Specifying high speed PCB stackups appropriately
Application Note AP517
Who should read this?
PCB Technologists, PCB Procurement Engineers, PCB supply chain managers and EEs with an interest in ensuring their designs work as intended.
How to specify high speed PCB stackups so you procure PCBs appropriate for your application
Increasingly, PCB supply chain managers and procurement engineers sometimes approach us at Polar with the following question: "Why, when we have specified a PCB, do we receive different builds from different suppliers and even sometimes multiple different builds from the same fabricator?"
The first question you must ask yourself is, "Does it matter?" Well, as is often the case, it depends – if the PCB functions as expected and the overall thickness and performance is acceptable then you may be able to relax and accept that the PCB has been fabricated to specification using two or more differing constructions.
PCB fabricators now have a host of different base material suppliers to work with and each supplier typically has a large suite of material families to choose from. If your PCB is designed for high speed or high reliability, or has specialist heat transmission properties, you may benefit from giving the stackup more attention than in the past. Is this a straightforward choice? Maybe – maybe not... by tightening up the specification and involving yourself more deeply in material properties you run the risk of increasing the cost of your boards. However, you as a procurement specialist need to work with your PCB procurement engineers to trade off possible additional cost – and contrast that with the larger potential risk that the boards do not work as designed. Clearly for some boards a detailed spec is not critical; for others, it can be a matter of the PCB functioning as promised – or not.
Speedstack PCB and Speedstack Si controlled impedance stackup design systems
Polar Speedstack PCB and Speedstack Si controlled impedance stackup design systems provide formal and professional stackup design and documentation for all parties involved in PCB design and fabrication.
Providing full documentation of the completed stackup to the board supplier helps reduce the chances of miscommunication between PCB manufacturers and brokers, conveying clearly to the PCB fabricator the board designers' intentions, ensuring, for example, that the correct board and layer heights and materials are used in the build process. Speedstack can dramatically reduce the time for documentation and information sharing to a fraction of the time taken when employing traditional methods such as spreadsheet, word processor or presentation software.
Supply chain management
PCB manufacturing supply chains are becoming longer – increasing in distance both in time and geography. In addition, modern circuit board designs are increasingly complex, multi-layer and often include controlled impedance structures – the more complex the stackup, the greater the potential for error. Board designers and fabricators are often not in direct communication, particularly where board fabrication is under the supervision of third party PCB brokers.
Speedstack PCB is especially tailored for PCB fabricators and PCB brokers – anyone with a requirement to design or communicate controlled impedance PCB stackups. Speedstack PCB customers are able to share stackups and read impedance requirements from designers who are using Speedstack Si. Where the requirement is, however, for printed documentation, Speedstack provides an elegant solution in the form of its technical reports.
Clear concise documentation
Speedstack technical reports provide for sharing and documenting stackup information in a clear and consistent format. Material usage and graphical stack up views can be printed or shared electronically in a format that is easily read by both interconnect designer and fabricator. High quality customisable printouts make it easy to discuss alternate builds and pricing impacts with fabricators.
Flexibility in specification
Speedstack provides the flexibility to specify as loosely or tightly as required. For example, the Speedstack printouts below (Figures 1a, 1b) show details of a Speedstack Technical Report for a stackup specified with just the generic layer/drill and controlled impedance information.
Figure 1a Generic stackup
This report specifies overall board height, base and processed thicknesses (Figure 1a) along with the essential controlled impedance information (Figure 1b) for each structure..
Figure 1b Controlled impedance structures
This will allow board fabricators to achieve the desired stackup with their own materials and provide boards meeting specification at the lowest cost.
The stackup below has been specified more closely, with the material types stipulated expressly; the constraints imposed on the board manufacturer will ensure consistency of board fabrication – but at a higher cost.
Figure 2 Stackup with material (glass weave) types specified
Speedstack is used by PCB designers and fabricators worldwide to provide professional stackup documentation from end to end of their global and internal supply chains.
Click the link below to view a sample in PDF format.