maintain constant impedance, and provide a robust mechanical connection. A standard 2.92 mm connector uses a customized insulator designed to maximize signal propagation and mechanical support. While this design is electrically efficient, it enables the possibility of introducing foreign object damage (FOD) into the air line during soldering, which causes unwanted reflections. To eliminate this possibility we recommend connectors that require no soldering, like our VLF family of vertical launch 2.92mm (K) connectors. Choosing Your Connector Launch Designer If you have the talent in-house to design and optimize the connector launch to what is probably your first foyer into the frequency of interest, good for you. But, in case you need help, here are some alternatives to consider: Option 1: Design a connector launch in-house Lots of customer complexity, so start the design in your stackup definition phase or soon after. High risk, high hassle Could be very expensive if optimized over several board turns. Option 2: Contract the launch design out and evaluate yourself. Vendor will design optimized launch based on customer supplied stackup and board materials. Medium risk. Vendor is only responsible for launch design and simulation. Measurement correspondence is the customer s responsibility. 2
Medium hassle. Constant correspondence with contractor during correspondence testing. Cost moderate Assumes you have proper equipment and skills to validate PCB Contact WRT for more information if you want to go this route. Option 3: Full service contract for optimized launch based on supplied stackup and board materials, fabricate an evaluation board (See Figure 1) and make TDR and S-parameter measurements, including starter kit in 3D EM solver Contractor assumes all risk Moderate cost increase to customer over Option 2 Best chance for correct launch on first fabrication of your PCB design. Contact WRT for more information. Figure 1 - Typical connector launch evaluation board. Stripline structures are on opposite side. TIP: Depending on the size and complexity of your board design, you may want to create a small test and measurement board before committing your launch designs to the final board design. A small test board is normally much cheaper to fabricate than the final board, and you can also add test structures for VNA calibration and material properties measurement. Wild River designs, fabricates and/or tests these types of boards for our customers when they require our expertise. 3
Mechanical Considerations Although not part of the connector launch design proper, a poor connector mounting design in your PCB can have a significant effect on measurement correspondence and launch signal quality consistency. There are two potential problems to consider: Misalignment of the connector s center pin to PCB pad Connector shift due to cable torque during and after installation. Here are some recommendation: Use the smallest drill diameter practical for the mounting holes. The usual drill hole size recommended for 0-80 screws can allow the connector to mount off center. Check to see if your fabrication vendor will guarantee a maximum plating thickness and use the smallest drill size possible. We use 60 mils with +/-3 mil tolerance. If your fabrication vendor will not guarantee their maximum plating thickness, use the recommended drill size and consider using flat head screws if your board is thick enough for the countersunk holes. Use Torx screws and a torque screwdriver for consistence connector mounting results. Phillips head screws have a way of fooling the user into thinking they are applying the correct torque, but remember that the Philips screwdriver is being forced upward while you are forcing it down. Using Torx screws remedies this and gives a better surface connection as a bonus. We use 1 in-lb. of torque on each screw. WRT can provide these items. Call us for details. Figure 3 - Torx Screw Figure 2 - Torque Screwdriver The Launch Design Checklist If you take nothing else away from this guide, let it be that you need to create a checklist of the materials and other information each connector launch design will need, be it for your internal 3D EM design engineer or for your outside design service. Here is what WRT requires for our launch design service: Connector Part Number, mechanical data, and s-parameter model 4
Stackup data (material, number of layers, etc.) Launch transition layer Frequency bandwidth Transmission line type: Microstrip, Stripline or Grounded Coplanar Waveguide (GCPW) Transmission line dimensions: - Customer supplied or provided by PCB fabrication vendor stack-up? - Optional: For MS transmission line, design frequency to calculate line width (e.g. dispersion effects) Specification sheet for substrate materials and provide dielectric constant (for some materials it isn't clear what the Dk is) Fabrication vendor capabilities (Smallest via size allowed, minimum copper feature gap, etc.) Words of Wisdom from an Experienced 3D EM Designer It typically takes xx hours to get the information, potential clarification, transmission line synthesis, simulation setup, optimization, CAD file and report. If you change the thickness or dielectric constant, it more or less takes the same amount of time to redo it. If the new file was quite similar to an existing file, it might take an hour less... But in my experience, that hasn't happened very often. The takeaway is, make sure your checklist is complete and accurate before submitting it to the 3D EM designer. PCB Landing Pad Design and Simulation PCB footprints should be designed to match an RF connector to a specific PCB stackup. The characteristics of the vertical launch connector that need to be considered are found in the connector mechanical document. The mechanical dimensions for the bottom center pin and ground ring for two types of connectors are shown below. 5
Figure 4 Typical 2.92mm vertical mount connector footprint. WRT part VLF-K+ shown Figure 5 - Typical SMA vertical mount connector footprint. WRT part VLF-KS shown. The characteristics of a printed circuit board that need to be considered for proper matching in a vertical launch are 1. Dielectric constant and thickness of prepreg and core layers (S1 and S2) 2. Signal pad diameter (D) 3. Top signal pad to ground fill gap width (G) 4. Signal transition via diameter 5. Copper thickness (T1, T2) 6. Top and bottom layer plating and masking 7. Plane void diameter(s). 8. Grounded guard vias - how many and their distance from the signal transition via. Modification to the items listed above to maintain 50 ohms impedance and minimized reflections can be modeled and analyzed in electromagnetic simulation software. Launch Verification Measurement - Optimization Attention should be paid to the correlation between theoretical performance of a simulation and measured performance of a finished product. The difference between an accurately matched connector and footprint and one that has not been optimized can be pronounced, especially at high frequencies. A paper that describes an approach to simulation to measurement correspondence can be found on Wild River Technology s web site in our Papers collection. DesignCon 2004 paper: A Hybrid Measurement and Electromagnetic Field Solver Approach for the Design of High-Performance Interconnects: An Investigation of Traces and SMA Transitions by Scott McMorrow and Alfred Neves 6
Abstract: As the cost of bringing complex high-performance systems to market increases and the time to market decreases, new methods must be used to insure first-pass design success. This paper describes a verifiable design method that establishes excellent signal integrity and enables leading-edge performance with common off-the-shelf materials. By integrating field solver and measurement-based modeling methods, the net process is verifiable. The optimum low-cost production design can be attained in the shortest time. This process yields unmatched time-domain and frequency-domain performance using off-the-shelf, generic, non-esoteric, low-cost materials. 7
About Wild River Technology Sandy River, Oregon Need help? Wild River Technology can provide Connector launch design Launch evaluation PCB design VNA training. Contact us for details. www.wildrivertech.com Office 503-718-7172 Mobile 503-679-2429 info@wildrivertech.com Wild River Technology markets products and services for the high-speed Signal Integrity practitioner an individual who addresses challenges in modeling interconnects, making better measurements, and solving signal integrity issues to 32 Gbpsec data rates and beyond. Our products and consulting are born over many years of working in high-speed design environments, and are adopted by much of the measurement community for improving measurements, characterizing SERDES, and optimizing communications channels all designed to elevate SI practices and methodology. 8